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Characterization of Marcellus Shale natural gas well drill cuttings
Journal of Unconventional Oil and Gas Resources 1–2 (2013) 9–17
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Journal of Unconventional Oil and Gas Resources journal homepage: www.elsevier.com/locate/juogr
Regular Article
Characterization of Marcellus Shale natural gas well drill cuttings B. Barry, M.S. Klima ⇑ John and Willie Leone Family Department of Energy and Mineral Engineering, Penn State University, University Park, PA, United States
a r t i c l e
i n f o
Article history: 18 October 2012 30 April 2013 10 May 2013 Available online 10 June 2013 Keywords: Marcellus Shale Drill cuttings Characterization Reuse
a b s t r a c t Drilling operations in preparation for natural gas extraction from the Marcellus Shale formation generate large amounts of rock cuttings, which return to the surface coated in drilling mud. Solids control is commonly implemented so that the mud can be recycled, but total removal of the cuttings is uneconomical, so any non-reclaimed cuttings are processed to reduce moisture and then deposited in landfills. Laboratory analyses were conducted to characterize two samples of drill cuttings and to present characterization methods that may be relevant in assessing the beneficial reuse potential of drill cuttings. A key aspect of this study was to evaluate several approaches for providing consistent size distribution data. In addition, degradation testing was performed by submitting cuttings to moderate forms of attrition and sonication. Analyses provided particle size distributions, ash values, moisture content, and total organic carbon content of the samples. Materials analyzed included cuttings from the vertical portion of a wellbore mixed with water-based mud as well as Marcellus Shale cuttings from the horizontal portion of the same wellbore, mixed with oil-based mud. It was found that the size distribution of the waterbased cuttings was much broader and finer than that of the oil-based cuttings for the samples analyzed in this study. Size degradation by attrition was minimal. Attempts to disperse the material using sonication were successful but lead to significant particle degradation. On a dry basis, the ash values of the waterbased cuttings ranged from 94% to 98% by weight compared to 85–89% by weight for the oil-based cuttings. Total organic carbon content of the oil-based cuttings was approximately 10.6%. Additional testing may be required to ensure compliance with applicable regulations for beneficial reuse of the cuttings. Ó 2013 Elsevier Ltd. All rights reserved.
Introduction Marcellus Shale is a black, Middle Devonian age sedimentary rock found in the Appalachian Basin Province. The shale contains oil and gas generated from the anaerobic decomposition of ancient, organic-rich material deposits. Adsorbed on the Marcellus Shale mineral grains and trapped in isolated pore spaces and fractures (Lee et al., 2011; United States Department of Environmental Conservation, 2011), the oil and gas are particularly difficult to extract. Extraction is further complicated by the tight, thinly layered nature of the shale. These thin strata are a result of compaction of sheetlike clay particles as the clay grains rotated to lie flat due to overburden pressure (Arthur and Bohm, 2008). Shale formation in this manner results in a fissile rock with very low permeability, typically between 0.1 and 0.00001 millidarcy (Lee et al., 2011; United States Department of Environmental Conservation, 2011). Shale matrix porosity values are also low, commonly on the order of 0–10% for black shales, and are estimated at 1–3% for Devonian shales in the Appalachian Basin (United States Department of Environmental Conservation, 2011). These characteristics have made ⇑ Corresponding author. Tel.: +1 814 863 7942. E-mail address: [email protected] (M.S. Klima). 2213-3976/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.juogr.2013.05.003
horizontal drilling and hydraulic fracturing of the Marcellus Shale a necessity for natural gas extraction, leading to its designation as an unconventional reservoir. Large quantities of drill cuttings are generated in preparation for natural gas extraction from Marcellus reservoir wells. A single typical, fully-cased horizontal well with a 7000 foot target depth and 4000 foot lateral section can produce over 200 yd3 of cuttings (United States Department of Environmental Conservation, 2011). As a substantial amount of heat is generated from friction on the drill bit during well drilling, drilling fluids commonly called muds are circulated through the hollow tip of the bit to maintain an adequate temperature as the bit travels through the ground (Lee et al., 2011; United States Department of Environmental Conservation, 2011; Arthur and Bohm, 2008). These muds, which are water-, oil- or synthetic-based, entrain and carry rock cuttings up through the wellbore to the surface. Currently, there is much uncertainty as to the best use of this material after it has emerged from the well. To a great extent, the cuttings are separated from the drilling mud using solids control equipment such as vibratory screens and hydrocyclones. Cuttings are usually then deposited in a landfill, while the reclaimed muds are reused for drilling. However, there is interest in beneficial reuse of the cuttings. At least one company is now permitted by the Pennsylvania Department of
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Fig. 1. Water-based cuttings, as-received.
Environmental Protection (DEP) to provide beneficial reuse services after subjecting the cuttings to a proprietary process (Clean Earth, 2012). This process involves addition of stabilization agents, namely cement, to cuttings which are typically untreated aside from processing for drilling mud recovery. These agents strengthen the material matrix to create a geotechnically sound fill material (D. Mueller, personal communication, April 23, 2013). Such stabilized material has proven to meet regulated fill standards and has been beneficially reused at several PA Act 2 Land Recycling Program sites including an abandoned mine and remediated steelworks (D. Mueller, personal communication, April 23, 2013). According to the Drilling Waste Management Information System (2012), drill cuttings can be used as road base and as fill in abandoned mines, for construction purposes, and as fuel if having significant oil content. This is certainly true of petroleum drill cuttings (Ball et al., 2012; Mohammed and Cheeseman, 2011), but the suitability of Marcellus Shale cuttings for beneficial reuse has yet to be thoroughly explored, at least in publication. Moreover, data and results pertaining to characterization of the cuttings are not readily available. The objective of this paper is to evaluate several approaches for characterizing Marcellus Shale natural gas drilling well cuttings which would be useful when assessing beneficial reuse potential. Specifically, the analyses will focus on particle size distributions, potential for size degradation, and attribute analyses
Fig. 3. Oil-based cuttings, as-received.
Fig. 4. WBC sampling method.
(i.e., ash value, moisture content, and carbon content) of the cuttings.
Fig. 2. Section view of Devonian formation (Milici and Swezey, 2006).
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Sample material
Wet screen at 150 m
+150 m material Combine and oven-dry at 217 F for 24 hours
-150 m material
Wet screen at 25 m
Repeat for 30 min of total sieve time
-150+25 m material Riffle material
-25 m material
Oven-dry at 217 F for 24 hours and weigh
(a) Analysis of size distribution Sieve in Ro-Tap for 25 min, deblinding at 5 and 10 min
Weigh material in size intervals
(b) Analysis of size distribution over time Sieve in Ro-Tap for 5 min
Weigh material in size intervals
Return material to each size interval
Fig. 5. Size characterization workflow chart.
