Direct methods first measure gas emitted by a sample. Next, that data is used to estimate gas lost by the sample during cutting, retrieval, and surface handling prior to being hermetically sealed. Then remaining gas is measured or estimated once desorption measurements are terminated. Early workers defined three categories of gas emitted by a coal sample.3 Gas emitted by the sample during coring or drilling, the subsequent trip up hole, and on the surface prior to being sealed in a desorption canister clearly cannot be measured. This gas volume, defined as lost gas, is estimated from the initial desorbed gas volumes. Once the sample has been sealed in an airtight container, desorbed gas volumes are measured over several days, weeks, or months until the rate drops below a somewhat arbitrary threshold value. Incremental desorbed gas volumes at each step must be corrected to standard temperature and pressure (STP). Cumulative measured gas volume, corrected to STP, is termed desorbed gas. When the gas emission rate for a given sample drops below a threshold value, desorption measurements are terminated, and the canister is opened. Any remaining gas, labeled residual gas, is determined either from laboratory procedures or analytic methods.
Sample collection. Desorption samples include whole core, either conventional or wireline retrieved, sidewall cores, and cuttings. Historically, the coalbed methane industry first used conventional, whole core samples.
This was soon followed by use of drill cuttings and continuous wireline retrieval cores. Rotary sidewall core technology was adapted to cut coals, but reliable gas contents from sidewalls requires painstaking field and laboratory techniques due to the small gas volumes released. More than one type of sample is often collected on a given well. For example, cuttings collected during coring provide alternative gas contents that can be especially useful if core recovery is poor or retrieval is problematic. Regardless of sample type, acquisition of high-quality coalbed gas content measurements is enhanced by involving the desorption company as early as possible in design of a drilling, coring, and testing program.
Conventional whole cores provide the largest samples and can be cost-effective when only one or two seams are to be cored. Picking the core point is often difficult, and frequently the top few feet of the target coal are drilled up before the drilling assembly is tripped out of the hole and the core barrel is run. Pulling times of conventional cores from 2,000 ft (600 m) can easily be three hours or more, leading to large lost gas volumes.
Current practice restricts whole cores to coal seam depths of less than about 3,000 ft (900 m).
Wireline retrieved cores, also called continuous cores, provide samples of bounding strata as well as the target coal seams. As a general rule, wireline cores cut faster and have higher recoveries than conventional cores, especially in friable coals. With retrieval rates on the order of 100 to 200 ft/min (30 to 60 m/min), pulling times of wireline retrieved cores are typically much less than those of conventional cores. Current technology limits wireline retrieved cores to depths less than about 4,000 ft (1,200 m).
Sidewall cores are frequently the most cost-effective sample type for coals more than 4,000 ft (1,200 m) in depth or when multiple seams are to be tested. Rotary cores are preferable to percussion cores, which can shatter a brittle coal. Factors affecting sidewall core recovery include hole washout and filtercake buildup. Service companies routinely achieve 80% sidewall core recoveries. Sidewall cores are, of course, cut after drilling and logging. If the well is being drilled underbalanced or with air or mist, gas begins to escape as soon as the coal is penetrated. Minimizing drilling below the coal as well as logging runs to identify sidewall core points help reduce lost gas time. Careful desorption of sidewall cores utilizing special equipment can yield coalbed gas contents as reliable as those obtained from conventional or wireline retrieved cores.
Determination of coalbed gas content from drill cuttings is quicker and cheaper than whole core or sidewall core desorption. Cuttings can be collected when drilling through coal seams to deeper targets, allowing assessment of the coal gas resource for small incremental cost. Gas contents obtained from desorption of drill cuttings are routinely less than those obtained by other methods. For reasons not entirely understood, particle size affects rate of gas release, and cuttings from wells drilled with air or foam are too small to desorb. In wells drilled with mud or water, the cuttings lag rate of 30 to 100 ft/min (10 to 30 m/min) limits use of cuttings desorption to coal depths less than about 4,000 ft (1,200 m).
