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Performance assessment of cement grout borehole plugs in basalt
ENGiNEERiNG GEOLOGY ELSEVIER
Engineering Geology 37 (1994) 137-148
Performance assessment of cement grout borehole plugs in basalt Haluk Akgun a, Jaak J.K. Daemen b " Geotech, Inc., 125 North Route 73, Maple Shade, NJ 08052, USA b Mining Engineering Department, Mackay School of Mines, University ofNevada--Reno, Reno, N V 89557, USA
(Received February 22, 1993; revised version accepted January 21, 1994)
Abstract
Flow tests have been conducted on expansive cement grout plugs with diameters of 160 mm and 200 mm, and lengthto-diameter ratios of one, in boreholes in basalt blocks and in steel pipes. Two types of flow tests have been performed: pseudo-constant head tests and transient pulse tests. Hydration temperatures of cement grout plugs have been monitored in steel pipes with inside diameters ranging from 110 mm to 200 mm. During flow tests, basalt blocks have fractured, presumably due to water injection pressure, cement grout expansion, packer pressure and temperature differences. Falling head tests performed on some block fractures indicate a complex interaction between a cement grout borehole plug and the rock, as determined from the hydraulic conductivities of fractures intersecting plugged boreholes.
1. Introduction
Construction and exploration related penetrations (e.g., boreholes, shafts or tunnels) associated with a high-level radioactive waste ( H L W ) repository must be sealed on abandonment to retard the migration of radionuclides to the accessible environment. Sealing of openings that penetrate a H L W repository geological barrier requires the use of materials that meet regulatory seal performance requirements. Expansive cement grouts are sealant materials considered by the U.S. Department of Energy ( U S D O E ) . We define plugs as structures emplaced in boreholes (or in other rock penetrations such as shafts or drifts) with the express purpose of greatly increasing the resistance to fluid flow along the penetrations. Most studies performed to date on borehole plug sealing have been on relatively small diameter boreholes (typically 25 mm) drilled in intact rock cylinders. Because of the relatively narrow range 0013-7952/94/$6.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0013-7952(94)00002-J
of plug diameters used, the results could not be extrapolated to larger plug sizes. The objective of this paper is to assess the effects of size on the performance of expansive cement grouts used to plug boreholes. Flow tests have been conducted on plugs with diameters of 160 m m and 200 mm, with length-to-diameter ratios of one. The plugs have been emplaced in boreholes cored in basalt blocks or in steel pipes. Differential water pressures of up to 3 M P a have been applied across the plugs. Cement grout hydration temperatures have been monitored at the top, middle and b o t t o m of plugs in steel pipes with inside diameters of l l 0 m m , 160 m m and 200 mm. The purpose of the hydration temperature measurements was to investigate the influence of cement plug size on hydration temperature generated. Temperature rise is an indicator of potential self-induced fracturing during cement curing, and is likely to be a function of cement volume. The rock blocks were taken from the P o m o n a
138
H. Akgun, J.J.K. Daemen/Engineering Geology 37 (1994) 137-148
Member of the Saddle Mountains Formation (a flow basalt unit of the Columbia River Basalt Group), located in south-central Washington. The expansive cement grout formulation, provided by Dowell-Schlumberger was composed of Ideal Type A Portland cement (from Tijeras Canyon, New Mexico), mixed with 50% distilled water, 10% D53 (an expansive agent) and 1% D65 (a dispersant). All additives are Dowell Schlumberger trademarks. All percentages were weight percent with respect to cement. Cement mixing was performed according to American Petroleum Institute (API) specification, API Standard No. RP-10B (1977). The plugs were cured under distilled water for eight days prior to flow testing.
2.2. Experimental procedure and set-up Figure 1 gives the flow test set-up. The system consists of eight major components: (1) a nitrogen tank; (2) a bladder accumulator; (3) a pressure gage; (4) a flowmeter; (5) an expandable mechanical packer; (6) a water outflow collection unit (recaptured flow unit); (7) stainless steel tubing; and (8) a steel frame. The bladder accumulator is pre-charged with nitrogen gas and then filled with distilled water. It is used to inject water under ~$UrQ
/
2. Flow testing
~r
2.1. Introduction The hydraulic conductivity of cement grout plugs poured and cured in relatively small diameter boreholes (usually 25 mm) ranges from 1 × 10 -14 m/s to 8×10 -11 m/s (e.g., Fuenkajorn and Daemen, 1986; South and Daemen, 1986; Adisoma and Daemen, 1988). The properties of both rock and sealing materials are size-dependent. This sizedependency introduces uncertainty in the extrapolation of the results to larger-diameter plugs. To assess the uncertainty involved, laboratory flow tests have been performed on plugs in larger diameter boreholes in basalt blocks and in steel pipes. Table 1 gives the dimensions of the plugged basalt block boreholes.
Flowmeter L_.~-----~Center rod
Nitroge tank~
~_,]~;f p.cker [ [~-;,~Upper end prate ~!~:} of p,.¢ker ~ . ' , , ,..;,'.~.;t~1~1Pa¢k• r (rubber)
-
.. _ ~ ...... J----~Rock K,q:~t;',':"?.t~5<~,:;':.:"~.~)11- II brock
V,/I',;5:',"-'~t'Pv".4+/"."J;'-',',~r:,'VT'T
I
II fiLL
I
!! B o " ^ m PVCdls-
Cement~ " : ~ ) ; , ' ° ] ~ L o w Ie r II m"o'packer el I I I//A~l:-'.,'J',~'-"-'-'~:';4;w-'~//l
II
+:.......
I IL;.;:,,.,.,,,o,, !
25cm
I
Fig. 1. Laboratory set-up for flow tests in a plugged Pomona Basalt block.
Table 1 Dimensions of the plugged basalt blocks Basalt block
D O (mm)
Dp (mm)
L1 (mm)
Lp (mm)
L 2 (mm)
Lt (mm)
No. No. No. No. No.
584 223 201 470 330
200 160 160 200 160
400 318 254 368 267
203 160 160 200 160
108 70 70 127 203
711 546 483 692 629
1 2 3 4 5
D o = m a x i m u m block diameter; Dp=plug (borehole) diameter; L l = t o p borehole length; Lp=cement plug length; L2=bottom borehole length; and Lt = total borehole length.
H. Akgun, J.J.K. Daemen/EngineeringGeology37 (1994) 137-148
pressure through the cured cement grout plug. The pressure gage measures the injection pressure. The volume of water injected is measured with the flowmeter. The pressurized water is injected through the axial hole of the expandable packer, and then, to the cement grout plug. The rubber disks of the mechanical packer expand when the packer is torqued in order to seal the top block borehole. A pipet is used to collect and measure the volume of water flowing through the cement plug (recaptured flow unit in Fig. 1). The bladder accumulator, pressure gage, flowmeter and packer are connected by stainless steel tubing. A steel flame around the rock block holds it vertically and stabilizes it during packer installation. Experimental details are given by Akgun and Daemen (1986). 2.3. Flow tests in boreholes in basalt blocks Introduction The flow tests led to the unintended fracturing of all five basalt blocks. In three blocks, fracturing occurred prior to flow testing, presumably due to a combination of the elevated hydration temperature of the cement grout (that led to temperature differences within the plug and between the plug and rock) and swelling of the expansive cement grout. Temperature differences between the curing cement grouts and the rock blocks reached up to 47°C. This temperature difference induced thermal shear stresses along the plug/rock interface and within the rock, which might have contributed to block fracturing. Akgun and Daemen (1991) give an analysis of this phenomenon. In the other two blocks, fracturing occurred during flow tests, most likely due to the combined effects of injection pressure, cement grout swelling pressure, packer pressure and temperature differences. The induced fractures of four blocks ran approximately parallel to the boreholes and through the entire lengths of the blocks. The other block fracture ran at about 45 ° to the borehole axis. The differences in fracture orientations are attributed to pre-existing microfractures although no attempt has been made at inspecting thin sections for petrographic evidence. Each fractured block had one or two dominant fractures along which flow tests have been con-
139
ducted. The results of these flow tests are reported in the following section. Cement grout swelling pressure was the main cause or at least one of the causes for the opening up of joints or hairline fractures, i.e., fractures visible to the naked eye after cement swelling and/or flow testing, but not detected before, in these basalt blocks. The blocks had been inspected carefully prior to testing, and along the borehole walls no cracks were apparent to the unaided eye. This observation confirms that a swelling sealing material can inadvertently enhance the hydraulic conductivity of the rock surrounding a sealed opening (as suggested, for example, by Apps et al., 1983). 2.4. Falling head tests on the basalt block fractures Introduction Development of fractures in the No. 1, No. 4 and No. 5 blocks provided a ready opportunity for the study of water flow through fractures. Given the limited data base presently available on relatively large-scale boreholes, and the already installed instrumentation, it was deemed appropriate and productive to divert somewhat from the main objective of this research, which was aimed at studying the size-effects on the hydraulic performance of cement grout plugs emplaced in drilled boreholes in rock blocks. Laboratory set-up for block fracture flow tests Figure 2 gives the schematic for performing falling head tests and for applying an approximately normal stress to the main fracture induced in the No. 1 Pomona Basalt block. The load was applied by hydraulic cylinders placed under the beams. The beams were connected by threaded rods. Wooden blocks and steel plates were placed under the beams to distribute the load uniformly across the block. The fracture flow was measured with a pipet. A saturated sponge was placed underneath the plug to prevent drying-induced plug shrinkage which could have led to cracking and to increased permeability along the plug/rock interface (as observed, for example, by Adisoma and Daemen, 1988). The Nos. 4 and 5 block fractures were flow tested with a configuration similar to that shown
H. Akgun, J . J . K . Daemen/Engineering Geology 37 (1994) 137 148
140
tst I Plpets
It
er disk Rock block S p t l e t ' ~ ~ ~
w,2.,, b... ~,_.G~
Irl~'~
#;,2"_'~
-
.