Materials and methods Sample acquisition and preparation Two samples of cuttings from two separate well drilling stages for Marcellus Shale natural gas extraction were provided by a natural gas producer in Pennsylvania. One sample was supplied in the form it emerged from the wellbore with minimal to no treatment and consisted of cuttings from the vertical portion of the borehole, above the Marcellus Shale strata, mixed with water-based drilling mud (Fig. 1). Due to the material’s appearance and because the Brallier and Scherr formations are the overlying stratigraphic units of the Marcellus Shale in western Pennsylvania (Fig. 2), it is expected that the water-based cuttings consist of siltstone. The other sample consisted of Marcellus Shale horizontal drill cuttings mixed with oil-based drilling mud (Fig. 3). This horizontal sample had been dried with an on-site physical drying process after emerging from the wellbore. Specific information regarding the drying process was not available. The two samples were provided in separate five-gallon buckets. The oil-based cuttings1 (OBC) were divided into 16, approximately 1000 g samples using a riffle splitter. Successive splitting was performed to ensure representative samples. Because of the high moisture content of the water-based cuttings2 (WBC), it was not possible to split the material using a riffle so smaller samples were obtained by scooping in layers in a downward, clockwise fashion within the 1 2
OBC – oil-based cuttings. WBC – water-based cuttings.
five-gallon bucket (Fig. 4). This produced 14, approximately 1700 g samples and minimized the effects of settling or stratification of the material. It should be noted that certain technical details regarding the cuttings were unavailable. For example, details concerning the sampling method with which the cuttings were gathered was not provided. Depending on the drill bit, subsurface position within the wellbore, and the nature of wellbore sampling, cuttings characteristics may differ. Additionally, there are known particle settling effects in non-Newtonian drilling fluids, which are utilized in operations such as this one (Omland et al., 2009; Eltilib et al., 2011). Furthermore, complete composition of the drilling muds was not available. However, water-based muds typically contain fine suspended solids (<50 lm) such as clays and barite, along with chemical additives like surfactants and polymers (Caenn et al., 2011). Most of these solids are for control of fluid viscosity and filtration properties, while barite is used as a weighting agent to increase overall mud density (Caenn et al., 2011). On the other hand, oilbased muds contain diesel or crude oil as well as emulsifying, suspending and filtration control agents (Caenn et al., 2011). Again, barite may be added for density control. For these reasons, results presented within this study directly pertain to the samples analyzed thus it should not be assumed similar drill cuttings exhibit the same characteristics. Size characterization Several types of size characterization tests were conducted to evaluate the effect of preparation technique on the resulting size
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Sample material
Combine with 1% (NaPO3)6 solution to achieve 20% solids by weight
Mix suspension at 200 rpm for 5 min using laboratory stirrer
Sonicate suspension at desired amplitude and for desired time period
Wet screen at 150 m
+150 m material Combine and oven-dry at 217 F for 24 hours
-150 m material
Wet screen at 25 m
-150+25 m material Riffle material
-25 m material Oven-dry at 217 F for 24 hours and weigh
Sieve in Ro-Tap for 25 min, deblinding at 5 and 10 min
Weigh material in size intervals Fig. 6. Sonication testing workflow chart.
distributions. Initially, a size analysis was conducted on one of the 14 samples (1700 g) of the WBC. The sample was wet screened by hand at 150 lm on a 100 US mesh sieve. The cuttings passing 150 lm were collected and then wet screened at 25 lm on a 500 mesh sieve. Screening was performed in this manner to reduce the solids loading on the 500 mesh sieve. The size fractions of material coarser than 150 lm (i.e., designated as +150 lm) and finer than 150 lm but coarser than 25 lm (i.e., designated as 150 + 25 lm) were then oven-dried at 217 °F for 24 h. The +25 lm dried WBC were riffled to obtain an approximately 200 g sample. This sample was then dry screened by loading a stack of sieves into a Ro-Tap sieve shaker and operating the unit for a total of 25 min. Eight-inch diameter sieves with square opening sizes of 12,500, 9500, 6300, 4750, 3350, 2360, 1700, 1180, 850, 600, 425, 300, 212, 150, 106, 75, 53, 38, and 25 lm were used for screening the WBC. The sieves were deblinded after 5 and 10 min by brushing the undersides of the woven-wire mesh sieves. The dry screening procedure was repeated for the remaining portion of the 1700 g sample (Fig. 5). The procedure for the OBC was similar to that of
the WBC but the process began with an approximately 1000 g sample. Because the OBC had a smaller top size, only sieves from 1700 lm through 25 lm were used. Another set of size analysis tests was carried out on the WBC and OBC to evaluate the change in size distribution with sieving time. For these tests, the previously discussed combination of wet/dry screening was used to remove material finer than 25 lm (i.e., designated as 25 lm) and to prepare the +25 lm material for dry sieving. An approximately 200 g sample of the +25 lm material was dry screened using the same sieve sizes. For these tests, the sieves were deblinded at 5 min after which the weight on each sieve was recorded. This approach was repeated at 5min intervals for a total of 30 min (Fig. 5). An additional set of size distribution tests was carried out on the OBC to observe the effects of sample dispersion by sonication and wet screening of particles less than 25 lm prior to sieving. An approximately 200 g sample was obtained by riffling one of the 16 samples (1000 g). These solids were mixed into a 1% (by weight) sodium hexametaphosphate [(NaPO3)6] and distilled water
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OBC. Previous work with mineral systems has shown that sonication can affect mineral separations (Celik, 1989; Franko and Klima, 2002). For each test, an approximately 30 g sample of the OBC was obtained by riffling and combined with a 1% sodium hexametaphosphate solution in a 400 mL glass beaker to achieve a final solids concentration of 20% (by weight). The suspension was then mixed at 200 rpm for 5 min using a laboratory stirrer. The mixed suspension was then sonicated at 20% amplitude and 35% power for 1 min using the sonicator. The suspension was then sized using the combination of wet/dry screening previously discussed. This procedure was repeated with separate 30 g OBC samples for sonication times of 2, 4, 8, and 16 min as well as without sonication. In addition, the procedure was repeated for the same sonication times but at 30% amplitude (Fig. 6). Testing was performed at a higher amplitude level (30%) to observe the effects of stronger sonication without exceeding the microtip probe limit of 40%. Attrition testing
Fig. 7a. Tumbling mill.
These tests were carried out to evaluate potential degradation of the cuttings by attrition. First, one of the 14 samples (1700 g) of WBC was oven-dried at 217 °F for 24 h. The attrition test procedure was modified from ASTM D441 (Standard Test Method of Tumbler Test for Coal). To obtain the 1000 cm3 sample needed for testing, the dried WBC were riffled and poured into a 1000 cm3 graduated cylinder in approximately 200 cm3 amounts. After each addition, the material was packed by firmly tapping the bottom of the cylinder until no change in volume occurred. The 1000 cm3 sample was placed into a 5-L (248 mm diameter by 172 mm deep) steel mill having six lifters (Fig. 7a) to produce a 20% volumetric mill loading. The mill was sealed and placed onto a roller table (Fig. 7b). The mill was rotated at 40 ± 1 rpm for 1 h. Following tumbling, the entire sample was wet screened to remove the 25 lm material and an approximately 200 g sample of the dried +25 lm material was obtained by riffling. The 200 g sample was then dry screened for 25 min with deblinding at 5 and 10 min. The full procedure was repeated for the OBC. It was necessary to dry two 1000 g samples to produce 1000 cm3 of the OBC. Moisture, ash, and TOC analyses
Fig. 7b. Tumbling mill on roller table.
solution. Sodium hexametaphosphate is a dispersing agent commonly used to assist in size analysis for mineral systems and soils as in ASTM D422 (Standard Test Method for Particle-Size Analysis of Soils). The mixed suspension was then sonicated at 20% amplitude and 35% power for 1 min using a Sonics & Material Inc. Vibra Cell VCX400 sonicator with CV26 probe. The entire sample was wet screened on a 500 US mesh sieve to remove the 25 lm material. The +25 lm material was dried and sieved on a Ro-Tap for 25 min with 5 and 10 min sieve deblindings, as before (Fig. 6). The procedure was repeated for another 200 g sample but without sonication. A third 200 g sample of the OBC was sieved without wet screening at 25 lm prior to dry screening.
