- Email: [email protected]
Blast Design Considerations for In-Situ Retorting of Oil Shale
(:OpH;)l11l
©
IF.V : .-\111''''' ''';011 to\'
:\lilleral Resollrce Den.:·lopIlH:' llI Qu ee nsland. Auslralia. I !IH.:l
Blast Design Considerations for In-Situ Retorting of Oil Shale G. HARRIES & G.G. PAINE ICI Australia Operations Pty Ltd . ICI House, PO Box 4311 , Melbourne
Abstract. The in-situ retorting of shallow oil shale deposits is an attractive alternative to the material handling problems of conventional open pit mining techniques, which involve removing overburden, fragmenting and transporting the shale to a fixed retort, and finally disposing of the hot spent shale. For in-situ retorting to be successful, the oil shale has to be fractured and the fragmented rock expanded into a void, to allow controlled propagation of a flame front through the in-situ retort. The principles of cratering with explosives have been used to develop a process for the in-situ fracturing of a thin oil shale deposit overlain by a formation of porous rock. The overlying porous rock formation is first compressed by explosive charges to create voids in the porous rock. The oil shale is then fragmented and pushed in to the voids with separate explosive charges. This paper discusses methods of creating voids in porous rocks with minimal surface disturbance, and then methods of obtaining the most uniform fragmentation of the shale. The whole process can be completed, in practice, in one blast. The results of an experimental blast are presented, and possible modifications are then discussed. Keywords.
Computer-aided design, mining in-situ, geology. blasting.
I NTRODUCTI ON An experimental in-situ retort was implemented at the Julia Creek Oil Shale Project of CSR Limited during 1981 (Marcovich, 1983; Paine, 1983). The major design considerations for the blasting were
Depth (m) 0-15 15-21 21-28 >28
Rock Unit Allaru Mudstone Coquina (limestone) Oil shale Runmoor Mudstone
The relevant rock properties required for the blast design are:
1. The properties of the oil shale and the overlying Coqu ina (limestone). 2. The location and mass of explosive charges to compress the overlying porous Coquina, forming a chamber into which the broken oil shale can expand to give a swell of 20%.
Dens ity (g. cm- 3 ) Porosity % Young's Modulus (GPa) Poisson's Ratio Compressive Strength (MPa)
Coqui na
Oi 1 Sha l e
1. 8 30
1.8 18 7.8 0.28 30
17 .5
0.44 4.1
3. The explosive charge configuration to fracture and expand the oil shale into the void created by blasting the overlying coquina. 4. Blast ind uced cracking should not extend to the surface. Computer mode llin g was used to predict the blast parameters, such as blasthole pattern and explosive charge mass and location for the experimental i n-s i tu retort. GEOLOGY The Toolebuc oil shale formation occurs as an extensive but relatively thin deposit over much of Northern and Central Queensland . The geological seqence at the site of the experimental in-situ retort is:
Thin, near surface deposits of oil shale have been exploited by Geokinetics at Kamp Kerogin using the in-situ retorting technique. This deposit is overlain with stronger rock types than those that occur at the Julia Creek site. To produce a retort, the whole deposit including the overburden is heaved up in large blocks. This gives well fragmented oil shale with sufficient swell to allow propagation of a flame front. It can be seen from the above rock properties that the shale and the overlying Coquina are porous. From experience in blasting porous
42
G. Harries and G. G. Paine
limestones similar to the Coquina, it is known that small explosive charges chamber holes rather than crack the surrounding rock. Extensive experience with cratering also suggested that the Coquina could be chambered without causing any surface disturbance. EXPLOSIVES CRATERING When short (quasi-spherical) charges of explosives are buried at a series of depths, as shown in Fig. I, the charges buried deeply cause no disturbance at the surface. As the depth of burial of the explosive charge is decreased, small radial cracks are observed at the surface. This depth of burial is known as the critical depth. As the depth of burial is further decreased, craters are formed, and it has been found that a depth known as the optimum depth a crater with the greatest volume is formed. With depths of burial less than the optimum, the volume of the crater decreases and more and more material is ejected from the crater. If the blasting can be carried out at depths greater than the critical depth, there will be little or no surface movement and little or no loss of vapour from the retort. To ensure that a flame front can propagate through the retort, the volume of the fractured shale has to be increased by about 20%. Voids then have to be made in the porous Coquina to accommodate this extra volume. A schematic diagram of the in-situ retort is shown in Fig. 2. BLAST DESIGN Explosives cratering principles can be applied to the process of chambering the Coquina to determine the limiting depth of blast induced cracking. In the absence of a suitable test site, the Harries Blasting Model (Harries, 1973, 1977) has been used to predict the depths of blast induced cracking for differing explosive types, charge lengths and di ameters. The following limits of blast induced cracking were predicted for ANFO explosives (ammonium nitrate, fuel oil mixture). Blasthole Diameter (mm) 200 152
Explosive Cha rge Length(m) 1.0 1.0
Depth Overburden (1) Required to Ensure No Surface Cracking(m) 17.2 13.3
Factors that could affect the efficiency of collection of liquid hydrocarbons were recognised as 1.