.
.
.
M-,~
-i, ..~~2 k,,*.:l-.',~Vll
IIII
ii I
. .~. ~,~.r~/
,pongo
.
~,.,"-:].t.._~w**d,n bl~k,
.~,t~.;_~. Ii
.
~ ~oncreto flu
~---~//A
Bot tom " l r / / / / ~ ] PVCdilc ~ , / / / J t / ~
I
l////Zl
| ~
I'////J
r////J
tic
Ground Level
Cement ptug
I
Fig. 2. Laboratory set-up for flow rate tests on the No. 1 Pomona Basalt block fracture. An approximatelynormal stress was applied to the fracture. W . L'racture lensth
in Fig. 2, but unconfined (i.e., without the loading frame).
Hydraulic properties of the block fractures Falling head tests on the No. 1, No. 4 and No. 5 basalt block fractures led to the determination of the fracture flow rate (Qf), fracture hydraulic conductivity (Kf), and the fracture hydraulic aperture (2b), as a function of time. The assumptions made in the derivations of the equations are: (1) a parallel, planar plate model representing the fracture; and (2) straight, horizontal and laminar flow in the borehole (i.e., no vertical flow). Figure 3 gives the flow parameters of a block fracture. The derivations of the equations are given in Akgun and Daemen (1986). The flow rate of the fracture (Qf) determined by the falling head test is given by:
I~I"UM NIl to l i l l e
Lf - f r a c t u r e width
hv - ~mter heed at iny point alon| the borehole u e l l h I = ~qLter held I n p i p e t st tlJle t 1
r o = pLpet radius
h 2 - vaker heed in p i p e t lit t i l e
I: 2
Fig. 3. Flow parameters of a Pomona Basalt block fracture. Flow lines are a schematic estimate, for illustrative purposes only. W= fracture length, Lf= fracture width, ro = pipet radius, hw=water head at any point along the borehole wall, hi= water head in pipet at time tl and h2=water head in pipet at time t2.
where: K f = fracture hydraulic conductivity; 2b =fracture aperture calculated from Eq. 3; L r = fracture width; W = f r a c t u r e length; h a = w a t e r head in pipet at time tl; h2=water head in pipet at time t2. Definition of the other variables are given with Eq. 1. The fracture aperture (2b) is determined from the falling head testing: "
Qf = ~r 2 dhw dt
2"
F12pLf~r 2 In (hl/h2)~ 1/3
(1)
where: Qf=fracture flow rate; r o = p i p e t radius; hw=water head at any point along the borehole wall; t = e l a p s e d time. The fracture hydraulic conductivity (Kf) is given by:
where: 2 b = fracture aperture width; 7w=unit weight of water; and, g = d y n a m i c viscosity of water. The other variables are defined with Eqs. 1 and 2.
LfTTr2 In (hl/h2) Kr= (2b) W(t2 - tl)
Falling head tests performed on the induced basalt block fractures allowed to study the
(2)
Results of the fracture flow tests and discussion
H. Akgun, J.J.K. Daemen/Engineering Geology37 (1994) 137-148
influence of the interaction between a cement grout borehole plug (e.g., swelling and shrinkage alternations) and the rock, and of the normal stress across the fracture on the hydraulic conductivity. The interaction between these various effects became complicated. Individual influences could not be separated out readily. The following counteracting effects needed to be considered: (1) Cement grout swelling. During cement grout swelling, the interface between the plug and the surrounding rock closed, i.e., the hydraulic conductivity along the plug/rock interface decreased. It was unclear whether the grout penetrated into the main induced fracture or not. The hydraulic conductivity of the fracture increased as a result of fracture aperture increase. (2) Cement grout shrinkage. During the testing sequences, the plugs have been allowed to dry out for a few hours to a maximum of ten days. During typical testing sequences, water could be maintained on top of the plugs for only about 3 to 8 hours per day, due to the emptying of the water reservoirs through the block fractures. During drying, shrinkage could have taken place (as observed, for example, by Adisoma and Daemen, 1988). This shrinkage could have resulted in an increased hydraulic conductivity along the interface between the plug and rock, and a decreased hydraulic conductivity along the fracture. (3) Cement grout permeability changes as a function of cement curing temperature. Beyond the initial hydration pulse, temperature changes have been modest (_2°C), and are not believed to have had a significant influence. (4) Cement grout-water-rock interaction. Chemical effects such as secondary mineralization and dissolution. Explicit identification of such causes were beyond the scope of this research. (5) Mechanical effects. Cement grout sedimentation along the fracture could have plugged a fracture, but also could have prevented re-seating and could have caused shearing along the fracture during an increase of the applied normal stress or during cement grout shrinkage, thus maintaining a higher hydraulic conductivity. Figure 4 gives the flow rate (Qf), hydraulic conductivity (Kf), aperture (2b), and the applied, approximately normal stress (on) on the No. 1
141
Pomona Basalt block fracture as a function of time. The falling head test was continued for about 5 months with nearly normal stresses ranging from 0 to 2.2 MPa. The average fracture aperture was estimated to be less than 3.8 x 10 -5 m when measured by an engineering filler gage after 5 months. During the first 4 hours of testing, the flow rate was inversely proportional to the normal stress on the block. Keeping the normal stress nearly constant for an additional 17 days led to a decrease in the fracture hydraulic conductivity presumably due to the continuing seating of the fracture. Over 17 days of flow testing, no monotonous relation was observed between the fracture hydraulic conductivity and the applied stress (Fig. 4). These cyclic and conflicting trends could have been due to the reasons listed above, namely: (1) fluctuations in the applied stress; (2) the opposing effects of the plug/rock interface flow path and the fracture flow path, e.g., plug swelling reduced interface flow but enhanced fracture flow; (3) the influence of drying-induced plug shrinkage being temporarily overridden by the influence of the increased normal stress or vice versa, e.g., if the plug shrinkage had been severe, the hydraulic conductivity along the interface could have increased more than the reduction in fracture hydraulic conductivity due to an increase in the normal stress; (4) the influence of the increased normal stress being overridden by the influence of plug swelling or vice versa, e.g., if the magnitude of the tensile stresses on the fracture caused by plug swelling was higher than the compressive stresses due to confinement, opening up of the fracture aperture and increased fracture hydraulic conductivity could have been observed; (5) cement grout sedimentation or secondary mineralization along the fracture could have prevented the re-seating of the fracture and could have caused shearing along the fracture with increased normal stress. This could have led to increased fracture hydraulic conductivity. Dissolutioning along the interface also could have led to increased hydraulic conductivity; and (6) recovery of the re-saturated plug upon drying could have reduced the interface hydraulic conductivity. In the absence of detailed investigations of individual effects, it is difficult to identify the relative influence of each effect.
H. Akgun, J.J.K. Daemen/Engineering Geology37 (1994) 137-148
142
3"
~2
O
8' 7.
o I,c
6 5
Eu
|
~
...................
---_
. . . = .1~ J/ - - ,,,Ill| I
4
,¢
3
l
2
u
I 0
I0 0
I01
IO 2
TIME
IO ~
~04
10 5
(minutes)
Fig. 4. Rate of flow (Qr) through the No. 1 Pomona Basalt block fracture, fracture hydraulic conductivity (Kr), and fracture aperture (2b) as functions of the applied, nearly normal stress (c~.), and time (1 day = 1440 min).
The No. 4 and No. 5 Pomona Basalt block fractures were flow- tested under unconfined conditions for up to 2 V2 months and showed an increase in all fracture hydraulic properties until the tests were terminated at the conclusion of the project. The manually measured average fracture apertures differ from those calculated analytically (from Eq. 3) for up to about five-folds. Both cyclic and conflicting trends were observed. These trends could have been due to: plug swelling and the resulting increase in the fracture aperture; slip along the fracture as a result of plug swelling or plug shrinkage; drying induced plug shrinkage; the recovery of the re-saturated plug; and the opposing effects between plug swelling and plug shrinkage.
Tensile strength of the Pomona basalt blocks and discussion The tensile fracturing of the basalt blocks allowed the determination of the tensile strength of basalt. The fractures of the No. 1, No. 3, No. 4 and No. 5 basalt blocks ran approximately parallel to the borehole axes. The tensile strengths were calculated from the equation for a hollow cylinder subjected to an internal pressure (e.g., Jaeger and Cook, 1979):
Pi(D~,-t-D~)) ~t -
2
2
Dp--D o
(4)
where: o t = tensile strength; Pi =internal (contact) pressure resulting from cement swelling, packer pressure (Pp), or water injection pressure (Pw); Dp=inside borehole (plug) diameter; and D o = outside block diameter. The tensile strength (%) of the No. 2 basalt block fracture (oriented at approximately 45 ° to the borehole axis) was calculated from:
¢~t-
Fw Af
(5)
where: Fw = water injection force on the fracture = (rt/4)D~,(Pw/2); and, Af= fracture surface area. Equations 4 and 5 require parameters such as cement grout swelling pressure, packer contact pressure, water injection pressure, inside and outside block diameter, and fracture surface area. The thermally-induced contact stresses are neglected in these equations because the blocks fractured after the grout curing temperatures were back to ambient. Akgun and Daemen (1991) measured the swelling pressure of a similar type of cement grout cast in 100 mm inside diameter steel pipes. The swelling pressure (Ps), measured with strain gages on the steel pipe outside walls, was 2.2 MPa after 8 days of cement curing. The No. 2 and No. 3 basalt blocks fractured at injection pressures (Pw) of 3.2 MPa and 0.7 MPa, respectively. The packers
143
H. Akgun, J.J.K. Daemen/Engineering Geology 37 (1994) 137-148
were torqued to 270 N.m, which led to an estimated packer contact pressure (Pc) of 2.1 MPa. The No. 2 block fracture surface area (Af) was 8 x 10 -2 m e . Equations 4 and 5 gave tensile strengths between 2.8 MPa to 9.5 MPa (based on cement grout swelling pressure), 9.2 MPa (based on packer contact pressure), and between 0.4 MPa to 3 MPa (based on water injection pressure). The tensile strengths calculated from Eq. 4 were high because this equation assumes that the pressurized length equals the cylinder (fracture) length. In actuality, the fracture lengths were considerably greater than the lengths of the pressurized intervals. The tensile strength calculated from Eq. 5 is low as compared to the results listed below because the normal stress is assumed to be uniform across the fracture. The calculated minimum and maximum tensile strengths were 0.4 MPa and 9.5 MPa, respectively. Fuenkajorn and Daemen (1986) performed ring tension tests on Pomona Basalt disks with outsideto-inside diameter ratios (i.e., stiffness, ratio of the radial displacement to the radial stress, Desai, 1979) similar to those of the basalt blocks. The blocks have fractured at internal pressures corresponding to tensile strengths significantly below (by a factor of two to sixty) those determined from ring tension tests. This might indicate a size effect on the tensile strength of basalt, the influence of prolonged loading, or basalt weakening under the influence of water-rock interaction, and might suggest that the long-term in situ strength of basalt may be much lower than that of samples conventionally tested in the laboratory. Several tensile fractures ran through the cement grout plugs and appeared to be direct continuations of the block fractures. This phenomenon suggests that the splitting fractures initiated within the cement grout plugs and propagated into the rock.