Any cuttings sample contains a variety of contaminants, with drilling solids and additives (such as lost circulation material) as well as uphole cavings being among the most common. Once desorption is complete, rinsing the cuttings sample with distilled water containing 2% KCl removes some contaminants. Separation of
coal cuttings from other cuttings in the sample was initially done with heavy brines. If this method of separation is employed, density of the heavy brine should be reported and compared against the bulk density used to define pay cutoff. A typical brine density for float/sink separation is 1.75 g/cm3, while many operators use a pay cutoff of 2.0 g/cm3. Such a practice will minimize coal mass and maximize coalbed gas content, and it can lead to erroneously high reserves when combined with a 2.0 g/cm3 pay cutoff on wireline logs. Heavy brines often compromise the coal cuttings for further laboratory testing. Current practice is to determine ash content, as is done in proximate analyses, and report the cuttings gas content on a dry, ash-free (daf) basis. Note that contamination of a cuttings sample with cavings from uphole coals is virtually undetectable and could result in computed gas contents lower than actual.
Pressure cores have sometimes been employed to determine coalbed gas content as they virtually eliminate any lost gas. However, they are expensive and result in a single sample over the cored interval, eliminating the redundancy of multiple samples, which is the most effective strategy against the inevitable gas leaks. While the single gas content from a pressure core characterizes the cored interval, that single in-situ gas content cannot be correlated with bulk density, precluding determination of gas content in offset wells with density logs.
Core and cuttings desorption sample canisters range from 6 in. to 3 ft in length, with the 1 ft size being by far the most popular, probably due to weight and tipping considerations in the field and laboratory. The Gas Research Institute (GRI) gives a more detailed discussion of coal sampling techniques (chapter 7) and representative collection protocols (chapter 8).4 The GRI summary of desorption sampling techniques is reproduced here as table 4–1.
Table 4–1. Summary of desorption sampling techniques
Recovery technique Advantages Disadvantages
Pressure Coring • Minimized lost gas volume.
• Direct measurement of producible gas from the cored interval.
• Minimized contamination by atmospheric gases.
• Evolution of gas species can be measured for the overall cored section.
• Intact, large-diameter “preserved” core samples are available for other flow or mechanical properties tests.
• Geological evaluation (cleat mapping, petrology, etc.) is possible.
• Most expensive option. Potential tool operation problems (sealing).
• Net gas content for the entire core barrel is measured, rather than for discrete zones. Do not know precisely where gas is coming from (can be approximated indirectly from proximate analysis).
• Cannot infer relationships between gas content and ash content for logging calibration (can be estimated indirectly from proximate analysis).
• Requires extensive field labor.
• Drilling fluid may impact desorption.
• Picking core point may be difficult.
Full Diameter Conventional Core (Usually 3.5 in.
diameter)
• Less expensive than pressure coring.
• Nominally intact, large-diameter core samples are available for other tests (i.e., permeability, mechanical properties, formation damage, etc.)
• Potential for lost gas is less than for drill cuttings.
• Possible to evaluate geology (cleat mapping, petrology, etc.).
• Least amount of contamination from drilling fluids.
• Possible to evaluate desorption characteristics of discrete lithologies.
• Potential for lost gas is greater than for pressure core.
• Potential for lost gas may be greater than for drilled or shot sidewalls or wireline core because of greater trip time.
• Probably more expensive than slimhole coring.
• Usually more expensive than sidewalls or drill cuttings.
• Picking core point may be difficult (may unintentionally drill through the zone of interest).
Slimhole or Wireline Recovery
(Continuous Coring) (e.g., NX core size)
• If recovered by wireline, lost gas time can be reduced significantly.
• May be less expensive than conventional coring.
• Identifying lithologic boundaries is easier (i.e., selection of core point is improved).
• Cleat mapping and petrology may be possible.
• Some additional testing (e.g., well testing) may be possible.
• Coring rigs may be possible.
• Possible to evaluate the desorption characteristics of discrete lithologies.
• Smaller diameter may lead to accelerated lost gas evolution.
• Slimhole logging tools may be required and may not be readily available. However, this is not true when using new generation oilfield continuous coring methods.
• Though it does not affect desorption, casing, completions, and production options may be seriously restricted, particularly if artificial lift is anticipated. However, this is not true when using new generation oilfield continuous coring methods.
• Complementary laboratory testing (i.e., permeability, mechanical properties) may be possible. However, tests such as these on coal should be performed on the largest samples possible.
Table 4–1. Cont.