The moisture content of the WBC was determined by weight loss after oven-drying one of the 1700 g samples at 217 °F for 24 h. The same approach was used to determine moisture content of the OBC by drying one of the 1000 g samples. Ash analysis was carried out on head samples and individual size fractions of the OBC and WBC by ASTM D5142 (Standard Test Methods for Proximate Analysis of the Analysis Sample of Coal and Coke by Instrumental Procedures) using a LECO TGA701 thermogravimetric analyzer. Prior to analysis, head samples of the OBC and dried WBC were riffled and ground in a Bleuler puck-and-ring mill, while the +100 lm size fractions were ground with a Fritsch vibratory micro ball mill. In addition, total organic carbon (TOC) analysis was carried out on head samples of the OBC. Three approximately 5 g OBC head samples were riffled and individually ground in a Fritsch vibratory micro ball mill then analyzed for TOC in a Shimadzu TOC-5000A. Results and discussion Size distribution
Sonication testing A series of tests was conducted to evaluate the effect of sonication treatment time and amplitude on the size distribution of the
Fig. 8 compares the size distributions of the WBC and the OBC. As can be seen, the WBC cover a broader range of sizes and contain particles much coarser than the OBC. Also, the WBC possess a top
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Cumulative % passing
100
10
1
WBC
0
OBC, no wet screening
10
100
OBC, wet screened 1000
10000
Sieve size, µm Fig. 8. Size distribution plot exhibiting the effects of wet screening and sonication.
Table 1 Cuttings gradation with increasing sieving time (wt.% in interval). Sieve size (lm)
12,500 + 9500 9500 + 6300 6300 + 4750 4750 + 3350 3350 + 2360 2360 + 1700 1700 + 1180 1180 + 850 850 + 600 600 + 425 425 + 300 300 + 212 212 + 150 150 + 106 106 + 75 75 + 53 53 + 38 38 + 25 25 Total
WBC Sieve time (min)
OBC Sieve time (min)
5
15
30
5
15
30
1.39 8.91 10.63 17.17 16.19 9.85 6.84 3.77 2.54 1.53 0.67 0.40 0.36 0.38 0.50 0.91 1.33 1.44 15.18 100
1.38 8.48 10.19 16.93 16.43 10.27 7.02 3.79 2.64 1.55 0.71 0.45 0.36 0.35 0.49 0.86 0.90 1.75 15.43 100
1.39 7.80 10.12 17.41 16.17 10.45 7.17 3.82 2.67 1.58 0.72 0.45 0.37 0.35 0.50 0.85 0.60 1.99 15.59 100
– – – – – 5.25 8.35 10.63 13.40 15.03 11.86 10.92 8.55 6.05 3.84 2.28 0.82 1.29 1.73 100
– – – – – 4.53 7.42 9.98 13.43 15.10 12.24 11.18 8.95 6.34 4.13 2.38 0.44 1.85 2.02 100
– – – – – 4.24 6.93 9.43 13.56 15.15 12.48 11.35 9.18 6.33 4.32 2.51 0.34 1.99 2.21 100
size of approximately 12,500 lm compared to 4750 lm for the OBC. Additionally, the percentage of fines was much greater for the WBC, which exhibited a bimodal particle size distribution compared to the unimodal distribution of the OBC. Fine barite is commonly added to water-based drilling muds as a weighting agent, so the 25 lm particles may be attributable to this material. The particle size distribution was positively skewed for both the WBC and OBC. Fig. 8 also includes the data obtained for the OBC when performing size analysis without wet screening to remove the 25 lm particles prior to dry sieving. As expected, the size distribution of the OBC was coarser when no wet screening was used, indicating that the fine particles were agglomerated and/or adhered to the surface of larger particles. This demonstrates the necessity for wet screening prior to size analysis. Furthermore, Pennsylvania Department of Transportation construction specifications indicate that the gradation of the WBC closely resembles AASHTO #10, which is a coarse aggregate roadstone (Commonwealth of Pennsylvania Department of Transportation, 2011). Also, the gradation of the OBC resembles bituminous concrete sand type B #2, which is
a fine aggregate (Commonwealth of Pennsylvania Department of Transportation, 2011). Table 1 compares the change in size distribution with time for the WBC and OBC. In both cases, there was little variability in the weight retained values with time across all sieves. The largest changes over the entire 30 min period were a 1.11% decrease in weight percentage retained in the 9500 + 6300 lm interval for the WBC and a 1.42% decrease in weight percentage retained in the 1700 + 1180 lm interval for the OBC. Hence, the action of the sieve shaker achieved a consistent size distribution in a relatively short time, while longer times produced minimal degradation for both materials. Wet screening to remove the 25 lm particles followed by dry screening of the coarser material appears to have minimized any agglomeration effects, even with the OBC. Based on these results, the selection of a total sieve shaking time of 25 min for size analyses was appropriate in that the interval weight percentage retained across all sieves seems to stabilize by this time. Sonication effects Fig. 9 shows the change in size distribution of the OBC as a function of sonication time. As can be seen, there is a significant increase in the weight percentage of 25 lm material as sonication time increases. Pictures taken under a laboratory microscope (Fig. 10) show that agglomerates were not present in the various size fractions. Therefore it is unlikely that dispersion of the agglomerates was responsible for the large increase in 25 lm material. Rather it appears that ultrasonic treatment not only scrubbed ultrafine particles from the surface of the coarser particles but also caused size reduction. Ultrasonics are capable of inducing cavitation in suspensions, which has been shown to cause surface cleaning and erosion as well as particle size reduction due to microjet and shock-wave surface impacts (Peters, 1996; Raman et al., 2011). In addition, these impacts can result in interparticle collisions and abrasion (Peters, 1996; Raman et al., 2011). Particle breakage through submission to ultrasonic treatment functions through a variety of mechanisms and kinetic models of this process have been developed (Raman et al., 2011). Nearly 20% of the 25 lm material generated appears to be coming from the 600 + 212 lm interval (Fig. 9). Furthermore, Fig. 10 reveals a clear reduction in the amount of surface fines on the 600 + 212 lm material with increasing sonication time. Although not shown
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40
0 min
1 min
2 min
4 min
8 min
15
16 min
35
% Weight retained
30 25 20 15 10 5 0 +600
-600+212
-212+75 -75+25 Sieve size inteval, µm
-25
Fig. 9. OBC gradation with increasing sonication time (20% amplitude).
Fig. 10.