The permeability of the oil shale.
2.
The viscosity of the liquid hydrocarbons.
3. The permeability of the underlying mudstone, particularly after blasting. EXPERIMENTAL IN-SITU RETORT BLAST A small scale test blast was fired initially to investigate the predicted explosives performance. Evaluation of increased fracturing in the oil shale by re-entry drilling (cored through oil shale) and pressure test for permeability proved inconclusive. The lack of surface disturbance showed that the explosive performance in design work had not been understated. The small scale test blast allowed the following modifications to the in-situ retort blast design. 1. The first firing blasthole could be increased to 200mm diameter. 2. All other blastholes could be increased to 171mm diameter. For the experimental in-situ retort blast the general blasthole charging procedure was to chargE the blasthole in the oil shale (numbers 1 to 11, Fig. 3) with explosive to approximately 2m from the Coquina, then place 2m of stemming (screenings) prior to placement of the explosive charge in the Coquina, and finally stemming to the surface. Explosive charge weights varied from 58 to 192 kg in the oil shale and 17 to 50 kg in the Coquina. The overall explosives powder factor for the blast was 1.37 kg.m- 3 of oil shale. All explosive charges in the Coquina were initiated with a No. 1 'L' Series millisecond delay detonator (nominal firing time 25 milliseconds) while explosive charges in the oil shale (see Fig. 2) were initiated with Nos 15 to 25 'L' Series millisecond delay detonators (nominal firing times 395 to 695 milliseconds).
(1) Assumes that the bottom of the explosive charge is located at the base of the Coquina.
After the blast the surface expression was shown in two ways.
Blasting to yield uniform cracking in the oil shale was simulated by means of the Harries Blasting Model. The blasthole pattern selected for the experimental in-situ retort was 6m equilateral triangular with a 152mm diameter blasthole. To blast a retort 18m in length and 10.4m in width, 11 blastholes dril led to the bottom of the oil shale were required (Fig. 3). For the chambering of the Coquina, another 14 blastholes, drilled to the base of the Coquina, were considered necessary in addition to the 11 blastholes drilled to the base of the oil shale (Fig. 3). All Coquina explosive charges need to be detonated instantaneously some 200 to 300 milliseconds before the individually detonated explosive charges in the oil shale. The design depths of blastholes were progressively increased from the blast initiation end of the retort with a view to providing a sloping base to facilitate the collection of liquid
1. Surface heave of approximately 30cm over the top of the retort. 2. Minor surface cracking 3 to 4m outside the confines of the blasthole. Subsequent re-entry drilling showed that the oil shale was well fractured but relatively undisturbed. The Coquina was not significantly compressed, however the underlying Ranmoor mudstone showed the effects of compression from the blast above, exhibiting crushed zones and opening of bedding planes.