Recovery Technique Advantages Disadvantages
Drilled Sidewall Core (less than 1 in. diameter)
• If run after logging, sampling zones can be high graded.
• Technology has rapidly improved to allow better sample recovery. However, operational difficulties can still be encountered
because of friability and cleating.
• Minimized lost gas due to reduced trip time.
• Reduced labor.
• Intact sampling may be difficult.
• Potential for “accelerated” lost gas evolution due to sample size.
• Small volume samples.
Shot Sidewall Core
(not recommended) • Relatively inexpensive.
• If run after logging, sampling zones can be high graded.
• Reduced labor in the field.
• May give significant underestimation of gas content values.
• Perforation gas and debris can contaminate the samples.
• Samples are completely disaggregated, with frequently poor recovery.
• Most other measurements (other than proximate and ultimate) are not possible.
• Large surface area accelerates the evolution of lost gas, despite reduced recovery time.
• Because of its many disadvantages, this method is not recommended.
Drill Cuttings • Least expensive.
• Can be considered as a viable methodology when calibrated with other desorption measurements (i.e., whole core, etc.).
• Gas content data typically are available in a shorter time period.
• May give significant underestimation of gas content values.
• Potential for substantial lost gas volume due to sample size and large exposed surface area.
• Great potential for contamination with adjacent formation material, drilling material, and moisture.
• Precise depth assessment is difficult.
• Selective concentration of some seam components.
• Potential plugging of canister valves.
Source: Gas Research Institute. 1995.
Measurement of desorbed gas volumes. Fresh coal samples emit gas rapidly. Whether the sample is whole core, sidewall core, or drill cuttings, determination of coalbed gas content entails capturing and measuring as much emitted gas as possible.
Regardless of sample type, it is important to promptly retrieve the samples to the surface, to minimize surface handling time, and to seal the samples in airtight canisters as quickly as possible. Typical desorption canisters for whole and sidewall core samples are shown in figure 4–1.5
Note the large canister for whole core samples can also be employed for cuttings desorption. Ideally, a sample should nearly fill the desorption container, leaving little headspace. Ulery and Hyman reported gas content errors on the order of 30% when headspace and desorbed gas volumes were roughly equal but minimal error when headspace volume was small compared to total desorbed gas volume.7 Service companies commonly minimize headspace error by filling void space in a desorption canister with produced formation water, plastic packing peanuts, or sand. Cuttings, especially those from gassy coals, can be lifted into the canister exhaust valve when it is opened for a desorption measurement. Sufficient cuttings can accumulate in the valve to form a pressure seal, necessitating opening of the canister to clear the valve. The attendant loss of early-time data often taints lost gas estimates. To prevent cuttings movement into the valve, service companies use steel wool or fine screens on top of the cuttings.
Rate of gas release depends on coal sample temperature. Mavor et al. reported gas contents from ambient temperature desorption tests were one-third lower than those obtained from reservoir temperature desorption tests.8 The importance of desorbing whole core samples at reservoir temperature was discussed by the GRI.9 During retrieval and surface handling, the core cools. Long pulling times and/or slow surface operations exacerbate temperature loss. Once a core sample is sealed in a desorption canister and placed in a temperature bath, it begins to return to reservoir temperature. Time required to restore thermal equilibrium is approximately equal to cooling time.10 Consequently, service companies now routinely desorb whole core samples at reservoir temperature using preheated canisters to minimize temperature recovery time. In contrast, current practice is to desorb drill cuttings at mud temperature, while no consensus has yet evolved for sidewall cores.
Fig. 4–1. Whole core and sidewall core desorption canisters6
As shown in figure 4–2a, desorbed gas volumes were first measured by periodically venting desorbed gas through a plastic tube into an inverted, water-filled graduated cylinder or burette. Subsequent workers refined the USBM method by adjusting the water-filled cylinder to align the fluid level with midpoint of the canister (fig. 4–2b), thus minimizing the pressure head from the water column on the sample. Additional improvements rapidly followed, and a current desorption apparatus is shown in figure 4–2c. Many service companies have now dispensed with the water-filled burettes in favor of continuous mass or flow rate meters maintained at atmospheric pressure.