600 + 212 lm OBC having received (a) 0 min (b) 2 min (c) 8 min (d) 16 min of sonication (20% amplitude).
here, additional microscopic observation indicated that the ultrafine particles coated the 600 + 212 lm particles more so than any other size interval. Fig. 11 compares the size distributions for sonication for the two amplitudes (20% and 30%) and several sonication times. As can be seen, the higher amplitude sonication resulted in a finer gradation of the OBC for the same sonication time. As stated previously, it is evident that sonication is liberating ultrafine particles from the surface of the coarser particles, but the nature of the curves in Fig. 11 suggests that particles are also breaking. This is affirmed by the continuous increase in the cumulative percent of
material passing the largest sieve (600 lm) as sonication amplitude and time exposure to sonication increases. Attrition effects It was mentioned that the WBC were presumably siltstone, which is a rock exhibiting great friability if weakly cemented. Also, the OBC consist of Marcellus Shale, which is fissile, meaning the rock tends to break in a sheet-like nature. These characteristics suggest the possibility of size degradation by attrition during material handling. Interestingly, the attrition test results proved
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80
Cumulative % passing
70 60 50
1 min., 20% amp. 1 min., 30% amp.
40
2 min., 20% amp. 2 min., 30% amp.
30
4 min., 20% amp. 4 min., 30% amp.
20
8 min., 20% amp. 8 min., 30% amp.
10 10
100 Sieve size, m
1000
Fig. 11. Variation of OBC size distributions with sonication time and amplitude.
Table 3 Ash values, dry basis (% by weight).
Table 2 Attrition testing cuttings size distributions (wt.% in interval). Sieve size (lm)
12,500 + 9500 9500 + 6300 6300 + 4750 4750 + 3350 3350 + 2360 2360 + 1700 1700 + 1180 1180 + 850 850 + 600 600 + 425 425 + 300 300 + 212 212 + 150 150 + 106 106 + 75 75 + 53 53 + 38 38 + 25 25 Total
WBC
OBC
Before attrition
After attrition
Before attrition
After attrition
1.39 8.17 9.78 17.45 16.28 10.39 7.15 3.83 2.67 1.58 0.72 0.44 0.36 0.35 0.49 0.84 0.65 1.92 15.55 100
0.57 6.82 12.73 19.59 15.64 9.48 6.54 3.27 2.39 1.36 0.73 0.52 0.36 0.34 0.47 0.99 1.11 1.45 15.64 100
– – – – – 4.31 7.07 9.62 13.52 15.11 12.39 11.31 9.10 6.32 4.29 2.48 0.37 1.93 2.17 100
– – – – – 4.60 8.50 11.68 12.03 14.00 12.24 10.43 8.25 5.53 3.36 2.02 1.70 0.83 4.82 100
Size interval (lm)
WBC
Nonsonicated OBC wet screened
Sonicated OBC
+9500 9500 + 6300 6300 + 4750 4750 + 3350 3350 + 2360 2360 + 1700 1700 + 1180 1180 + 850 850 + 600 600 + 425 425 + 300 300 + 212 212 + 150 150 + 106 106 + 75 75 + 53 53 + 38 38 + 25 25
96.8 96.2 96.6 96.4 96.5 96.4 96.4 96.3 96.2 95.9 95.6 95.2 94.8 94.0 95.1 96.7 97.3 97.4 95.4
– – – – 85.1 88.6 88.7 87.2 87.7 87.8 86.5 86.0 86.0 89.0 89.2 89.6 87.9 89.3 87.3
– – – – 89.0 88.8 89.3 89.1 88.0 88.4 88.1 88.3 88.3 87.2 87.1 87.2 86.0 87.5 89.3
Head 1 Head 2
96.4 96.4
87.0 87.3
– –
Additional cuttings characteristics otherwise and in fact, the effects of agglomeration seem more pronounced than those of attrition. As can be seen in Table 2, tumbling of the WBC in the rotating mill had very minor effects on the particle size distribution. Notably, a 2.95% increase in weight percentage retained in the 6300 + 4750 lm size interval and a 1.35% decrease in weight percentage retained in the 9500 + 6300 lm material was observed. Similar effects occurred when testing the OBC (Table 2). There was a 2.64% increase in weight percentage retained of 25 lm material and a 1.49% decrease in weight percentage retained in the 850 + 600 lm size interval. Autogenous tumbling mills cause particle breakage by fracturing, chipping, and abrasion (Austin et al., 1987). As tumbling progresses, generation of fine particles creates a cushioning effect which reduces breakage (Austin et al., 1987). For this study, the presence of ultrafine particles in the mill promoted agglomeration of the cuttings and led to cushioning. This was especially true for the WBC due to the larger weight percentage of 25 lm material. Agglomeration effects could in fact be more prominent than indicated here as wet screening after removal of the cuttings from the mill may have unavoidably separated some adhered particles.
Oven-drying of the composite or head samples showed that the WBC had a moisture content of 18.8% compared to 5.3% for the OBC. Because the OBC were treated using a physical drying process, this difference was expected. Using the weights of the 1000 cm3 samples of packed material from the attrition testing allowed the bulk densities to be calculated. These were 1.59 g/cm3 and 1.63 g/cm3 for the WBC and OBC, respectively. Also, three head samples of the OBC were tested for organic carbon, producing TOC values of 10.8%, 10.6% and 10.6%. Sageman et al. (2003) found that the TOC content for Marcellus Shale ranged from 6.5% up to 18%, which compares favorably to the results in this study. Table 3 gives the ash values for the head samples and size fractions of the WBC and OBC tested. As expected, the average ash values for the WBC were very high, ranging between 94% and 98% (by weight) on a dry basis. The high ash values indicate the presence of very little, if any, combustible material. The ash values for the OBC were somewhat lower, with values ranging from 85% to 89% (by weight) on a dry basis. As Marcellus Shale is comprised of organic matter more so than siltstone, the OBC were expected to have low-
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er ash values. Also, the data indicate that the organic matter did not concentrate in any particular size fraction. Summary and conclusions Samples of WBC and OBC were obtained from a Marcellus Shale drilling operation in Pennsylvania. These materials were characterized to determine various properties with a focus on the particle size distributions. Observing the gradation of the cuttings presented in Fig. 8 and Table 1, the WBC exhibited a broader and finer size distribution than the OBC. It is possible these materials could be suited to serve as structural fill following additional testing to meet applicable regulations. For example, the gradation of the WBC resembles coarse aggregate AASHTO #10 and that of the OBC resembles fine aggregate bituminous concrete sand type B #2. However, several quality requirements which were not assessed in this study must be met to fully satisfy these designations. Furthermore, the cuttings may meet regulated fill standards for Act 2 sites as discussed in the Introduction. Also, both the WBC and OBC maintained reasonably consistent size distributions (i.e., low variation in wt.% values) when submitted to moderate forms of attrition in a sieve shaker and during tumbling in a rotating cylinder. Because both materials maintained their size distributions, it is likely that transportation or moderate handling would lead to minimal particle size degradation. On the other hand, significant degradation could occur under more severe forms of treatment such as ultrasonic treatment. Furthermore, because of the very low carbon content of the OBC, their use in combustion applications would be inappropriate. The procedures presented in this study would be useful in future characterization of drilling well cuttings to determine potential for beneficial reuse. Results of this study serve to provide expected properties and size distributions of such cuttings. All results pertain specifically to the samples analyzed and conclusions drawn may not necessarily be true of all Marcellus drilling well cuttings.