2.5. Flow tests in steel pipes Introduction The fracturing of the basalt blocks led to a decision to determine the hydraulic conductivities of cement grout plugs emplaced in steel pipes. The pipes had inside diameters of 160 mm and 200 mm.
The cement grout plugs had length-to-diameter ratios of approximately 1. They were cured for 8 days prior to flow testing. Pseudo-constant head and transient pulse tests were performed. The laboratory set-up was similar to that shown in Fig. 1.
2.6. Determination of the hydraulic conductivity of cement grout plugs cured in steel pipes Pseudo-constant head tests The tests are referred to as pseudo-constant head tests because water could not be injected at a constant pressure throughout testing; the injection pressures decreased with time. The hydraulic conductivities were calculated by using total flows, i.e., both flow through the plug body, and flow along the plug/rock interface, if any. The hydraulic conductivity of the cement grout plug obtained from pseudo-constant head testing can be expressed as (Akgun and Daemen, 1986): a(h2 - h i )
kAF 1 t2
zL ,lP at + L(te-
(6)
;t h
dt 1
where: k = the hydraulic conductivity of the cement grout plug; P = w a t e r injection pressure; A =plug cross-sectional area; L = p l u g length; a = p i p e t cross-sectional area; h = total head at the bottom of the plug; hi and he=water levels in the outflow collection pipet at times tl and t2, respectively; and, Yw= u n i t weight of water.
Transient hydraulic conductivity analys& Hsieh et al. (1981) used the transient pulse test to measure very small hydraulic conductivities. The apparatus consists of a cement grout plug with direct connections to an upstream and downstream fluid reservoir. At the start of the experiment, the fluid pressure in the upstream reservoir is suddenly increased. The fluid flows from the upstream to the downstream reservoir. The pressure in the upstream reservoir decays. The hydraulic conductivity of the cement grout plug is calculated from the pressure decrease in the upstream reservoir. The initial and boundary conditions describing the hydraulic head in the plug,
H. Akgun, J.J.K. Daemen/Engineering Geology 37 (1994) 137 148
144
and in the upstream and downstream reservoirs are solved by the Laplace transform method. The procedure is given by Hsieh and others (1981, Appendix I).
6 x 10-13 m/s, with fluctuations over time of about two orders of magnitude. Figure 5 gives the result of one transient permeability pulse test on the cement grout plug in the 200 mm diameter pipe. The pressure, as a percentage of the initial pressure, is plotted as a function of time. The initial and final pressure in this 200 mm diameter pipe was 0.7 MPa and 0.5 MPa, respectively. Figure 6 gives the type curve that fits the experimental results given in Fig. 5. The hydraulic conductivity determined by matching the experimental and type curves is 2 × l 0 -x4 m / s . Three sets of decay tests were run in each pipe for up to 1 month. The average hydraulic conductivities of the 160mm and 2 0 0 m m plugs were 6 x 10 -15 m/s and 2 x 10 -14 m/s, respectively.
Results of the pipe flow tests The pseudo-constant head tests on the 160 mm and 200 mm diameter pipe plugs were performed for over 2 months, with water injection pressures ranging from 0.7 MPa to 3 MPa. A total of 64 and 66 sets of individual flow tests, with testing periods for individual tests ranging from 15 minutes to 6 days were run in each pipe. Readings were taken at intervals ranging from 2 minutes to about 20 hours. Akgun and Daemen (1986) give the details. The cement grout plug in the 160 mm diameter pipe gave a minimum and maximum hydraulic conductivity of 6 x 1 0 -15 m/s and 7 x 10 -xz m/s. The minimum and maximum flow rates were 2 x 10 -13 m3/s and 7 x 10 -11 m3/s. The minimum hydraulic conductivity and outflow rate of the cement grout plug in the 200 mm diameter pipe was 2 x 10 -14 m / s and 7 x 10 -13 m3/s, respectively. The maximum hydraulic conductivity was 8 x 10 -12 m/s. The maximum outflow rate was 3 x 10-lo m3/s. The average hydraulic conductivity of the cement grout plugs in each pipe was
Discussion of the results of the pipe flow tests No obvious dependency of hydraulic conductivity on plug size has been identified. The average hydraulic conductivity of the plugs in the 160 mm and 2 0 0 m m pipes varied from 6× 10 -15 m/s to 6 × 10 -13 m/s and did not differ drastically from the hydraulic conductivities of the much smaller (25 mm) plugs tested (e.g., by Fuenkajorn and Daemen, 1986; South and Daemen, 1986; Adisoma and Daemen, 1988).
I00
O
90
• °
80-
0
Q
• ".'.",
t,i
~ro(n
~:~o.
50-
40-
~)10 IO
'
~ ~ J '~'"110
'
'
' ' ''"~210
'
' ' ' '''"310
'
' ' ''""410
I
.....
;()'5
TIME (minutes) Fig. 5. Pressure decay for a cement grout plug in a 200-mm steel pipe obtained through transient pulse testing ( 1 day = 1440 min).
H.Akgun,J.J.K.Daemen/EngineerinGeol g ogy37(1994)137-148 I• 0 o ~ ~ I =0.0604
145
J
I ,:°.,
.90 .80 •70.60.~-
.40.
.30
i
,
| 1 1 1 , , ,
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f
i
i l l , , , ,
i
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,
~,11,11
i
IO I
,
, , , , i , i
,
,
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,
, l l , l ~
103
oc8
(HJH)
Fig. 6. Analytical transient pulse test type curve for a decrease in the upstream reservoir hydraulic head versus ct~ (~t= dimensionless time; 13= ratio of the compressive storage within the sample to that in the upstream reservoir; y = ratio of the compressive storage of the downstream reservoir to that of the upstream reservoir).
3. Cement grout hydration temperature 3.1. Introduction
This section describes the temperatures measured in cement grout plugs cast in steel pipes. The cement grout plugs had diameters of 110 mm, 160mm and 200mm, and length-to-diameter ratios of about 2. The main purpose of this study was to investigate the influence of plug size on the hydration temperature. According to Neville (1981 ) the interior of a large concrete mass (e.g., gravity dam) produces a rise in temperature due to the heat of hydration, while the exterior loses heat to the surrounding atmosphere. If the temperature difference between the interior and the exterior of the concrete mass becomes too large (e.g., 20°C or greater), cracking may develop. The objective of this section is to determine whether considerable rises in temperature that occur during cement grout curing could damage the plugs significantly. Elevated temperatures due to the hydration of the curing cement grouts might also have led to excessive thermal shear stresses along the plug/rock interfaces, and within the rock blocks. These shear
stresses might have contributed to the fracturing of the basalt blocks. Akgun and Daemen (1991) have analyzed this mechanism. The hydration temperature was measured by temperature sensors connected to a temperature indicator• Three temperature sensors were installed in each cement grout plug: one near the top of the plug, one at the middle, and the third near the bottom of the plug. Akgun and Daemen (1986) give the experimental details. 3.2. Results and discussion
The hydration temperatures are summarized in Table 2. A similar trend was observed for all tests. Immediately after the cement grout was poured, temperatures were in the range of 25 ° to 38°C. Two to five hours later, the temperatures decreased to 20 ° to 33°C, probably due to the cement pastes coming into thermal equilibrium with the steel pipes and the room environment. The temperatures then gradually increased until the maximum hydration temperatures were reached. Following this, the curing temperatures decreased to room temperatures (22 °_4°C) after 27 to 46 hours. The results of the cement grout hydration
H. Akgun, J.J.K. Daemen/Engineering Geology37 (1994) 137-148
146
Table 2 Results of the cement grout hydration temperature measurements in steel pipes Cement grout plug diameter
Cement grout plug length
Maximum hydration temperature
(mm)
(mm)
(°C)
Maximum hydration temperature difference (°C)
110 110 160 160 200 200
220 220 320 320 400 400
63 53 77 78 85 96
15 20 23 26 31 47
temperature tests implied some significant observations. (1) The specimens showed maximum hydration temperatures ranging from 53° to 96°C, depending on the plug size, 6 to 12 hours following cement grout emplacement. The maximum hydration temperature differences within the plugs ranged from 15° to 47°C. (2) Larger (i.e., 200 mm diameter) plugs showed larger hydration temperatures and larger temperature differences. (3) The surface of the plug cast in one of the largest diameter steel pipes contained cracks, presumably due to the large temperature difference (up to 47°C) during curing. As the hydraulic conductivities of the plugs cast in larger diameter boreholes (e.g., 200 mm) do not differ drastically from those of the much smaller plugs (e.g., 25 mm diameter), this may suggest that the considerable temperature rises that occurred during hydration have not significantly impaired the sealing performance of the plugs. However, as analyzed by Akgun and Daemen (1991 ), these elevated temperatures might have contributed to block fracturing.