Some coals contain significant amounts of CO2, which is readily soluble in water. The Standards Association of Australia recommended use of an acidified brine in the desorption apparatus to mitigate CO2 dissolution.12 However, acidified brine may simply prevent formation of carbonate ions, with little impact on CO2 dissolution.13 Once sufficient data have been acquired to estimate lost gas, many service companies will heat samples to 50°C (122°F) in order to accelerate gas release and shorten test time. This practice results in a second hump in the gas desorption plot, and the data cannot be used in determination of lost gas.
Desorbed gas volumes are measured as a function of time, with frequent sampling early in the desorption test and decreased sampling at late times. To provide clear and sufficient data, desorbed gas volumes should be measured at least every 3 to 5 min for the first hour of desorption. Regardless of the method employed to analyze the desorption data, early-time data is critical for estimation of lost gas. As the rate of gas release slows, the frequency of desorbed gas measurements also relaxes. After the first few hours of desorption, sample frequency drops off to every 15 min. After a few more hours, gas readings are taken once per hour. In the latter part of the test, desorbed gas volumes are typically measured once per day. Experience plays a large part in knowing when to take another gas reading. When in doubt, a gas measurement should be taken. Sparse data collection has ruined many desorption tests. A typical desorption data set (courtesy of HWA) from a wireline retrieved whole
Table 4–2. Desorption data set (courtesy of HWA) ABC Desorption & Isotherm Company
123 Main Street Denver, Colorado 80122 Phone: 303-123-4567 Website:
*Coalbed Methane Desorption Specialists*
Coalbed Methane Desorption Report
Company: Coal Gas Forever, Inc Sample Type: Wireline Core
Well Name: Bad Idea # 13 Geologist: Col. E. Drake
Location: SWNW, SEC 37, T1N-R185W, Sorption County, NJ Surface Elevation: 1234
Canister Number: CGF-BI#13-122 Coalbed Encountered: 8/24/2001 20:34:00
Seam Identification: Old Swamp Time Coal Started Out of Hole: 8/24/2001 21:28:00
Top of Test Interval: 1257.6 feet Time Coal Reached Reservoir Pressure: 8/24/2001 21:28:31
Bottom of Test Interval: 1259.3 feet Coal Half Way to Surface: 8/24/2001 21:33:16
Interval Thickness: 1.7 feet Coal Reached Surface: 8/24/2001 21:38:00
Bulk Sample Weight: 2177.0 grams Coal Sealed in Canister: 8/24/2001 21:54:00
Air-Dried Weight of Coal: 1959.3 grams DAF Weight of Coal: 1830.8 grams
DMMF Weight of Coal: 1791.8 grams Gas Content Results:
Moisture (Residual) 1.97% Polynomial Fit
for Lost Gas Linear Fit for Lost Gas
Moisture (Total) 11.77% Time Lost: 20.7 minutes 20.7 minutes
Ash Content (As Received) 4.13% Desorbed Gas: 15931 cc, STP 15931 cc, STP
Sulfur Content (As Received) 2.66% Lost Gas: 1904 cc, STP 1647 cc, STP
Bit Size: 5.50 inches Projected Residual Gas: 362.0 cc, STP 362.0 cc, STP
Core Diameter: 3.00 inches Measured Residual Gas: na na
Drilling Medium: Mud Total Gas: 18197 cc, STP 17940 cc, STP
Mud Weight: 8.800 lb/gal Total Gas Volume/Bulk Coal Weight: 8.3587 cc/gram 8.2408 cc/gram
Canister Size: 2.0 feet GAS CONTENT-Bulk Sample - des + lost 262.5 scf/ton 258.7 scf/ton
Canister Headspace Volume: 3488.4 cc GAS CONTENT-Bulk - des + lost + proj resid 267.8 scf/ton 264.0 scf/ton Depth Core at Reservoir Press: 1193 feet GAS CONTENT-Bulk - des + lost + meas resid na na
Flowline Temperature: 25.6 deg C GAS CONTENT-Dry Ash-free 318.4 scf/ton 313.9 scf/ton
Outdoor Temperature: 16.7 deg C GAS CONTENT-Dry Mineral Matter-free 325.4 scf/ton 320.8 scf/ton
Reservoir Temperature: 35.0 deg C
Lithology: Coal, blk, vit, brit, hd, vert fracs, g face cleats, occ butt cleats
Comments:
Table 4–2. cont.