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References Arthur, J.D., Bohm, B., 2008. Hydraulic fracturing considerations for natural gas wells of the Marcellus Shale. The Ground Water Protection Council Annual Forum, Cincinnati, OH. Austin, L.G., Barahona, C.A., Menacho, J.M., 1987. Investigations of autogenous and semi-autogenous grinding in tumbling mills. Power Technol. 51 (3), 283–294. Ball, A.S., Stewart, R.J., Schliephake, K., 2012. A review of the current options for the treatment and safe disposal of drill cuttings. Waste Manage. Res. 30 (5), 457– 473. Caenn, R., Darley, H.C.H., Gray, G.R., 2011. Composition and Properties of Drilling and Completion Fluids. Gulf Professional Publishing, Boston. Celik, M.S., 1989. Effect of ultrasonic treatment on the floatability of coal and galena. Sep. Sci. Technol. 24, 1159–1166. Clean Earth, Inc., 2012. Available from:
Contents lists available at SciVerse ScienceDirect
Journal of Unconventional Oil and Gas Resources journal homepage: www.elsevier.com/locate/juogr
Regular Article
Characterization of Marcellus Shale natural gas well drill cuttings B. Barry, M.S. Klima ⇑ John and Willie Leone Family Department of Energy and Mineral Engineering, Penn State University, University Park, PA, United States
a r t i c l e
i n f o
Article history: 18 October 2012 30 April 2013 10 May 2013 Available online 10 June 2013 Keywords: Marcellus Shale Drill cuttings Characterization Reuse
a b s t r a c t Drilling operations in preparation for natural gas extraction from the Marcellus Shale formation generate large amounts of rock cuttings, which return to the surface coated in drilling mud. Solids control is commonly implemented so that the mud can be recycled, but total removal of the cuttings is uneconomical, so any non-reclaimed cuttings are processed to reduce moisture and then deposited in landfills. Laboratory analyses were conducted to characterize two samples of drill cuttings and to present characterization methods that may be relevant in assessing the beneficial reuse potential of drill cuttings. A key aspect of this study was to evaluate several approaches for providing consistent size distribution data. In addition, degradation testing was performed by submitting cuttings to moderate forms of attrition and sonication. Analyses provided particle size distributions, ash values, moisture content, and total organic carbon content of the samples. Materials analyzed included cuttings from the vertical portion of a wellbore mixed with water-based mud as well as Marcellus Shale cuttings from the horizontal portion of the same wellbore, mixed with oil-based mud. It was found that the size distribution of the waterbased cuttings was much broader and finer than that of the oil-based cuttings for the samples analyzed in this study. Size degradation by attrition was minimal. Attempts to disperse the material using sonication were successful but lead to significant particle degradation. On a dry basis, the ash values of the waterbased cuttings ranged from 94% to 98% by weight compared to 85–89% by weight for the oil-based cuttings. Total organic carbon content of the oil-based cuttings was approximately 10.6%. Additional testing may be required to ensure compliance with applicable regulations for beneficial reuse of the cuttings. Ó 2013 Elsevier Ltd. All rights reserved.
Introduction Marcellus Shale is a black, Middle Devonian age sedimentary rock found in the Appalachian Basin Province. The shale contains oil and gas generated from the anaerobic decomposition of ancient, organic-rich material deposits. Adsorbed on the Marcellus Shale mineral grains and trapped in isolated pore spaces and fractures (Lee et al., 2011; United States Department of Environmental Conservation, 2011), the oil and gas are particularly difficult to extract. Extraction is further complicated by the tight, thinly layered nature of the shale. These thin strata are a result of compaction of sheetlike clay particles as the clay grains rotated to lie flat due to overburden pressure (Arthur and Bohm, 2008). Shale formation in this manner results in a fissile rock with very low permeability, typically between 0.1 and 0.00001 millidarcy (Lee et al., 2011; United States Department of Environmental Conservation, 2011). Shale matrix porosity values are also low, commonly on the order of 0–10% for black shales, and are estimated at 1–3% for Devonian shales in the Appalachian Basin (United States Department of Environmental Conservation, 2011). These characteristics have made ⇑ Corresponding author. Tel.: +1 814 863 7942. E-mail address: [email protected] (M.S. Klima). 2213-3976/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.juogr.2013.05.003
horizontal drilling and hydraulic fracturing of the Marcellus Shale a necessity for natural gas extraction, leading to its designation as an unconventional reservoir. Large quantities of drill cuttings are generated in preparation for natural gas extraction from Marcellus reservoir wells. A single typical, fully-cased horizontal well with a 7000 foot target depth and 4000 foot lateral section can produce over 200 yd3 of cuttings (United States Department of Environmental Conservation, 2011). As a substantial amount of heat is generated from friction on the drill bit during well drilling, drilling fluids commonly called muds are circulated through the hollow tip of the bit to maintain an adequate temperature as the bit travels through the ground (Lee et al., 2011; United States Department of Environmental Conservation, 2011; Arthur and Bohm, 2008). These muds, which are water-, oil- or synthetic-based, entrain and carry rock cuttings up through the wellbore to the surface. Currently, there is much uncertainty as to the best use of this material after it has emerged from the well. To a great extent, the cuttings are separated from the drilling mud using solids control equipment such as vibratory screens and hydrocyclones. Cuttings are usually then deposited in a landfill, while the reclaimed muds are reused for drilling. However, there is interest in beneficial reuse of the cuttings. At least one company is now permitted by the Pennsylvania Department of
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Fig. 1. Water-based cuttings, as-received.
Environmental Protection (DEP) to provide beneficial reuse services after subjecting the cuttings to a proprietary process (Clean Earth, 2012). This process involves addition of stabilization agents, namely cement, to cuttings which are typically untreated aside from processing for drilling mud recovery. These agents strengthen the material matrix to create a geotechnically sound fill material (D. Mueller, personal communication, April 23, 2013). Such stabilized material has proven to meet regulated fill standards and has been beneficially reused at several PA Act 2 Land Recycling Program sites including an abandoned mine and remediated steelworks (D. Mueller, personal communication, April 23, 2013). According to the Drilling Waste Management Information System (2012), drill cuttings can be used as road base and as fill in abandoned mines, for construction purposes, and as fuel if having significant oil content. This is certainly true of petroleum drill cuttings (Ball et al., 2012; Mohammed and Cheeseman, 2011), but the suitability of Marcellus Shale cuttings for beneficial reuse has yet to be thoroughly explored, at least in publication. Moreover, data and results pertaining to characterization of the cuttings are not readily available. The objective of this paper is to evaluate several approaches for characterizing Marcellus Shale natural gas drilling well cuttings which would be useful when assessing beneficial reuse potential. Specifically, the analyses will focus on particle size distributions, potential for size degradation, and attribute analyses
Fig. 3. Oil-based cuttings, as-received.
Fig. 4. WBC sampling method.
(i.e., ash value, moisture content, and carbon content) of the cuttings.
Fig. 2. Section view of Devonian formation (Milici and Swezey, 2006).
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Sample material
Wet screen at 150 m
+150 m material Combine and oven-dry at 217 F for 24 hours
-150 m material
Wet screen at 25 m
Repeat for 30 min of total sieve time
-150+25 m material Riffle material
-25 m material
Oven-dry at 217 F for 24 hours and weigh
(a) Analysis of size distribution Sieve in Ro-Tap for 25 min, deblinding at 5 and 10 min
Weigh material in size intervals
(b) Analysis of size distribution over time Sieve in Ro-Tap for 5 min
Weigh material in size intervals
Return material to each size interval
Fig. 5. Size characterization workflow chart.