4. Research recommendations
On the basis of the results obtained in this research, aimed at identifying size effects on the hydraulic conductivity of cement, grout borehole plugs, it would be desirable to test plugs with three significantly different diameters, e.g., 25mm, 100 mm and 250 mm. Ideally, the plugs should be installed in relatively large intact blocks of poten-
tial host rock (i.e., with dimensions of at least 3-4 times the plug diameters) and stressed to a level likely to correspond to in situ stresses, or at least sufficiently large to minimize the risk of tensile rock fracturing as a result of grout swelling, or injection pressures. An experimental program along these lines poses exceedingly difficult implementation requirements. An adequate substitute for some aspects of the tests, particularly for flow through the plug itself, would consist of performing the flow tests on plugs in steel pipes. This will change the hydraulic and mechanical interface effects, but will correspond to the configuration for grouts around shaft liners or within or around borehole casings. The stiffness of the steel pipe should be selected to simulate in situ confinement. Continuous high precision temperature and strain monitoring needs to be performed during flow tests, given that flow tests are highly sensitive to these parameters. In all tests, a constant pressure injection of sufficient duration to assure steadystate conditions would be desirable prior to the initiation of transient flow testing, although it diverts from the purpose of transient tests, i.e., accelerating the experimental program. The fracturing of the basalt blocks at internal pressures significantly lower than the tensile strength of the same rock determined in shortterm standard laboratory experiments leads to a recommendation to perform internal pressurization tensile strength tests, over extended periods of time, on hollow rock cylinders. These tests will allow the utilization of similar geometries and internal pressurization mechanisms that were involved in the fracturing of the blocks, and would enable to correlate the tensile strengths more reliably. Such internal pressures can readily be maintained, thus allowing a study of the influence of pressure duration, as well as of the effects of water, on tensile strengths and on stress corrosion. Simultaneous heating up of some basalt samples during internal pressurization tests would be desirable to study the influence of elevated temperature on basalt tensile strength. This would allow to investigate how detrimental elevated temperatures might be on the sealing performance. More attention needs to be given to the longitudinal size effects. All testing reported on here has
H. Akgun, J.J.K. Daemen/Engineering Geology 37 (1994) 137-148
been performed on plugs with a length not more than twice the diameter. Longer plugs, especially in the larger diameter ratios, would be difficult to test. Of particular concern with regard to the performance of long plugs are cement grout channeling, segregation, settlement and bleeding, which may significantly detract from the sealing performance of longer plugs. The channeling and bleeding that may occur in very long plugs might suggest that hydraulic conductivities determined on very small volumes, i.e., nearly pointwise material properties, might not be representative o f true in situ hydraulic conductivities. A detailed systematic investigation of channeling (interface flowpath induced by water rising through uncured grout), erosion (physical removal of cement grout particles) and piping (open flow paths, e.g., induced by high water pressure gradients), i.e., the various mechanisms that can cause highly preferential flow paths through sections of plugs, and particularly along plug-rock interfaces, therefore, would be highly desirable. The most direct approach to clarifying this issue would consist of casting long plugs in pipes, slicing the plugs, and testing their hydraulic properties by some type of hydraulic conductivity testing. Visual determination and mathematical description of preferential flow paths will be necessary, and will assist in analyzing the conditions that cause the developments of such flow paths. Experiments of this type need to be conducted for a range of installation, mixing and curing conditions. Analysis of the results in light of the bleeding mechanics should assist in optimizing the mix and the installation to minimize problems of this nature. The complex interaction between plug and host rock deserves further investigation. Swelling of a plug reduces the hydraulic conductivity along the interface between plug and rock, but increases the conductivity of unfavorably oriented fractures in the host rock. Some swelling almost certainly is needed to provide adequate bond strength. Excessive swelling may damage the host rock. A detailed numerical simulation, validated by experiments on extensively instrumented plugged blocks, would be desirable to develop a better understanding of the optimum swelling.
147
5. Summary and conclusions The objective of this paper is to assess the size effects on expansive cement grout borehole plug performance. Flow tests have been conducted on cement grout plugs with diameters of 160 mm and 200 mm, and length-to-diameter ratios of one, in boreholes in basalt blocks and in steel pipes. The hydraulic conductivity of the plugs has been determined by pseudo-constant head tests and by transient pulse testing. Differential water pressures of up to 3 MPa have been applied across the plugs. Cement grout hydration temperatures have been monitored on plugs in steel pipes having diameters ranging from 110 mm to 200 mm. During flow tests, the basalt blocks have fractured, presumably due to a combination of water injection pressure, cement grout expansion, packer contact pressure, and temperature differences. These fractures have been induced at stress levels significantly lower than those measured in conventional short-term laboratory experiments, which might indicate a size effect, a time effect, and/or a corrosion effect on the tensile strength of basalt. As a result of the unintentional block fracturing, it became opportune to study the hydraulic conductivity of the ~block fractures and the bypass flow paths around the plugs. A frame has been installed around one fractured rock block to allow the application of a normal stress across the fracture. Hydraulic conductivity tests performed on this and on the other, not externally loaded, block fractures allowed to study the influence of the complicated interaction between a cement grout borehole plug (e.g., swelling and shrinkage alternations) and the rock, and of the normal stress applied across a fracture on the hydraulic conductivity of a fracture intersecting a plugged borehole. Interpretation of the results was complicated by the opposing effects of the plug/rock interface flow path and the fracture flow path, e.g., plug swelling reduced interface flow but enhanced fracture flow. The fluctuations in the fracture hydraulic conductivity might have been due to: plug expansion and the resulting increase in fracture aperture width; slip along the fracture; drying induced shrinkage; recovery of the re-saturated plug; accumulation of cement grout particles in
148
H. Akgun, J.J.K. Daemen/Engineering Geology 37 (1994) 137-148
the fracture; and, change in the applied, approximately n o r m a l stress. The hydraulic conductivity o f the cement grout plugs cast in steel pipes ranged f r o m 6 × 10 -15 m/s to 6 × 10 -13 m/s, with fluctuations over time o f about two orders o f magnitude. The results o f the flow tests implied that no p r o n o u n c e d size effect on cement grout plug hydraulic conductivities existed. The hydraulic conductivity measured on small diameter (25 mm) plugs might well have been representative o f the relatively large diameter (200 mm) plugs. Cement grout hydration temperatures increased from small to large diameter plugs. Visible cracks have been observed on the surface o f the largest (200 m m diameter) plugs, in which temperature differences a p p r o a c h e d 50°C.
Acknowledgements This w o r k is part o f a research effort on rock mass sealing, C o n t r a c t NRC-04-78-271, supported by the U.S. Nuclear Regulatory Commission. Permission to publish this paper is gratefully acknowledged, as are invaluable discussions with N R C contract m o n i t o r Mr. J. Philip, P.E. D o w e l l - S c h l u m b e r g e r provided the tested cement. Considerable assistance f r o m Mr. E.B. Nelson o f D o w e l l - S c h l u m b e r g e r is gratefully acknowledged. T h a n k s are due to Dr. Vedat D o y u r a n , Chairman, Geological Engineering Department, Middle East Technical University, A n k a r a , Turkey, for his comments on the draft o f this paper.
References Adisoma, G.S. and Daemen, J.J.K., 1988. Experimental assessment of the influence of dynamic loading on the permeability of wet and of dried cement borehole seals. Tech. Rep. NUREG/CR-5129, U.S. Nuclear Regulatory Commission, Washington, D.C., 218 pp. Akgun, H. and Daemen, J.J.K., 1986. Size influence on the sealing performance of cementitious borehole plugs. Tech. Rep. NUREG/CR-4738, U . S . Nuclear Regulatory Commission, Washington, D.C., 301 pp. Akgun, H. and Daemen, J.J.K., 1991. Bond strength of cementitious borehole plugs in welded tuff. Tech. Rep. NUREG/CR-4295, U.S. Nuclear Regulatory Commission, Washington, D.C., 290 pp. American Petroleum Institute, 1977. Testing Oil-Well Cements and Cement Additives, 20th ed. Am. Pet. Inst., Dallas, Tex., 61 pp. Apps, J.A., Carnahan, C.L., Lichtner, P.C., Michel, M.C., Perry, D., Silva, R.J., Weres, O. and White, A.F., 1983. Status of geochemical problems relating to the burial of high-level radioactive waste. Tech. Rep. NUREG/CR-3062, LBL-15103, Lawrence Berkeley Laboratory, Berkeley, Calif., 365 pp. Desai, C.S., 1979. Elementary Finite Element Method. PrenticeHall, Inc., Englewood Cliffs, N.J., 434 pp. Fuenkajorn, K. and Daemen, J.J.K., 1986. Experimental assessment of borehole wall drilling damage in basaltic rocks. Tech. Rep. NUREG/CR-4641, U.S. Nuclear Regulatory Commission, Washington, D.C., 287 pp. Hsieh, P.A., Tracy J.V., Neuzil, C.E., Bredehoeft, J.D. and Silliman, S.E., 1981. A transient method for determining the hydraulic properties of 'tight' rocks - - I. Theory. Int. J. Rock Mech. Min. Sci.; Geomech. Abstr., 18(3): 245-252. Jaeger, J.C. and Cook, N.G.W., 1979. Fundamentals of Rock Mechanics, 3rd ed. Chapman and Hall, London, 593 pp. Neville, A.M., 1981. Properties of Concrete, 3rd ed. Pitman, London, 779 pp. South, D.L. and Daemen, J.J.K., 1986. Permeameter studies of water flow through cement and clay borehole seals in granite, basalt and tuff. Tech. Rep. NUREG/CR-4748, U.S. Nuclear Regulatory Commission, Washington, D.C., 280 pp.