Reading
Number Date Time (hr : min)Minutes
Elapsed
Desorbed Gas Ambient Cond. (cc)
DESORPTION READINGS:
Desorbed Gas, STP (HS Corrected, cc) SQRT
Time
Total Gas Desorbed,
(STP, cc) Ambient
Temp
(deg C) Comments
Headspace Correction, STP (cc)
Canister Temp (deg C)
Ambient Pressure
(in. Hg)
1 8/24/2001 22 : 5 32 478 0.0 35.4 23.64 364 5.6340 364 26.8
2 8/24/2001 22 : 14 41 342 –1.7 35.2 23.64 262 6.3829 625 27.1 blue flame
3 8/24/2001 22 : 24 51 346 –0.3 34.9 23.62 263 7.1233 889 26.7
4 8/24/2001 22 : 34 61 286 –1.7 34.7 23.62 219 7.7937 1108 26.6
5 8/24/2001 22 : 44 71 309 –0.9 34.6 23.62 236 8.4108 1344 26.4
6 8/24/2001 22 : 54 81 270 5.5 35.5 23.64 200 8.9856 1544 26.2
7 8/24/2001 23 : 4 91 244 –0.9 35.4 23.64 187 9.5258 1731 26.0
8 8/24/2001 23 : 14 101 245 –0.9 35.3 23.64 188 10.0370 1919 26.0
9 8/24/2001 23 : 24 111 242 –1.7 35.1 23.64 186 10.5234 2105 25.9
10 8/24/2001 23 : 34 121 206 –1.7 34.9 23.64 159 10.9883 2264 25.8
11 8/24/2001 23 : 44 131 211 –1.7 34.7 23.64 163 11.4342 2427 25.8
12 8/24/2001 23 : 54 141 220 –0.9 34.6 23.64 169 11.8635 2596 25.7
13 8/25/2001 0 : 4 151 188 6.1 35.3 23.64 137 12.2777 2733 25.7
14 8/25/2001 0 : 14 161 190 0.9 35.4 23.64 144 12.6784 2877 25.6
15 8/25/2001 0 : 24 171 161 0.0 35.4 23.64 123 13.0668 3000 25.6
16 8/25/2001 0 : 34 181 180 –1.7 35.2 23.64 139 13.4440 3139 25.6
17 8/25/2001 0 : 44 191 148 –1.7 35.0 23.64 115 13.8109 3254 25.7
18 8/25/2001 0 : 54 201 155 –1.7 34.8 23.64 120 14.1683 3374 25.7
24 8/25/2001 2 : 4 271 121 –1.7 34.7 23.64 94 16.4542 4118 25.2
25 8/25/2001 2 : 44 311 484 6.1 35.4 23.64 364 17.6279 4483 24.9 Gas sample A, reading estimated
26 8/25/2001 3 : 4 331 199 –4.6 35.0 23.65 157 18.1863 4639 25.0
73 8/26/2001 21 : 1 2,848 157 1.2 35.4 23.66 119 53.3642 10769 24.2
74 8/27/2001 0 : 46 3,073 285 –5.4 34.9 23.67 223 55.4323 10992 25.6 Gas sample B, reading estimated
75 8/27/2001 3 : 36 3,243 206 6.1 35.6 23.67 152 56.9451 11144 25.3
78 8/27/2001 12 : 45 3,792 146 2.5 35.4 23.66 109 61.5771 11557 24.9
79 8/27/2001 21 : 46 4,333 247 –138.5 34.9 24.86 335 65.8236 11892 27.8 Move to Denver 80 8/28/2001 10 : 34 5,101 390 –8.2 34.0 24.86 323 71.4195 12215 24.4
86 8/31/2001 9 : 38 9,365 241 0.7 34.7 24.88 194 96.7716 13575 24.4
87 9/1/2001 14 : 30 11,097 310 12.9 35.0 24.79 236 105.3411 13811 24.5 Gas sample C, reading estimated 88 9/2/2001 16 : 18 12,645 305 –2.5 35.1 24.82 245 112.4488 14056 28.4
89 9/3/2001 9 : 49 13,696 152 –20.8 34.3 24.94 143 117.0288 14199 25.5 90 9/4/2001 11 : 26 15,233 210 0.5 34.6 24.96 169 123.4210 14368 25.6 91 9/5/2001 11 : 4 16,651 195 –10.5 32.1 24.85 167 129.0378 14536 24.8 92 9/6/2001 13 : 30 18,237 180 49.0 34.1 24.58 95 135.0435 14630 24.3 93 9/7/2001 10 : 21 19,488 115 –9.0 34.6 24.70 101 139.5985 14731 24.5 94 9/8/2001 15 : 5 21,212 130 –25.4 34.8 24.94 131 145.6425 14862 24.2 95 9/9/2001 16 : 16 22,723 125 4.5 34.8 24.90 96 150.7406 14958 25.1 96 9/10/2001 11 : 15 23,862 120 –16.0 33.8 24.96 114 154.4725 15072 23.2 97 9/11/2001 16 : 59 25,646 118 0.6 33.5 24.93 95 160.1429 15166 24.0 98 9/12/2001 17 : 18 27,105 117 9.1 34.0 24.89 85 164.6352 15252 24.0 99 9/13/2001 11 : 17 28,184 80 3.7 34.4 24.89 61 167.8801 15313 23.6 100 9/15/2001 16 : 46 31,393 108 4.8 34.8 24.88 82 177.1800 15395 25.4 101 9/17/2001 10 : 31 33,898 92 –4.1 34.6 24.90 78 184.1134 15473 24.4 102 9/19/2001 10 : 43 36,790 50 –1.4 34.2 24.88 42 191.8065 15515 23.8