Blast Design Considerations
CONCLUSIONS The experimental in-situ retort blast and subsequent retort commissioning has shown 1. The oil shale can be satisfactorily fractured in-situ with explosives. 2. The relatively undisturbed nature of the blasted oil shale and the subsequent problems with maintenance of combustion in the retort indicate that the explosive charges in the Coquina did not provide sufficient expansion void. 3. The damage to the underlying mudstone is most likely due to insufficient expansion void for the blasted oil shale, however consideration should also be given to explosive charge placement in the oil shale. 4. Blast design modifications are required to increase the explosives distribution in the Coquina for an increased expansion void to provide better permeability in the oil shale and reduced damage to the underlying mudstone. 5. The properties of the underlying mudstone should be considered in more detail.
ACKNOWLEDGEMENTS The authors extend their thanks to ICI Australia Operations Pty Ltd for permission to publish this paper. REFERENCES Marcovich, B.J. (1983). In-situ Retorting of Julia Creek Oil Shale. Proceedings of the First Australian Workshop on Oil Shale Lucas Heights. pp. 47. Paine, G.G. (1983). Explosives Fracturing of Oi 1 Shale for an Experimental In-Situ Retort at the Julia Creek Project. Proceedings of the First Australian Workshop on Oil Shale Lucas Heights. pp. 51. Harries, G. (1973). A Mathematical Model of Cratering and Blasting Proc. Nat. Symp. On Rock Fragmentation (Aust. Geomech. Soc.) Adelaide. pp. 41. Harries, G. (1977). The Calculation of the Fragmentation of Rock from Cratering, Proc. 15th APCOM Symp, Brisbane. pp. 325.
43
C. Harries an d C. C. Paine
44
Tensi l e Spal li ng
Fig. 2.
Schematic of Field In-S i tu Retort (Marovi t ch 1983)
o bl astho l e dri ll ed to bottom of oil shale • bl ast hol e dr ill ed to bottom of coqu i na (15) mil l i second de l ay numbers for oil shale charges Bl astho l e Pattern for Exper imental In- Situ Retor t Bl ast
Fi g. 3.
ow "
.
.
AllafU
Mudslone
- '- '-' CO QU l na
,
l
. r; I, ' ;:; -- .
~
0,' Shale
- -- -
- - .-.-
M u dst o ne
Fig. 4.
Longitudinal Section of Experimental In - Situ Retort Blast
©
IF.V : .-\111''''' ''';011 to\'
:\lilleral Resollrce Den.:·lopIlH:' llI Qu ee nsland. Auslralia. I !IH.:l
Blast Design Considerations for In-Situ Retorting of Oil Shale G. HARRIES & G.G. PAINE ICI Australia Operations Pty Ltd . ICI House, PO Box 4311 , Melbourne
Abstract. The in-situ retorting of shallow oil shale deposits is an attractive alternative to the material handling problems of conventional open pit mining techniques, which involve removing overburden, fragmenting and transporting the shale to a fixed retort, and finally disposing of the hot spent shale. For in-situ retorting to be successful, the oil shale has to be fractured and the fragmented rock expanded into a void, to allow controlled propagation of a flame front through the in-situ retort. The principles of cratering with explosives have been used to develop a process for the in-situ fracturing of a thin oil shale deposit overlain by a formation of porous rock. The overlying porous rock formation is first compressed by explosive charges to create voids in the porous rock. The oil shale is then fragmented and pushed in to the voids with separate explosive charges. This paper discusses methods of creating voids in porous rocks with minimal surface disturbance, and then methods of obtaining the most uniform fragmentation of the shale. The whole process can be completed, in practice, in one blast. The results of an experimental blast are presented, and possible modifications are then discussed. Keywords.
Computer-aided design, mining in-situ, geology. blasting.
I NTRODUCTI ON An experimental in-situ retort was implemented at the Julia Creek Oil Shale Project of CSR Limited during 1981 (Marcovich, 1983; Paine, 1983). The major design considerations for the blasting were
Depth (m) 0-15 15-21 21-28 >28
Rock Unit Allaru Mudstone Coquina (limestone) Oil shale Runmoor Mudstone
The relevant rock properties required for the blast design are:
1. The properties of the oil shale and the overlying Coqu ina (limestone). 2. The location and mass of explosive charges to compress the overlying porous Coquina, forming a chamber into which the broken oil shale can expand to give a swell of 20%.