Engineering Geology 37 (1994) 137-148
Performance assessment of cement grout borehole plugs in basalt Haluk Akgun a, Jaak J.K. Daemen b " Geotech, Inc., 125 North Route 73, Maple Shade, NJ 08052, USA b Mining Engineering Department, Mackay School of Mines, University ofNevada--Reno, Reno, N V 89557, USA
(Received February 22, 1993; revised version accepted January 21, 1994)
Abstract
Flow tests have been conducted on expansive cement grout plugs with diameters of 160 mm and 200 mm, and lengthto-diameter ratios of one, in boreholes in basalt blocks and in steel pipes. Two types of flow tests have been performed: pseudo-constant head tests and transient pulse tests. Hydration temperatures of cement grout plugs have been monitored in steel pipes with inside diameters ranging from 110 mm to 200 mm. During flow tests, basalt blocks have fractured, presumably due to water injection pressure, cement grout expansion, packer pressure and temperature differences. Falling head tests performed on some block fractures indicate a complex interaction between a cement grout borehole plug and the rock, as determined from the hydraulic conductivities of fractures intersecting plugged boreholes.
1. Introduction
Construction and exploration related penetrations (e.g., boreholes, shafts or tunnels) associated with a high-level radioactive waste ( H L W ) repository must be sealed on abandonment to retard the migration of radionuclides to the accessible environment. Sealing of openings that penetrate a H L W repository geological barrier requires the use of materials that meet regulatory seal performance requirements. Expansive cement grouts are sealant materials considered by the U.S. Department of Energy ( U S D O E ) . We define plugs as structures emplaced in boreholes (or in other rock penetrations such as shafts or drifts) with the express purpose of greatly increasing the resistance to fluid flow along the penetrations. Most studies performed to date on borehole plug sealing have been on relatively small diameter boreholes (typically 25 mm) drilled in intact rock cylinders. Because of the relatively narrow range 0013-7952/94/$6.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0013-7952(94)00002-J
of plug diameters used, the results could not be extrapolated to larger plug sizes. The objective of this paper is to assess the effects of size on the performance of expansive cement grouts used to plug boreholes. Flow tests have been conducted on plugs with diameters of 160 m m and 200 mm, with length-to-diameter ratios of one. The plugs have been emplaced in boreholes cored in basalt blocks or in steel pipes. Differential water pressures of up to 3 M P a have been applied across the plugs. Cement grout hydration temperatures have been monitored at the top, middle and b o t t o m of plugs in steel pipes with inside diameters of l l 0 m m , 160 m m and 200 mm. The purpose of the hydration temperature measurements was to investigate the influence of cement plug size on hydration temperature generated. Temperature rise is an indicator of potential self-induced fracturing during cement curing, and is likely to be a function of cement volume. The rock blocks were taken from the P o m o n a
138
H. Akgun, J.J.K. Daemen/Engineering Geology 37 (1994) 137-148
Member of the Saddle Mountains Formation (a flow basalt unit of the Columbia River Basalt Group), located in south-central Washington. The expansive cement grout formulation, provided by Dowell-Schlumberger was composed of Ideal Type A Portland cement (from Tijeras Canyon, New Mexico), mixed with 50% distilled water, 10% D53 (an expansive agent) and 1% D65 (a dispersant). All additives are Dowell Schlumberger trademarks. All percentages were weight percent with respect to cement. Cement mixing was performed according to American Petroleum Institute (API) specification, API Standard No. RP-10B (1977). The plugs were cured under distilled water for eight days prior to flow testing.
2.2. Experimental procedure and set-up Figure 1 gives the flow test set-up. The system consists of eight major components: (1) a nitrogen tank; (2) a bladder accumulator; (3) a pressure gage; (4) a flowmeter; (5) an expandable mechanical packer; (6) a water outflow collection unit (recaptured flow unit); (7) stainless steel tubing; and (8) a steel frame. The bladder accumulator is pre-charged with nitrogen gas and then filled with distilled water. It is used to inject water under ~$UrQ
/
2. Flow testing
~r
2.1. Introduction The hydraulic conductivity of cement grout plugs poured and cured in relatively small diameter boreholes (usually 25 mm) ranges from 1 × 10 -14 m/s to 8×10 -11 m/s (e.g., Fuenkajorn and Daemen, 1986; South and Daemen, 1986; Adisoma and Daemen, 1988). The properties of both rock and sealing materials are size-dependent. This sizedependency introduces uncertainty in the extrapolation of the results to larger-diameter plugs. To assess the uncertainty involved, laboratory flow tests have been performed on plugs in larger diameter boreholes in basalt blocks and in steel pipes. Table 1 gives the dimensions of the plugged basalt block boreholes.
Flowmeter L_.~-----~Center rod
Nitroge tank~
~_,]~;f p.cker [ [~-;,~Upper end prate ~!~:} of p,.¢ker ~ . ' , , ,..;,'.~.;t~1~1Pa¢k• r (rubber)
-
.. _ ~ ...... J----~Rock K,q:~t;',':"?.t~5<~,:;':.:"~.~)11- II brock
V,/I',;5:',"-'~t'Pv".4+/"."J;'-',',~r:,'VT'T
I
II fiLL
I
!! B o " ^ m PVCdls-
Cement~ " : ~ ) ; , ' ° ] ~ L o w Ie r II m"o'packer el I I I//A~l:-'.,'J',~'-"-'-'~:';4;w-'~//l
II
+:.......
I IL;.;:,,.,.,,,o,, !
25cm
I
Fig. 1. Laboratory set-up for flow tests in a plugged Pomona Basalt block.
Table 1 Dimensions of the plugged basalt blocks Basalt block
D O (mm)
Dp (mm)
L1 (mm)
Lp (mm)
L 2 (mm)
Lt (mm)
No. No. No. No. No.
584 223 201 470 330
200 160 160 200 160
400 318 254 368 267
203 160 160 200 160
108 70 70 127 203
711 546 483 692 629
1 2 3 4 5
D o = m a x i m u m block diameter; Dp=plug (borehole) diameter; L l = t o p borehole length; Lp=cement plug length; L2=bottom borehole length; and Lt = total borehole length.
H. Akgun, J.J.K. Daemen/EngineeringGeology37 (1994) 137-148
pressure through the cured cement grout plug. The pressure gage measures the injection pressure. The volume of water injected is measured with the flowmeter. The pressurized water is injected through the axial hole of the expandable packer, and then, to the cement grout plug. The rubber disks of the mechanical packer expand when the packer is torqued in order to seal the top block borehole. A pipet is used to collect and measure the volume of water flowing through the cement plug (recaptured flow unit in Fig. 1). The bladder accumulator, pressure gage, flowmeter and packer are connected by stainless steel tubing. A steel flame around the rock block holds it vertically and stabilizes it during packer installation. Experimental details are given by Akgun and Daemen (1986). 2.3. Flow tests in boreholes in basalt blocks Introduction The flow tests led to the unintended fracturing of all five basalt blocks. In three blocks, fracturing occurred prior to flow testing, presumably due to a combination of the elevated hydration temperature of the cement grout (that led to temperature differences within the plug and between the plug and rock) and swelling of the expansive cement grout. Temperature differences between the curing cement grouts and the rock blocks reached up to 47°C. This temperature difference induced thermal shear stresses along the plug/rock interface and within the rock, which might have contributed to block fracturing. Akgun and Daemen (1991) give an analysis of this phenomenon. In the other two blocks, fracturing occurred during flow tests, most likely due to the combined effects of injection pressure, cement grout swelling pressure, packer pressure and temperature differences. The induced fractures of four blocks ran approximately parallel to the boreholes and through the entire lengths of the blocks. The other block fracture ran at about 45 ° to the borehole axis. The differences in fracture orientations are attributed to pre-existing microfractures although no attempt has been made at inspecting thin sections for petrographic evidence. Each fractured block had one or two dominant fractures along which flow tests have been con-
139
ducted. The results of these flow tests are reported in the following section. Cement grout swelling pressure was the main cause or at least one of the causes for the opening up of joints or hairline fractures, i.e., fractures visible to the naked eye after cement swelling and/or flow testing, but not detected before, in these basalt blocks. The blocks had been inspected carefully prior to testing, and along the borehole walls no cracks were apparent to the unaided eye. This observation confirms that a swelling sealing material can inadvertently enhance the hydraulic conductivity of the rock surrounding a sealed opening (as suggested, for example, by Apps et al., 1983). 2.4. Falling head tests on the basalt block fractures Introduction Development of fractures in the No. 1, No. 4 and No. 5 blocks provided a ready opportunity for the study of water flow through fractures. Given the limited data base presently available on relatively large-scale boreholes, and the already installed instrumentation, it was deemed appropriate and productive to divert somewhat from the main objective of this research, which was aimed at studying the size-effects on the hydraulic performance of cement grout plugs emplaced in drilled boreholes in rock blocks. Laboratory set-up for block fracture flow tests Figure 2 gives the schematic for performing falling head tests and for applying an approximately normal stress to the main fracture induced in the No. 1 Pomona Basalt block. The load was applied by hydraulic cylinders placed under the beams. The beams were connected by threaded rods. Wooden blocks and steel plates were placed under the beams to distribute the load uniformly across the block. The fracture flow was measured with a pipet. A saturated sponge was placed underneath the plug to prevent drying-induced plug shrinkage which could have led to cracking and to increased permeability along the plug/rock interface (as observed, for example, by Adisoma and Daemen, 1988). The Nos. 4 and 5 block fractures were flow tested with a configuration similar to that shown
H. Akgun, J . J . K . Daemen/Engineering Geology 37 (1994) 137 148
140
tst I Plpets
It
er disk Rock block S p t l e t ' ~ ~ ~
w,2.,, b... ~,_.G~
Irl~'~
#;,2"_'~
-
.