103 9/27/2001 11 : 52 48,379 200 –1.1 34.2 24.89 163 219.9517 15678 24.5 Gas sample D, reading estimated 104 9/29/2001 14 : 25 51,412 90 13.6 35.8 24.90 59 226.7416 15737 23.4
105 10/3/2001 12 : 23 57,050 120 7.0 35.7 24.83 90 238.8509 15827 23.3 106 10/8/2001 15 : 24 64,431 70 11.6 35.1 24.68 45 253.8321 15872 23.1 107 10/12/2001 11 : 42 69,969 50 –19.0 35.0 24.84 59 264.5161 15931 23.8
Analysis of desorption data. Raw desorbed gas volumes from each step must be corrected to standard temperature and pressure (STP) conditions. Note that English STP conditions are defined as 60°F and 14.7 or 14.65 psia, whereas metric STP conditions are 0°C and 101 kPaa. For each and every data point, it is important to record ambient temperature and atmospheric pressure to allow correction to STP. As desorbed gas typically contains some water vapor, especially early in the test, most service companies also correct measured gas volumes for the presence of water vapor. Note that at 100°F, vapor pressure of water is on the order of 1 psia, making a saturated gas mixture at 14.7 psia approximately 7% water vapor. If the samples are being desorbed at reservoir temperature, canister temperature should also be reported. Corrected incremental volumes from each step are summed to provide desorbed gas as a function of time with the final, cumulative value being termed desorbed gas.
Inspection of raw desorption data in table 4–2 indicates the coal seam was penetrated about 8:30 p.m., and the core was off bottom approximately an hour later. Trip time was 10 min. Surface operations (removing the core from the barrel, inspecting it to select samples, breaking the core into samples of the appropriate lengths, and canistering them) required an additional 16 min. Table 4–2 contains desorbed gas volumes corrected to STP, as well as coalbed gas contents calculated from the raw desorption data. Cumulative desorbed gas volume, corrected to STP, is plotted in figure 4–3.
Fig. 4–3. Cumulative desorbed gas—sample CGF-BI#13-122
Coal gas is typically methane, with minor amounts of nitrogen, CO2, and heavier hydrocarbons. The average coal gas composition is 93% methane, 3% each of nitrogen and CO2, and 1% heavier hydrocarbons.14 Gas composition varies during both sample desorption and reservoir depletion. Efforts to relate desorbed gas composition to in-situ sorbed gas composition have been problematic. Variation in gas composition during depletion was first studied by Ulery and Hyman.15 Analysis of gas sampled at each desorption step showed nitrogen came off early in the test, while CO2 and heavy hydrocarbons were released late in the test. Early workers, unaware of such compositional variations, often assumed an unchanging gas composition and calculated in-situ gas volume based on one or two desorbed gas analyses. Ulery and Hyman utilized gas