Dens ity (g. cm- 3 ) Porosity % Young's Modulus (GPa) Poisson's Ratio Compressive Strength (MPa)
Coqui na
Oi 1 Sha l e
1. 8 30
1.8 18 7.8 0.28 30
17 .5
0.44 4.1
3. The explosive charge configuration to fracture and expand the oil shale into the void created by blasting the overlying coquina. 4. Blast ind uced cracking should not extend to the surface. Computer mode llin g was used to predict the blast parameters, such as blasthole pattern and explosive charge mass and location for the experimental i n-s i tu retort. GEOLOGY The Toolebuc oil shale formation occurs as an extensive but relatively thin deposit over much of Northern and Central Queensland . The geological seqence at the site of the experimental in-situ retort is:
Thin, near surface deposits of oil shale have been exploited by Geokinetics at Kamp Kerogin using the in-situ retorting technique. This deposit is overlain with stronger rock types than those that occur at the Julia Creek site. To produce a retort, the whole deposit including the overburden is heaved up in large blocks. This gives well fragmented oil shale with sufficient swell to allow propagation of a flame front. It can be seen from the above rock properties that the shale and the overlying Coquina are porous. From experience in blasting porous
42
G. Harries and G. G. Paine
limestones similar to the Coquina, it is known that small explosive charges chamber holes rather than crack the surrounding rock. Extensive experience with cratering also suggested that the Coquina could be chambered without causing any surface disturbance. EXPLOSIVES CRATERING When short (quasi-spherical) charges of explosives are buried at a series of depths, as shown in Fig. I, the charges buried deeply cause no disturbance at the surface. As the depth of burial of the explosive charge is decreased, small radial cracks are observed at the surface. This depth of burial is known as the critical depth. As the depth of burial is further decreased, craters are formed, and it has been found that a depth known as the optimum depth a crater with the greatest volume is formed. With depths of burial less than the optimum, the volume of the crater decreases and more and more material is ejected from the crater. If the blasting can be carried out at depths greater than the critical depth, there will be little or no surface movement and little or no loss of vapour from the retort. To ensure that a flame front can propagate through the retort, the volume of the fractured shale has to be increased by about 20%. Voids then have to be made in the porous Coquina to accommodate this extra volume. A schematic diagram of the in-situ retort is shown in Fig. 2. BLAST DESIGN Explosives cratering principles can be applied to the process of chambering the Coquina to determine the limiting depth of blast induced cracking. In the absence of a suitable test site, the Harries Blasting Model (Harries, 1973, 1977) has been used to predict the depths of blast induced cracking for differing explosive types, charge lengths and di ameters. The following limits of blast induced cracking were predicted for ANFO explosives (ammonium nitrate, fuel oil mixture). Blasthole Diameter (mm) 200 152
Explosive Cha rge Length(m) 1.0 1.0
Depth Overburden (1) Required to Ensure No Surface Cracking(m) 17.2 13.3
Factors that could affect the efficiency of collection of liquid hydrocarbons were recognised as 1.
The permeability of the oil shale.
2.
The viscosity of the liquid hydrocarbons.
3. The permeability of the underlying mudstone, particularly after blasting. EXPERIMENTAL IN-SITU RETORT BLAST A small scale test blast was fired initially to investigate the predicted explosives performance. Evaluation of increased fracturing in the oil shale by re-entry drilling (cored through oil shale) and pressure test for permeability proved inconclusive. The lack of surface disturbance showed that the explosive performance in design work had not been understated. The small scale test blast allowed the following modifications to the in-situ retort blast design. 1. The first firing blasthole could be increased to 200mm diameter. 2. All other blastholes could be increased to 171mm diameter. For the experimental in-situ retort blast the general blasthole charging procedure was to chargE the blasthole in the oil shale (numbers 1 to 11, Fig. 3) with explosive to approximately 2m from the Coquina, then place 2m of stemming (screenings) prior to placement of the explosive charge in the Coquina, and finally stemming to the surface. Explosive charge weights varied from 58 to 192 kg in the oil shale and 17 to 50 kg in the Coquina. The overall explosives powder factor for the blast was 1.37 kg.m- 3 of oil shale. All explosive charges in the Coquina were initiated with a No. 1 'L' Series millisecond delay detonator (nominal firing time 25 milliseconds) while explosive charges in the oil shale (see Fig. 2) were initiated with Nos 15 to 25 'L' Series millisecond delay detonators (nominal firing times 395 to 695 milliseconds).