.
.
.
M-,~
-i, ..~~2 k,,*.:l-.',~Vll
IIII
ii I
. .~. ~,~.r~/
,pongo
.
~,.,"-:].t.._~w**d,n bl~k,
.~,t~.;_~. Ii
.
~ ~oncreto flu
~---~//A
Bot tom " l r / / / / ~ ] PVCdilc ~ , / / / J t / ~
I
l////Zl
| ~
I'////J
r////J
tic
Ground Level
Cement ptug
I
Fig. 2. Laboratory set-up for flow rate tests on the No. 1 Pomona Basalt block fracture. An approximatelynormal stress was applied to the fracture. W . L'racture lensth
in Fig. 2, but unconfined (i.e., without the loading frame).
Hydraulic properties of the block fractures Falling head tests on the No. 1, No. 4 and No. 5 basalt block fractures led to the determination of the fracture flow rate (Qf), fracture hydraulic conductivity (Kf), and the fracture hydraulic aperture (2b), as a function of time. The assumptions made in the derivations of the equations are: (1) a parallel, planar plate model representing the fracture; and (2) straight, horizontal and laminar flow in the borehole (i.e., no vertical flow). Figure 3 gives the flow parameters of a block fracture. The derivations of the equations are given in Akgun and Daemen (1986). The flow rate of the fracture (Qf) determined by the falling head test is given by:
I~I"UM NIl to l i l l e
Lf - f r a c t u r e width
hv - ~mter heed at iny point alon| the borehole u e l l h I = ~qLter held I n p i p e t st tlJle t 1
r o = pLpet radius
h 2 - vaker heed in p i p e t lit t i l e
I: 2
Fig. 3. Flow parameters of a Pomona Basalt block fracture. Flow lines are a schematic estimate, for illustrative purposes only. W= fracture length, Lf= fracture width, ro = pipet radius, hw=water head at any point along the borehole wall, hi= water head in pipet at time tl and h2=water head in pipet at time t2.
where: K f = fracture hydraulic conductivity; 2b =fracture aperture calculated from Eq. 3; L r = fracture width; W = f r a c t u r e length; h a = w a t e r head in pipet at time tl; h2=water head in pipet at time t2. Definition of the other variables are given with Eq. 1. The fracture aperture (2b) is determined from the falling head testing: "
Qf = ~r 2 dhw dt
2"
F12pLf~r 2 In (hl/h2)~ 1/3
(1)
where: Qf=fracture flow rate; r o = p i p e t radius; hw=water head at any point along the borehole wall; t = e l a p s e d time. The fracture hydraulic conductivity (Kf) is given by:
where: 2 b = fracture aperture width; 7w=unit weight of water; and, g = d y n a m i c viscosity of water. The other variables are defined with Eqs. 1 and 2.
LfTTr2 In (hl/h2) Kr= (2b) W(t2 - tl)
Falling head tests performed on the induced basalt block fractures allowed to study the
(2)
Results of the fracture flow tests and discussion
H. Akgun, J.J.K. Daemen/Engineering Geology37 (1994) 137-148
influence of the interaction between a cement grout borehole plug (e.g., swelling and shrinkage alternations) and the rock, and of the normal stress across the fracture on the hydraulic conductivity. The interaction between these various effects became complicated. Individual influences could not be separated out readily. The following counteracting effects needed to be considered: (1) Cement grout swelling. During cement grout swelling, the interface between the plug and the surrounding rock closed, i.e., the hydraulic conductivity along the plug/rock interface decreased. It was unclear whether the grout penetrated into the main induced fracture or not. The hydraulic conductivity of the fracture increased as a result of fracture aperture increase. (2) Cement grout shrinkage. During the testing sequences, the plugs have been allowed to dry out for a few hours to a maximum of ten days. During typical testing sequences, water could be maintained on top of the plugs for only about 3 to 8 hours per day, due to the emptying of the water reservoirs through the block fractures. During drying, shrinkage could have taken place (as observed, for example, by Adisoma and Daemen, 1988). This shrinkage could have resulted in an increased hydraulic conductivity along the interface between the plug and rock, and a decreased hydraulic conductivity along the fracture. (3) Cement grout permeability changes as a function of cement curing temperature. Beyond the initial hydration pulse, temperature changes have been modest (_2°C), and are not believed to have had a significant influence. (4) Cement grout-water-rock interaction. Chemical effects such as secondary mineralization and dissolution. Explicit identification of such causes were beyond the scope of this research. (5) Mechanical effects. Cement grout sedimentation along the fracture could have plugged a fracture, but also could have prevented re-seating and could have caused shearing along the fracture during an increase of the applied normal stress or during cement grout shrinkage, thus maintaining a higher hydraulic conductivity. Figure 4 gives the flow rate (Qf), hydraulic conductivity (Kf), aperture (2b), and the applied, approximately normal stress (on) on the No. 1
141
Pomona Basalt block fracture as a function of time. The falling head test was continued for about 5 months with nearly normal stresses ranging from 0 to 2.2 MPa. The average fracture aperture was estimated to be less than 3.8 x 10 -5 m when measured by an engineering filler gage after 5 months. During the first 4 hours of testing, the flow rate was inversely proportional to the normal stress on the block. Keeping the normal stress nearly constant for an additional 17 days led to a decrease in the fracture hydraulic conductivity presumably due to the continuing seating of the fracture. Over 17 days of flow testing, no monotonous relation was observed between the fracture hydraulic conductivity and the applied stress (Fig. 4). These cyclic and conflicting trends could have been due to the reasons listed above, namely: (1) fluctuations in the applied stress; (2) the opposing effects of the plug/rock interface flow path and the fracture flow path, e.g., plug swelling reduced interface flow but enhanced fracture flow; (3) the influence of drying-induced plug shrinkage being temporarily overridden by the influence of the increased normal stress or vice versa, e.g., if the plug shrinkage had been severe, the hydraulic conductivity along the interface could have increased more than the reduction in fracture hydraulic conductivity due to an increase in the normal stress; (4) the influence of the increased normal stress being overridden by the influence of plug swelling or vice versa, e.g., if the magnitude of the tensile stresses on the fracture caused by plug swelling was higher than the compressive stresses due to confinement, opening up of the fracture aperture and increased fracture hydraulic conductivity could have been observed; (5) cement grout sedimentation or secondary mineralization along the fracture could have prevented the re-seating of the fracture and could have caused shearing along the fracture with increased normal stress. This could have led to increased fracture hydraulic conductivity. Dissolutioning along the interface also could have led to increased hydraulic conductivity; and (6) recovery of the re-saturated plug upon drying could have reduced the interface hydraulic conductivity. In the absence of detailed investigations of individual effects, it is difficult to identify the relative influence of each effect.
H. Akgun, J.J.K. Daemen/Engineering Geology37 (1994) 137-148
142
3"
~2
O
8' 7.
o I,c
6 5
Eu
|
~
...................
---_
. . . = .1~ J/ - - ,,,Ill| I
4
,¢
3
l
2
u
I 0
I0 0
I01
IO 2
TIME
IO ~
~04
10 5
(minutes)
Fig. 4. Rate of flow (Qr) through the No. 1 Pomona Basalt block fracture, fracture hydraulic conductivity (Kr), and fracture aperture (2b) as functions of the applied, nearly normal stress (c~.), and time (1 day = 1440 min).
The No. 4 and No. 5 Pomona Basalt block fractures were flow- tested under unconfined conditions for up to 2 V2 months and showed an increase in all fracture hydraulic properties until the tests were terminated at the conclusion of the project. The manually measured average fracture apertures differ from those calculated analytically (from Eq. 3) for up to about five-folds. Both cyclic and conflicting trends were observed. These trends could have been due to: plug swelling and the resulting increase in the fracture aperture; slip along the fracture as a result of plug swelling or plug shrinkage; drying induced plug shrinkage; the recovery of the re-saturated plug; and the opposing effects between plug swelling and plug shrinkage.
Tensile strength of the Pomona basalt blocks and discussion The tensile fracturing of the basalt blocks allowed the determination of the tensile strength of basalt. The fractures of the No. 1, No. 3, No. 4 and No. 5 basalt blocks ran approximately parallel to the borehole axes. The tensile strengths were calculated from the equation for a hollow cylinder subjected to an internal pressure (e.g., Jaeger and Cook, 1979):
Pi(D~,-t-D~)) ~t -
2
2
Dp--D o
(4)
where: o t = tensile strength; Pi =internal (contact) pressure resulting from cement swelling, packer pressure (Pp), or water injection pressure (Pw); Dp=inside borehole (plug) diameter; and D o = outside block diameter. The tensile strength (%) of the No. 2 basalt block fracture (oriented at approximately 45 ° to the borehole axis) was calculated from:
¢~t-
Fw Af
(5)
where: Fw = water injection force on the fracture = (rt/4)D~,(Pw/2); and, Af= fracture surface area. Equations 4 and 5 require parameters such as cement grout swelling pressure, packer contact pressure, water injection pressure, inside and outside block diameter, and fracture surface area. The thermally-induced contact stresses are neglected in these equations because the blocks fractured after the grout curing temperatures were back to ambient. Akgun and Daemen (1991) measured the swelling pressure of a similar type of cement grout cast in 100 mm inside diameter steel pipes. The swelling pressure (Ps), measured with strain gages on the steel pipe outside walls, was 2.2 MPa after 8 days of cement curing. The No. 2 and No. 3 basalt blocks fractured at injection pressures (Pw) of 3.2 MPa and 0.7 MPa, respectively. The packers
143
H. Akgun, J.J.K. Daemen/Engineering Geology 37 (1994) 137-148
were torqued to 270 N.m, which led to an estimated packer contact pressure (Pc) of 2.1 MPa. The No. 2 block fracture surface area (Af) was 8 x 10 -2 m e . Equations 4 and 5 gave tensile strengths between 2.8 MPa to 9.5 MPa (based on cement grout swelling pressure), 9.2 MPa (based on packer contact pressure), and between 0.4 MPa to 3 MPa (based on water injection pressure). The tensile strengths calculated from Eq. 4 were high because this equation assumes that the pressurized length equals the cylinder (fracture) length. In actuality, the fracture lengths were considerably greater than the lengths of the pressurized intervals. The tensile strength calculated from Eq. 5 is low as compared to the results listed below because the normal stress is assumed to be uniform across the fracture. The calculated minimum and maximum tensile strengths were 0.4 MPa and 9.5 MPa, respectively. Fuenkajorn and Daemen (1986) performed ring tension tests on Pomona Basalt disks with outsideto-inside diameter ratios (i.e., stiffness, ratio of the radial displacement to the radial stress, Desai, 1979) similar to those of the basalt blocks. The blocks have fractured at internal pressures corresponding to tensile strengths significantly below (by a factor of two to sixty) those determined from ring tension tests. This might indicate a size effect on the tensile strength of basalt, the influence of prolonged loading, or basalt weakening under the influence of water-rock interaction, and might suggest that the long-term in situ strength of basalt may be much lower than that of samples conventionally tested in the laboratory. Several tensile fractures ran through the cement grout plugs and appeared to be direct continuations of the block fractures. This phenomenon suggests that the splitting fractures initiated within the cement grout plugs and propagated into the rock.