Materials and methods Sample acquisition and preparation Two samples of cuttings from two separate well drilling stages for Marcellus Shale natural gas extraction were provided by a natural gas producer in Pennsylvania. One sample was supplied in the form it emerged from the wellbore with minimal to no treatment and consisted of cuttings from the vertical portion of the borehole, above the Marcellus Shale strata, mixed with water-based drilling mud (Fig. 1). Due to the material’s appearance and because the Brallier and Scherr formations are the overlying stratigraphic units of the Marcellus Shale in western Pennsylvania (Fig. 2), it is expected that the water-based cuttings consist of siltstone. The other sample consisted of Marcellus Shale horizontal drill cuttings mixed with oil-based drilling mud (Fig. 3). This horizontal sample had been dried with an on-site physical drying process after emerging from the wellbore. Specific information regarding the drying process was not available. The two samples were provided in separate five-gallon buckets. The oil-based cuttings1 (OBC) were divided into 16, approximately 1000 g samples using a riffle splitter. Successive splitting was performed to ensure representative samples. Because of the high moisture content of the water-based cuttings2 (WBC), it was not possible to split the material using a riffle so smaller samples were obtained by scooping in layers in a downward, clockwise fashion within the 1 2
OBC – oil-based cuttings. WBC – water-based cuttings.
five-gallon bucket (Fig. 4). This produced 14, approximately 1700 g samples and minimized the effects of settling or stratification of the material. It should be noted that certain technical details regarding the cuttings were unavailable. For example, details concerning the sampling method with which the cuttings were gathered was not provided. Depending on the drill bit, subsurface position within the wellbore, and the nature of wellbore sampling, cuttings characteristics may differ. Additionally, there are known particle settling effects in non-Newtonian drilling fluids, which are utilized in operations such as this one (Omland et al., 2009; Eltilib et al., 2011). Furthermore, complete composition of the drilling muds was not available. However, water-based muds typically contain fine suspended solids (<50 lm) such as clays and barite, along with chemical additives like surfactants and polymers (Caenn et al., 2011). Most of these solids are for control of fluid viscosity and filtration properties, while barite is used as a weighting agent to increase overall mud density (Caenn et al., 2011). On the other hand, oilbased muds contain diesel or crude oil as well as emulsifying, suspending and filtration control agents (Caenn et al., 2011). Again, barite may be added for density control. For these reasons, results presented within this study directly pertain to the samples analyzed thus it should not be assumed similar drill cuttings exhibit the same characteristics. Size characterization Several types of size characterization tests were conducted to evaluate the effect of preparation technique on the resulting size
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Sample material
Combine with 1% (NaPO3)6 solution to achieve 20% solids by weight
Mix suspension at 200 rpm for 5 min using laboratory stirrer
Sonicate suspension at desired amplitude and for desired time period
Wet screen at 150 m
+150 m material Combine and oven-dry at 217 F for 24 hours
-150 m material
Wet screen at 25 m
-150+25 m material Riffle material
-25 m material Oven-dry at 217 F for 24 hours and weigh
Sieve in Ro-Tap for 25 min, deblinding at 5 and 10 min
Weigh material in size intervals Fig. 6. Sonication testing workflow chart.
distributions. Initially, a size analysis was conducted on one of the 14 samples (1700 g) of the WBC. The sample was wet screened by hand at 150 lm on a 100 US mesh sieve. The cuttings passing 150 lm were collected and then wet screened at 25 lm on a 500 mesh sieve. Screening was performed in this manner to reduce the solids loading on the 500 mesh sieve. The size fractions of material coarser than 150 lm (i.e., designated as +150 lm) and finer than 150 lm but coarser than 25 lm (i.e., designated as 150 + 25 lm) were then oven-dried at 217 °F for 24 h. The +25 lm dried WBC were riffled to obtain an approximately 200 g sample. This sample was then dry screened by loading a stack of sieves into a Ro-Tap sieve shaker and operating the unit for a total of 25 min. Eight-inch diameter sieves with square opening sizes of 12,500, 9500, 6300, 4750, 3350, 2360, 1700, 1180, 850, 600, 425, 300, 212, 150, 106, 75, 53, 38, and 25 lm were used for screening the WBC. The sieves were deblinded after 5 and 10 min by brushing the undersides of the woven-wire mesh sieves. The dry screening procedure was repeated for the remaining portion of the 1700 g sample (Fig. 5). The procedure for the OBC was similar to that of
the WBC but the process began with an approximately 1000 g sample. Because the OBC had a smaller top size, only sieves from 1700 lm through 25 lm were used. Another set of size analysis tests was carried out on the WBC and OBC to evaluate the change in size distribution with sieving time. For these tests, the previously discussed combination of wet/dry screening was used to remove material finer than 25 lm (i.e., designated as 25 lm) and to prepare the +25 lm material for dry sieving. An approximately 200 g sample of the +25 lm material was dry screened using the same sieve sizes. For these tests, the sieves were deblinded at 5 min after which the weight on each sieve was recorded. This approach was repeated at 5min intervals for a total of 30 min (Fig. 5). An additional set of size distribution tests was carried out on the OBC to observe the effects of sample dispersion by sonication and wet screening of particles less than 25 lm prior to sieving. An approximately 200 g sample was obtained by riffling one of the 16 samples (1000 g). These solids were mixed into a 1% (by weight) sodium hexametaphosphate [(NaPO3)6] and distilled water
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OBC. Previous work with mineral systems has shown that sonication can affect mineral separations (Celik, 1989; Franko and Klima, 2002). For each test, an approximately 30 g sample of the OBC was obtained by riffling and combined with a 1% sodium hexametaphosphate solution in a 400 mL glass beaker to achieve a final solids concentration of 20% (by weight). The suspension was then mixed at 200 rpm for 5 min using a laboratory stirrer. The mixed suspension was then sonicated at 20% amplitude and 35% power for 1 min using the sonicator. The suspension was then sized using the combination of wet/dry screening previously discussed. This procedure was repeated with separate 30 g OBC samples for sonication times of 2, 4, 8, and 16 min as well as without sonication. In addition, the procedure was repeated for the same sonication times but at 30% amplitude (Fig. 6). Testing was performed at a higher amplitude level (30%) to observe the effects of stronger sonication without exceeding the microtip probe limit of 40%. Attrition testing
Fig. 7a. Tumbling mill.
These tests were carried out to evaluate potential degradation of the cuttings by attrition. First, one of the 14 samples (1700 g) of WBC was oven-dried at 217 °F for 24 h. The attrition test procedure was modified from ASTM D441 (Standard Test Method of Tumbler Test for Coal). To obtain the 1000 cm3 sample needed for testing, the dried WBC were riffled and poured into a 1000 cm3 graduated cylinder in approximately 200 cm3 amounts. After each addition, the material was packed by firmly tapping the bottom of the cylinder until no change in volume occurred. The 1000 cm3 sample was placed into a 5-L (248 mm diameter by 172 mm deep) steel mill having six lifters (Fig. 7a) to produce a 20% volumetric mill loading. The mill was sealed and placed onto a roller table (Fig. 7b). The mill was rotated at 40 ± 1 rpm for 1 h. Following tumbling, the entire sample was wet screened to remove the 25 lm material and an approximately 200 g sample of the dried +25 lm material was obtained by riffling. The 200 g sample was then dry screened for 25 min with deblinding at 5 and 10 min. The full procedure was repeated for the OBC. It was necessary to dry two 1000 g samples to produce 1000 cm3 of the OBC. Moisture, ash, and TOC analyses
Fig. 7b. Tumbling mill on roller table.
solution. Sodium hexametaphosphate is a dispersing agent commonly used to assist in size analysis for mineral systems and soils as in ASTM D422 (Standard Test Method for Particle-Size Analysis of Soils). The mixed suspension was then sonicated at 20% amplitude and 35% power for 1 min using a Sonics & Material Inc. Vibra Cell VCX400 sonicator with CV26 probe. The entire sample was wet screened on a 500 US mesh sieve to remove the 25 lm material. The +25 lm material was dried and sieved on a Ro-Tap for 25 min with 5 and 10 min sieve deblindings, as before (Fig. 6). The procedure was repeated for another 200 g sample but without sonication. A third 200 g sample of the OBC was sieved without wet screening at 25 lm prior to dry screening.