(1) Assumes that the bottom of the explosive charge is located at the base of the Coquina.
After the blast the surface expression was shown in two ways.
Blasting to yield uniform cracking in the oil shale was simulated by means of the Harries Blasting Model. The blasthole pattern selected for the experimental in-situ retort was 6m equilateral triangular with a 152mm diameter blasthole. To blast a retort 18m in length and 10.4m in width, 11 blastholes dril led to the bottom of the oil shale were required (Fig. 3). For the chambering of the Coquina, another 14 blastholes, drilled to the base of the Coquina, were considered necessary in addition to the 11 blastholes drilled to the base of the oil shale (Fig. 3). All Coquina explosive charges need to be detonated instantaneously some 200 to 300 milliseconds before the individually detonated explosive charges in the oil shale. The design depths of blastholes were progressively increased from the blast initiation end of the retort with a view to providing a sloping base to facilitate the collection of liquid
1. Surface heave of approximately 30cm over the top of the retort. 2. Minor surface cracking 3 to 4m outside the confines of the blasthole. Subsequent re-entry drilling showed that the oil shale was well fractured but relatively undisturbed. The Coquina was not significantly compressed, however the underlying Ranmoor mudstone showed the effects of compression from the blast above, exhibiting crushed zones and opening of bedding planes.
Blast Design Considerations
CONCLUSIONS The experimental in-situ retort blast and subsequent retort commissioning has shown 1. The oil shale can be satisfactorily fractured in-situ with explosives. 2. The relatively undisturbed nature of the blasted oil shale and the subsequent problems with maintenance of combustion in the retort indicate that the explosive charges in the Coquina did not provide sufficient expansion void. 3. The damage to the underlying mudstone is most likely due to insufficient expansion void for the blasted oil shale, however consideration should also be given to explosive charge placement in the oil shale. 4. Blast design modifications are required to increase the explosives distribution in the Coquina for an increased expansion void to provide better permeability in the oil shale and reduced damage to the underlying mudstone. 5. The properties of the underlying mudstone should be considered in more detail.
ACKNOWLEDGEMENTS The authors extend their thanks to ICI Australia Operations Pty Ltd for permission to publish this paper. REFERENCES Marcovich, B.J. (1983). In-situ Retorting of Julia Creek Oil Shale. Proceedings of the First Australian Workshop on Oil Shale Lucas Heights. pp. 47. Paine, G.G. (1983). Explosives Fracturing of Oi 1 Shale for an Experimental In-Situ Retort at the Julia Creek Project. Proceedings of the First Australian Workshop on Oil Shale Lucas Heights. pp. 51. Harries, G. (1973). A Mathematical Model of Cratering and Blasting Proc. Nat. Symp. On Rock Fragmentation (Aust. Geomech. Soc.) Adelaide. pp. 41. Harries, G. (1977). The Calculation of the Fragmentation of Rock from Cratering, Proc. 15th APCOM Symp, Brisbane. pp. 325.
43
C. Harries an d C. C. Paine
44
Tensi l e Spal li ng
Fig. 2.
Schematic of Field In-S i tu Retort (Marovi t ch 1983)
o bl astho l e dri ll ed to bottom of oil shale • bl ast hol e dr ill ed to bottom of coqu i na (15) mil l i second de l ay numbers for oil shale charges Bl astho l e Pattern for Exper imental In- Situ Retor t Bl ast
Fi g. 3.
ow "
.
.
AllafU
Mudslone
- '- '-' CO QU l na
,
l
. r; I, ' ;:; -- .
~
0,' Shale
- -- -
- - .-.-
M u dst o ne
Fig. 4.
Longitudinal Section of Experimental In - Situ Retort Blast