2.5. Flow tests in steel pipes Introduction The fracturing of the basalt blocks led to a decision to determine the hydraulic conductivities of cement grout plugs emplaced in steel pipes. The pipes had inside diameters of 160 mm and 200 mm.
The cement grout plugs had length-to-diameter ratios of approximately 1. They were cured for 8 days prior to flow testing. Pseudo-constant head and transient pulse tests were performed. The laboratory set-up was similar to that shown in Fig. 1.
2.6. Determination of the hydraulic conductivity of cement grout plugs cured in steel pipes Pseudo-constant head tests The tests are referred to as pseudo-constant head tests because water could not be injected at a constant pressure throughout testing; the injection pressures decreased with time. The hydraulic conductivities were calculated by using total flows, i.e., both flow through the plug body, and flow along the plug/rock interface, if any. The hydraulic conductivity of the cement grout plug obtained from pseudo-constant head testing can be expressed as (Akgun and Daemen, 1986): a(h2 - h i )
kAF 1 t2
zL ,lP at + L(te-
(6)
;t h
dt 1
where: k = the hydraulic conductivity of the cement grout plug; P = w a t e r injection pressure; A =plug cross-sectional area; L = p l u g length; a = p i p e t cross-sectional area; h = total head at the bottom of the plug; hi and he=water levels in the outflow collection pipet at times tl and t2, respectively; and, Yw= u n i t weight of water.
Transient hydraulic conductivity analys& Hsieh et al. (1981) used the transient pulse test to measure very small hydraulic conductivities. The apparatus consists of a cement grout plug with direct connections to an upstream and downstream fluid reservoir. At the start of the experiment, the fluid pressure in the upstream reservoir is suddenly increased. The fluid flows from the upstream to the downstream reservoir. The pressure in the upstream reservoir decays. The hydraulic conductivity of the cement grout plug is calculated from the pressure decrease in the upstream reservoir. The initial and boundary conditions describing the hydraulic head in the plug,
H. Akgun, J.J.K. Daemen/Engineering Geology 37 (1994) 137 148
144
and in the upstream and downstream reservoirs are solved by the Laplace transform method. The procedure is given by Hsieh and others (1981, Appendix I).
6 x 10-13 m/s, with fluctuations over time of about two orders of magnitude. Figure 5 gives the result of one transient permeability pulse test on the cement grout plug in the 200 mm diameter pipe. The pressure, as a percentage of the initial pressure, is plotted as a function of time. The initial and final pressure in this 200 mm diameter pipe was 0.7 MPa and 0.5 MPa, respectively. Figure 6 gives the type curve that fits the experimental results given in Fig. 5. The hydraulic conductivity determined by matching the experimental and type curves is 2 × l 0 -x4 m / s . Three sets of decay tests were run in each pipe for up to 1 month. The average hydraulic conductivities of the 160mm and 2 0 0 m m plugs were 6 x 10 -15 m/s and 2 x 10 -14 m/s, respectively.
Results of the pipe flow tests The pseudo-constant head tests on the 160 mm and 200 mm diameter pipe plugs were performed for over 2 months, with water injection pressures ranging from 0.7 MPa to 3 MPa. A total of 64 and 66 sets of individual flow tests, with testing periods for individual tests ranging from 15 minutes to 6 days were run in each pipe. Readings were taken at intervals ranging from 2 minutes to about 20 hours. Akgun and Daemen (1986) give the details. The cement grout plug in the 160 mm diameter pipe gave a minimum and maximum hydraulic conductivity of 6 x 1 0 -15 m/s and 7 x 10 -xz m/s. The minimum and maximum flow rates were 2 x 10 -13 m3/s and 7 x 10 -11 m3/s. The minimum hydraulic conductivity and outflow rate of the cement grout plug in the 200 mm diameter pipe was 2 x 10 -14 m / s and 7 x 10 -13 m3/s, respectively. The maximum hydraulic conductivity was 8 x 10 -12 m/s. The maximum outflow rate was 3 x 10-lo m3/s. The average hydraulic conductivity of the cement grout plugs in each pipe was
Discussion of the results of the pipe flow tests No obvious dependency of hydraulic conductivity on plug size has been identified. The average hydraulic conductivity of the plugs in the 160 mm and 2 0 0 m m pipes varied from 6× 10 -15 m/s to 6 × 10 -13 m/s and did not differ drastically from the hydraulic conductivities of the much smaller (25 mm) plugs tested (e.g., by Fuenkajorn and Daemen, 1986; South and Daemen, 1986; Adisoma and Daemen, 1988).
I00
O
90
• °
80-
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Q
• ".'.",
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~:~o.
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~)10 IO
'
~ ~ J '~'"110
'
'
' ' ''"~210
'
' ' ' '''"310
'
' ' ''""410
I
.....
;()'5
TIME (minutes) Fig. 5. Pressure decay for a cement grout plug in a 200-mm steel pipe obtained through transient pulse testing ( 1 day = 1440 min).
H.Akgun,J.J.K.Daemen/EngineerinGeol g ogy37(1994)137-148 I• 0 o ~ ~ I =0.0604
145
J
I ,:°.,
.90 .80 •70.60.~-
.40.
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i
,
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i
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i
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i
IO I
,
, , , , i , i
,
,
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,
, l l , l ~
103
oc8
(HJH)
Fig. 6. Analytical transient pulse test type curve for a decrease in the upstream reservoir hydraulic head versus ct~ (~t= dimensionless time; 13= ratio of the compressive storage within the sample to that in the upstream reservoir; y = ratio of the compressive storage of the downstream reservoir to that of the upstream reservoir).
3. Cement grout hydration temperature 3.1. Introduction
This section describes the temperatures measured in cement grout plugs cast in steel pipes. The cement grout plugs had diameters of 110 mm, 160mm and 200mm, and length-to-diameter ratios of about 2. The main purpose of this study was to investigate the influence of plug size on the hydration temperature. According to Neville (1981 ) the interior of a large concrete mass (e.g., gravity dam) produces a rise in temperature due to the heat of hydration, while the exterior loses heat to the surrounding atmosphere. If the temperature difference between the interior and the exterior of the concrete mass becomes too large (e.g., 20°C or greater), cracking may develop. The objective of this section is to determine whether considerable rises in temperature that occur during cement grout curing could damage the plugs significantly. Elevated temperatures due to the hydration of the curing cement grouts might also have led to excessive thermal shear stresses along the plug/rock interfaces, and within the rock blocks. These shear
stresses might have contributed to the fracturing of the basalt blocks. Akgun and Daemen (1991) have analyzed this mechanism. The hydration temperature was measured by temperature sensors connected to a temperature indicator• Three temperature sensors were installed in each cement grout plug: one near the top of the plug, one at the middle, and the third near the bottom of the plug. Akgun and Daemen (1986) give the experimental details. 3.2. Results and discussion
The hydration temperatures are summarized in Table 2. A similar trend was observed for all tests. Immediately after the cement grout was poured, temperatures were in the range of 25 ° to 38°C. Two to five hours later, the temperatures decreased to 20 ° to 33°C, probably due to the cement pastes coming into thermal equilibrium with the steel pipes and the room environment. The temperatures then gradually increased until the maximum hydration temperatures were reached. Following this, the curing temperatures decreased to room temperatures (22 °_4°C) after 27 to 46 hours. The results of the cement grout hydration
H. Akgun, J.J.K. Daemen/Engineering Geology37 (1994) 137-148
146
Table 2 Results of the cement grout hydration temperature measurements in steel pipes Cement grout plug diameter
Cement grout plug length
Maximum hydration temperature
(mm)
(mm)
(°C)
Maximum hydration temperature difference (°C)
110 110 160 160 200 200
220 220 320 320 400 400
63 53 77 78 85 96
15 20 23 26 31 47
temperature tests implied some significant observations. (1) The specimens showed maximum hydration temperatures ranging from 53° to 96°C, depending on the plug size, 6 to 12 hours following cement grout emplacement. The maximum hydration temperature differences within the plugs ranged from 15° to 47°C. (2) Larger (i.e., 200 mm diameter) plugs showed larger hydration temperatures and larger temperature differences. (3) The surface of the plug cast in one of the largest diameter steel pipes contained cracks, presumably due to the large temperature difference (up to 47°C) during curing. As the hydraulic conductivities of the plugs cast in larger diameter boreholes (e.g., 200 mm) do not differ drastically from those of the much smaller plugs (e.g., 25 mm diameter), this may suggest that the considerable temperature rises that occurred during hydration have not significantly impaired the sealing performance of the plugs. However, as analyzed by Akgun and Daemen (1991 ), these elevated temperatures might have contributed to block fracturing.