The moisture content of the WBC was determined by weight loss after oven-drying one of the 1700 g samples at 217 °F for 24 h. The same approach was used to determine moisture content of the OBC by drying one of the 1000 g samples. Ash analysis was carried out on head samples and individual size fractions of the OBC and WBC by ASTM D5142 (Standard Test Methods for Proximate Analysis of the Analysis Sample of Coal and Coke by Instrumental Procedures) using a LECO TGA701 thermogravimetric analyzer. Prior to analysis, head samples of the OBC and dried WBC were riffled and ground in a Bleuler puck-and-ring mill, while the +100 lm size fractions were ground with a Fritsch vibratory micro ball mill. In addition, total organic carbon (TOC) analysis was carried out on head samples of the OBC. Three approximately 5 g OBC head samples were riffled and individually ground in a Fritsch vibratory micro ball mill then analyzed for TOC in a Shimadzu TOC-5000A. Results and discussion Size distribution
Sonication testing A series of tests was conducted to evaluate the effect of sonication treatment time and amplitude on the size distribution of the
Fig. 8 compares the size distributions of the WBC and the OBC. As can be seen, the WBC cover a broader range of sizes and contain particles much coarser than the OBC. Also, the WBC possess a top
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Cumulative % passing
100
10
1
WBC
0
OBC, no wet screening
10
100
OBC, wet screened 1000
10000
Sieve size, µm Fig. 8. Size distribution plot exhibiting the effects of wet screening and sonication.
Table 1 Cuttings gradation with increasing sieving time (wt.% in interval). Sieve size (lm)
12,500 + 9500 9500 + 6300 6300 + 4750 4750 + 3350 3350 + 2360 2360 + 1700 1700 + 1180 1180 + 850 850 + 600 600 + 425 425 + 300 300 + 212 212 + 150 150 + 106 106 + 75 75 + 53 53 + 38 38 + 25 25 Total
WBC Sieve time (min)
OBC Sieve time (min)
5
15
30
5
15
30
1.39 8.91 10.63 17.17 16.19 9.85 6.84 3.77 2.54 1.53 0.67 0.40 0.36 0.38 0.50 0.91 1.33 1.44 15.18 100
1.38 8.48 10.19 16.93 16.43 10.27 7.02 3.79 2.64 1.55 0.71 0.45 0.36 0.35 0.49 0.86 0.90 1.75 15.43 100
1.39 7.80 10.12 17.41 16.17 10.45 7.17 3.82 2.67 1.58 0.72 0.45 0.37 0.35 0.50 0.85 0.60 1.99 15.59 100
– – – – – 5.25 8.35 10.63 13.40 15.03 11.86 10.92 8.55 6.05 3.84 2.28 0.82 1.29 1.73 100
– – – – – 4.53 7.42 9.98 13.43 15.10 12.24 11.18 8.95 6.34 4.13 2.38 0.44 1.85 2.02 100
– – – – – 4.24 6.93 9.43 13.56 15.15 12.48 11.35 9.18 6.33 4.32 2.51 0.34 1.99 2.21 100
size of approximately 12,500 lm compared to 4750 lm for the OBC. Additionally, the percentage of fines was much greater for the WBC, which exhibited a bimodal particle size distribution compared to the unimodal distribution of the OBC. Fine barite is commonly added to water-based drilling muds as a weighting agent, so the 25 lm particles may be attributable to this material. The particle size distribution was positively skewed for both the WBC and OBC. Fig. 8 also includes the data obtained for the OBC when performing size analysis without wet screening to remove the 25 lm particles prior to dry sieving. As expected, the size distribution of the OBC was coarser when no wet screening was used, indicating that the fine particles were agglomerated and/or adhered to the surface of larger particles. This demonstrates the necessity for wet screening prior to size analysis. Furthermore, Pennsylvania Department of Transportation construction specifications indicate that the gradation of the WBC closely resembles AASHTO #10, which is a coarse aggregate roadstone (Commonwealth of Pennsylvania Department of Transportation, 2011). Also, the gradation of the OBC resembles bituminous concrete sand type B #2, which is
a fine aggregate (Commonwealth of Pennsylvania Department of Transportation, 2011). Table 1 compares the change in size distribution with time for the WBC and OBC. In both cases, there was little variability in the weight retained values with time across all sieves. The largest changes over the entire 30 min period were a 1.11% decrease in weight percentage retained in the 9500 + 6300 lm interval for the WBC and a 1.42% decrease in weight percentage retained in the 1700 + 1180 lm interval for the OBC. Hence, the action of the sieve shaker achieved a consistent size distribution in a relatively short time, while longer times produced minimal degradation for both materials. Wet screening to remove the 25 lm particles followed by dry screening of the coarser material appears to have minimized any agglomeration effects, even with the OBC. Based on these results, the selection of a total sieve shaking time of 25 min for size analyses was appropriate in that the interval weight percentage retained across all sieves seems to stabilize by this time. Sonication effects Fig. 9 shows the change in size distribution of the OBC as a function of sonication time. As can be seen, there is a significant increase in the weight percentage of 25 lm material as sonication time increases. Pictures taken under a laboratory microscope (Fig. 10) show that agglomerates were not present in the various size fractions. Therefore it is unlikely that dispersion of the agglomerates was responsible for the large increase in 25 lm material. Rather it appears that ultrasonic treatment not only scrubbed ultrafine particles from the surface of the coarser particles but also caused size reduction. Ultrasonics are capable of inducing cavitation in suspensions, which has been shown to cause surface cleaning and erosion as well as particle size reduction due to microjet and shock-wave surface impacts (Peters, 1996; Raman et al., 2011). In addition, these impacts can result in interparticle collisions and abrasion (Peters, 1996; Raman et al., 2011). Particle breakage through submission to ultrasonic treatment functions through a variety of mechanisms and kinetic models of this process have been developed (Raman et al., 2011). Nearly 20% of the 25 lm material generated appears to be coming from the 600 + 212 lm interval (Fig. 9). Furthermore, Fig. 10 reveals a clear reduction in the amount of surface fines on the 600 + 212 lm material with increasing sonication time. Although not shown
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40
0 min
1 min
2 min
4 min
8 min
15
16 min
35
% Weight retained
30 25 20 15 10 5 0 +600
-600+212
-212+75 -75+25 Sieve size inteval, µm
-25
Fig. 9. OBC gradation with increasing sonication time (20% amplitude).
Fig. 10.