4. Research recommendations
On the basis of the results obtained in this research, aimed at identifying size effects on the hydraulic conductivity of cement, grout borehole plugs, it would be desirable to test plugs with three significantly different diameters, e.g., 25mm, 100 mm and 250 mm. Ideally, the plugs should be installed in relatively large intact blocks of poten-
tial host rock (i.e., with dimensions of at least 3-4 times the plug diameters) and stressed to a level likely to correspond to in situ stresses, or at least sufficiently large to minimize the risk of tensile rock fracturing as a result of grout swelling, or injection pressures. An experimental program along these lines poses exceedingly difficult implementation requirements. An adequate substitute for some aspects of the tests, particularly for flow through the plug itself, would consist of performing the flow tests on plugs in steel pipes. This will change the hydraulic and mechanical interface effects, but will correspond to the configuration for grouts around shaft liners or within or around borehole casings. The stiffness of the steel pipe should be selected to simulate in situ confinement. Continuous high precision temperature and strain monitoring needs to be performed during flow tests, given that flow tests are highly sensitive to these parameters. In all tests, a constant pressure injection of sufficient duration to assure steadystate conditions would be desirable prior to the initiation of transient flow testing, although it diverts from the purpose of transient tests, i.e., accelerating the experimental program. The fracturing of the basalt blocks at internal pressures significantly lower than the tensile strength of the same rock determined in shortterm standard laboratory experiments leads to a recommendation to perform internal pressurization tensile strength tests, over extended periods of time, on hollow rock cylinders. These tests will allow the utilization of similar geometries and internal pressurization mechanisms that were involved in the fracturing of the blocks, and would enable to correlate the tensile strengths more reliably. Such internal pressures can readily be maintained, thus allowing a study of the influence of pressure duration, as well as of the effects of water, on tensile strengths and on stress corrosion. Simultaneous heating up of some basalt samples during internal pressurization tests would be desirable to study the influence of elevated temperature on basalt tensile strength. This would allow to investigate how detrimental elevated temperatures might be on the sealing performance. More attention needs to be given to the longitudinal size effects. All testing reported on here has
H. Akgun, J.J.K. Daemen/Engineering Geology 37 (1994) 137-148
been performed on plugs with a length not more than twice the diameter. Longer plugs, especially in the larger diameter ratios, would be difficult to test. Of particular concern with regard to the performance of long plugs are cement grout channeling, segregation, settlement and bleeding, which may significantly detract from the sealing performance of longer plugs. The channeling and bleeding that may occur in very long plugs might suggest that hydraulic conductivities determined on very small volumes, i.e., nearly pointwise material properties, might not be representative o f true in situ hydraulic conductivities. A detailed systematic investigation of channeling (interface flowpath induced by water rising through uncured grout), erosion (physical removal of cement grout particles) and piping (open flow paths, e.g., induced by high water pressure gradients), i.e., the various mechanisms that can cause highly preferential flow paths through sections of plugs, and particularly along plug-rock interfaces, therefore, would be highly desirable. The most direct approach to clarifying this issue would consist of casting long plugs in pipes, slicing the plugs, and testing their hydraulic properties by some type of hydraulic conductivity testing. Visual determination and mathematical description of preferential flow paths will be necessary, and will assist in analyzing the conditions that cause the developments of such flow paths. Experiments of this type need to be conducted for a range of installation, mixing and curing conditions. Analysis of the results in light of the bleeding mechanics should assist in optimizing the mix and the installation to minimize problems of this nature. The complex interaction between plug and host rock deserves further investigation. Swelling of a plug reduces the hydraulic conductivity along the interface between plug and rock, but increases the conductivity of unfavorably oriented fractures in the host rock. Some swelling almost certainly is needed to provide adequate bond strength. Excessive swelling may damage the host rock. A detailed numerical simulation, validated by experiments on extensively instrumented plugged blocks, would be desirable to develop a better understanding of the optimum swelling.
147
5. Summary and conclusions The objective of this paper is to assess the size effects on expansive cement grout borehole plug performance. Flow tests have been conducted on cement grout plugs with diameters of 160 mm and 200 mm, and length-to-diameter ratios of one, in boreholes in basalt blocks and in steel pipes. The hydraulic conductivity of the plugs has been determined by pseudo-constant head tests and by transient pulse testing. Differential water pressures of up to 3 MPa have been applied across the plugs. Cement grout hydration temperatures have been monitored on plugs in steel pipes having diameters ranging from 110 mm to 200 mm. During flow tests, the basalt blocks have fractured, presumably due to a combination of water injection pressure, cement grout expansion, packer contact pressure, and temperature differences. These fractures have been induced at stress levels significantly lower than those measured in conventional short-term laboratory experiments, which might indicate a size effect, a time effect, and/or a corrosion effect on the tensile strength of basalt. As a result of the unintentional block fracturing, it became opportune to study the hydraulic conductivity of the ~block fractures and the bypass flow paths around the plugs. A frame has been installed around one fractured rock block to allow the application of a normal stress across the fracture. Hydraulic conductivity tests performed on this and on the other, not externally loaded, block fractures allowed to study the influence of the complicated interaction between a cement grout borehole plug (e.g., swelling and shrinkage alternations) and the rock, and of the normal stress applied across a fracture on the hydraulic conductivity of a fracture intersecting a plugged borehole. Interpretation of the results was complicated by the opposing effects of the plug/rock interface flow path and the fracture flow path, e.g., plug swelling reduced interface flow but enhanced fracture flow. The fluctuations in the fracture hydraulic conductivity might have been due to: plug expansion and the resulting increase in fracture aperture width; slip along the fracture; drying induced shrinkage; recovery of the re-saturated plug; accumulation of cement grout particles in
148
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the fracture; and, change in the applied, approximately n o r m a l stress. The hydraulic conductivity o f the cement grout plugs cast in steel pipes ranged f r o m 6 × 10 -15 m/s to 6 × 10 -13 m/s, with fluctuations over time o f about two orders o f magnitude. The results o f the flow tests implied that no p r o n o u n c e d size effect on cement grout plug hydraulic conductivities existed. The hydraulic conductivity measured on small diameter (25 mm) plugs might well have been representative o f the relatively large diameter (200 mm) plugs. Cement grout hydration temperatures increased from small to large diameter plugs. Visible cracks have been observed on the surface o f the largest (200 m m diameter) plugs, in which temperature differences a p p r o a c h e d 50°C.
Acknowledgements This w o r k is part o f a research effort on rock mass sealing, C o n t r a c t NRC-04-78-271, supported by the U.S. Nuclear Regulatory Commission. Permission to publish this paper is gratefully acknowledged, as are invaluable discussions with N R C contract m o n i t o r Mr. J. Philip, P.E. D o w e l l - S c h l u m b e r g e r provided the tested cement. Considerable assistance f r o m Mr. E.B. Nelson o f D o w e l l - S c h l u m b e r g e r is gratefully acknowledged. T h a n k s are due to Dr. Vedat D o y u r a n , Chairman, Geological Engineering Department, Middle East Technical University, A n k a r a , Turkey, for his comments on the draft o f this paper.
References Adisoma, G.S. and Daemen, J.J.K., 1988. Experimental assessment of the influence of dynamic loading on the permeability of wet and of dried cement borehole seals. Tech. Rep. NUREG/CR-5129, U.S. Nuclear Regulatory Commission, Washington, D.C., 218 pp. Akgun, H. and Daemen, J.J.K., 1986. Size influence on the sealing performance of cementitious borehole plugs. Tech. Rep. NUREG/CR-4738, U . S . Nuclear Regulatory Commission, Washington, D.C., 301 pp. Akgun, H. and Daemen, J.J.K., 1991. Bond strength of cementitious borehole plugs in welded tuff. Tech. Rep. NUREG/CR-4295, U.S. Nuclear Regulatory Commission, Washington, D.C., 290 pp. American Petroleum Institute, 1977. Testing Oil-Well Cements and Cement Additives, 20th ed. Am. Pet. Inst., Dallas, Tex., 61 pp. Apps, J.A., Carnahan, C.L., Lichtner, P.C., Michel, M.C., Perry, D., Silva, R.J., Weres, O. and White, A.F., 1983. Status of geochemical problems relating to the burial of high-level radioactive waste. Tech. Rep. NUREG/CR-3062, LBL-15103, Lawrence Berkeley Laboratory, Berkeley, Calif., 365 pp. Desai, C.S., 1979. Elementary Finite Element Method. PrenticeHall, Inc., Englewood Cliffs, N.J., 434 pp. Fuenkajorn, K. and Daemen, J.J.K., 1986. Experimental assessment of borehole wall drilling damage in basaltic rocks. Tech. Rep. NUREG/CR-4641, U.S. Nuclear Regulatory Commission, Washington, D.C., 287 pp. Hsieh, P.A., Tracy J.V., Neuzil, C.E., Bredehoeft, J.D. and Silliman, S.E., 1981. A transient method for determining the hydraulic properties of 'tight' rocks - - I. Theory. Int. J. Rock Mech. Min. Sci.; Geomech. Abstr., 18(3): 245-252. Jaeger, J.C. and Cook, N.G.W., 1979. Fundamentals of Rock Mechanics, 3rd ed. Chapman and Hall, London, 593 pp. Neville, A.M., 1981. Properties of Concrete, 3rd ed. Pitman, London, 779 pp. South, D.L. and Daemen, J.J.K., 1986. Permeameter studies of water flow through cement and clay borehole seals in granite, basalt and tuff. Tech. Rep. NUREG/CR-4748, U.S. Nuclear Regulatory Commission, Washington, D.C., 280 pp.