600 + 212 lm OBC having received (a) 0 min (b) 2 min (c) 8 min (d) 16 min of sonication (20% amplitude).
here, additional microscopic observation indicated that the ultrafine particles coated the 600 + 212 lm particles more so than any other size interval. Fig. 11 compares the size distributions for sonication for the two amplitudes (20% and 30%) and several sonication times. As can be seen, the higher amplitude sonication resulted in a finer gradation of the OBC for the same sonication time. As stated previously, it is evident that sonication is liberating ultrafine particles from the surface of the coarser particles, but the nature of the curves in Fig. 11 suggests that particles are also breaking. This is affirmed by the continuous increase in the cumulative percent of
material passing the largest sieve (600 lm) as sonication amplitude and time exposure to sonication increases. Attrition effects It was mentioned that the WBC were presumably siltstone, which is a rock exhibiting great friability if weakly cemented. Also, the OBC consist of Marcellus Shale, which is fissile, meaning the rock tends to break in a sheet-like nature. These characteristics suggest the possibility of size degradation by attrition during material handling. Interestingly, the attrition test results proved
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80
Cumulative % passing
70 60 50
1 min., 20% amp. 1 min., 30% amp.
40
2 min., 20% amp. 2 min., 30% amp.
30
4 min., 20% amp. 4 min., 30% amp.
20
8 min., 20% amp. 8 min., 30% amp.
10 10
100 Sieve size, m
1000
Fig. 11. Variation of OBC size distributions with sonication time and amplitude.
Table 3 Ash values, dry basis (% by weight).
Table 2 Attrition testing cuttings size distributions (wt.% in interval). Sieve size (lm)
12,500 + 9500 9500 + 6300 6300 + 4750 4750 + 3350 3350 + 2360 2360 + 1700 1700 + 1180 1180 + 850 850 + 600 600 + 425 425 + 300 300 + 212 212 + 150 150 + 106 106 + 75 75 + 53 53 + 38 38 + 25 25 Total
WBC
OBC
Before attrition
After attrition
Before attrition
After attrition
1.39 8.17 9.78 17.45 16.28 10.39 7.15 3.83 2.67 1.58 0.72 0.44 0.36 0.35 0.49 0.84 0.65 1.92 15.55 100
0.57 6.82 12.73 19.59 15.64 9.48 6.54 3.27 2.39 1.36 0.73 0.52 0.36 0.34 0.47 0.99 1.11 1.45 15.64 100
– – – – – 4.31 7.07 9.62 13.52 15.11 12.39 11.31 9.10 6.32 4.29 2.48 0.37 1.93 2.17 100
– – – – – 4.60 8.50 11.68 12.03 14.00 12.24 10.43 8.25 5.53 3.36 2.02 1.70 0.83 4.82 100
Size interval (lm)
WBC
Nonsonicated OBC wet screened
Sonicated OBC
+9500 9500 + 6300 6300 + 4750 4750 + 3350 3350 + 2360 2360 + 1700 1700 + 1180 1180 + 850 850 + 600 600 + 425 425 + 300 300 + 212 212 + 150 150 + 106 106 + 75 75 + 53 53 + 38 38 + 25 25
96.8 96.2 96.6 96.4 96.5 96.4 96.4 96.3 96.2 95.9 95.6 95.2 94.8 94.0 95.1 96.7 97.3 97.4 95.4
– – – – 85.1 88.6 88.7 87.2 87.7 87.8 86.5 86.0 86.0 89.0 89.2 89.6 87.9 89.3 87.3
– – – – 89.0 88.8 89.3 89.1 88.0 88.4 88.1 88.3 88.3 87.2 87.1 87.2 86.0 87.5 89.3
Head 1 Head 2
96.4 96.4
87.0 87.3
– –
Additional cuttings characteristics otherwise and in fact, the effects of agglomeration seem more pronounced than those of attrition. As can be seen in Table 2, tumbling of the WBC in the rotating mill had very minor effects on the particle size distribution. Notably, a 2.95% increase in weight percentage retained in the 6300 + 4750 lm size interval and a 1.35% decrease in weight percentage retained in the 9500 + 6300 lm material was observed. Similar effects occurred when testing the OBC (Table 2). There was a 2.64% increase in weight percentage retained of 25 lm material and a 1.49% decrease in weight percentage retained in the 850 + 600 lm size interval. Autogenous tumbling mills cause particle breakage by fracturing, chipping, and abrasion (Austin et al., 1987). As tumbling progresses, generation of fine particles creates a cushioning effect which reduces breakage (Austin et al., 1987). For this study, the presence of ultrafine particles in the mill promoted agglomeration of the cuttings and led to cushioning. This was especially true for the WBC due to the larger weight percentage of 25 lm material. Agglomeration effects could in fact be more prominent than indicated here as wet screening after removal of the cuttings from the mill may have unavoidably separated some adhered particles.
Oven-drying of the composite or head samples showed that the WBC had a moisture content of 18.8% compared to 5.3% for the OBC. Because the OBC were treated using a physical drying process, this difference was expected. Using the weights of the 1000 cm3 samples of packed material from the attrition testing allowed the bulk densities to be calculated. These were 1.59 g/cm3 and 1.63 g/cm3 for the WBC and OBC, respectively. Also, three head samples of the OBC were tested for organic carbon, producing TOC values of 10.8%, 10.6% and 10.6%. Sageman et al. (2003) found that the TOC content for Marcellus Shale ranged from 6.5% up to 18%, which compares favorably to the results in this study. Table 3 gives the ash values for the head samples and size fractions of the WBC and OBC tested. As expected, the average ash values for the WBC were very high, ranging between 94% and 98% (by weight) on a dry basis. The high ash values indicate the presence of very little, if any, combustible material. The ash values for the OBC were somewhat lower, with values ranging from 85% to 89% (by weight) on a dry basis. As Marcellus Shale is comprised of organic matter more so than siltstone, the OBC were expected to have low-
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er ash values. Also, the data indicate that the organic matter did not concentrate in any particular size fraction. Summary and conclusions Samples of WBC and OBC were obtained from a Marcellus Shale drilling operation in Pennsylvania. These materials were characterized to determine various properties with a focus on the particle size distributions. Observing the gradation of the cuttings presented in Fig. 8 and Table 1, the WBC exhibited a broader and finer size distribution than the OBC. It is possible these materials could be suited to serve as structural fill following additional testing to meet applicable regulations. For example, the gradation of the WBC resembles coarse aggregate AASHTO #10 and that of the OBC resembles fine aggregate bituminous concrete sand type B #2. However, several quality requirements which were not assessed in this study must be met to fully satisfy these designations. Furthermore, the cuttings may meet regulated fill standards for Act 2 sites as discussed in the Introduction. Also, both the WBC and OBC maintained reasonably consistent size distributions (i.e., low variation in wt.% values) when submitted to moderate forms of attrition in a sieve shaker and during tumbling in a rotating cylinder. Because both materials maintained their size distributions, it is likely that transportation or moderate handling would lead to minimal particle size degradation. On the other hand, significant degradation could occur under more severe forms of treatment such as ultrasonic treatment. Furthermore, because of the very low carbon content of the OBC, their use in combustion applications would be inappropriate. The procedures presented in this study would be useful in future characterization of drilling well cuttings to determine potential for beneficial reuse. Results of this study serve to provide expected properties and size distributions of such cuttings. All results pertain specifically to the samples analyzed and conclusions drawn may not necessarily be true of all Marcellus drilling well cuttings.
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