Gas contents at 10°C were estimated from equation (5.4) using micropore gas volumes and energy constants obtained from fitting the 20°C data. Reported and calculated gas contents are shown in figure 5–12. Gas contents based on either saturation vapor pressure, Dubinin or reduced Kirchoff, are in good agreement with reported values at low pressures. Above 400 psia, calculated gas contents fall below measured values, with the reduced Kirchoff saturation vapor pressure yielding more accurate gas contents than that of Dubinin.
Repeating the exercise for 50°C, calculated and reported gas contents are in figure 5–13. Regardless of the saturation pressure employed, plots of gas contents from the Dubinin-Radushkevich equation show unrealistic flexure at low pressures. Again, the reduced Kirchoff saturation vapor pressure relation gave more accurate results than did that from the Dubinin equation.
Fig. 5–15. Daf methane sorption isotherms—Canyon coal, Powder River Basin 47
Number of isotherms to characterize multiple seams
Coal wells are frequently completed in more than one seam, yet calculation of original gas in place, recovery factors, and reserves are overly simplified by characterizing all seams with a single isotherm. Such a simplification is valid when differences in in-situ sorption capacity are minimal due to coal seams with roughly the same maturity and little variation in coal ash content, quality, and depth.
Mavor et al. reported isotherms from samples taken from depths of 876 and 943 ft in the USM #1 well, an Appalachian Basin coal well.48 As the samples were of the same rank, low-volatile bituminous, and were from seams only 67 ft apart, it is not surprising the isotherms, displayed in figure 5–16, are similar. Use of a single isotherm to describe sorption behavior in both seams is clearly justified.
Fig. 5–16. Isotherm variation between seams—USM#1 well, Appalachian Basin, West Virginia49
Contrast this with the daf isotherms from the CO 3277-9 well in the San Juan Basin, shown in figure 5–17.50 Both samples were low-volatile bituminous in rank and differed in depth by only 39 ft, yet the daf gas contents at a reservoir pressure of 1,200 psia differ by nearly 30%.
A third example of multiseam isotherms comes from the Hamilton #3 well studied by Mavor et al.51 In addition to the isotherms discussed above, an isotherm was also measured on an uphole coal at a depth of 2,677 ft. This isotherm is plotted in figure 5–18 along with those from figure 5–14. As seen in the figure, the daf isotherms of the two seams differ little, even though the seams are separated by 203 ft.
To properly characterize the gas resource in a multiseam coal well completion, isotherms should be measured for every coal seam of interest. Timing of sorption isotherm measurements can vary from the initial exploration phase of prospect development to full-field development and depends on the degree of uncertainty with which the operator is comfortable. A good rule of thumb for condensing isotherms from multiple seams into a single average isotherm is when isotherm variation between seams is less than isotherm variation within the seams themselves.
0 100 200 300 400 500 600 700 800
0 200 400 600 800 1000 1200 1400 1600 1800 2000
gas content, scf/ton
pressure, psia Metcalfe, et al Mavor, et al
Fig. 5–18. Methane isotherms—Cedar Hill Field, San Juan Basin53
Number of isotherms to characterize a project
The number of isotherms necessary to characterize a project depends on its geologic complexity. As an example of a single-seam project, consider the Cedar Hill Field in San Juan County, New Mexico, where the basal Fruitland coal is being produced. As noted above, Metcalfe et al. reported three dmmf isotherms for this field, and all three collapsed to the same daf isotherm.54 The Hamilton #3 well, studied by Mavor et al., is also from this field.55 The Metcalfe isotherm and average Hamilton isotherm are plotted in figure 5–18. At initial reservoir pressure, 1,600 psia, gas content of the Metcalfe et al. isotherm is 15% higher than that of the Mavor et al. isotherm. Gas contents of the two isotherms are nearly equal when reservoir pressure has been halved to 800 psia. At 200 psia, current reservoir pressure of many San Juan fairway wells, gas content of the Mavor isotherm is 36% higher than that of the Metcalfe isotherm. The Cedar Hill Field has been studied extensively, and great care was taken in measuring the isotherms reported here. The wells were separated by two to three miles, and differences in the daf isotherms are ascribed to lateral variation across the coal deposit. Dry, ash-free isotherms for a given project often vary by 25% to 30%, and volumetric gas in place, recovery factor, and estimated ultimate recovery calculations should reflect this variation.
A project with multiple coals seams, complicated structure, or rank and coal quality variations will require substantially more than the five isotherms measured for the basal Fruitland coal of the San Juan Basin discussed above. Conversely, measurement of only a few isotherms from exploration wildcats probing a complicated coal deposit can provide bewildering results. To reflect the uncertainty in the gas resource of such a prospect, bounding calculations for maximum and minimum gas in place or through Monte Carlo simulations are often useful.
Number of isotherms to characterize a basin
Isotherms vary a great deal over any given basin, reflecting coal deposit heterogeneity. Attempts to characterize an entire basin with a single isotherm are as foolhardy as they are frequent. However, rank exploration plays frequently have only one isotherm available for estimation of the coal gas resource. The three examples discussed below illustrate isotherm variation between formations within a basin and across a basin.
As an example of isotherm variation between formations, consider coals of the Raton Basin, which occur primarily in the Raton and Vermejo formations. Coals in both formations were formed from deposits in floodplains and swamps following a retreating sea.56 Coals of the overlying Raton Formation are relatively thinner and less laterally extensive than the older, more thermally mature Vermejo coals. Hydrologies, and hence reservoir pressures, are distinctly different in the two formations. Currently, most commercial coal wells in the basin are completed in Vermejo coals. Generic in-situ isotherms for coals of both formations are shown in figure 5–19.57 Note that at 1,000 psia, a Vermejo coal sorbs 50% more gas than does a Raton coal, illustrating the need for separate isotherms for separate formations.
Coals of the Powder River Basin were laid down in a fluvial environment in Paleocene time and are nearly all subbituminous in rank. An average Powder River coal isotherm was calculated by Crockett and Meyer for the east-central Powder River Basin.58 Individual isotherms used to construct the average isotherm were not identified, but high and low isotherms that were “one standard deviation” off the average were also calculated.
All three isotherms are plotted in figure 5–20. The high and low isotherms differ from the average isotherm at any given pressure by roughly 30%. Although these three isotherms were developed for the eastern margin of the Powder River play, current indications are that for reservoir engineering purposes, they can be used for many seams across the basin in the absence of specific isotherms.
The majority of coal gas produced in the United States has been produced from the San Juan Basin, and many public domain sorption isotherms are available for the Fruitland coals of this basin. Eleven in-situ methane isotherms reported for San Juan Fruitland coals are collected in figure 5–21, with location and isotherm details given in table 5–3. This collection is not meant to be exhaustive but rather illustrative of the variation of Fruitland coal sorption characteristics across the basin. Coal rank and quality variations across the basin are reflected in the fourfold difference in sorption capacity of the Fruitland coal.
As a general rule, for a basin of even moderate complexity, over a dozen isotherms are required to adequately capture basinwide sorption behavior.
0 100 200 300 400 500 600
0 200 400 600 800 1000 1200 1400 1600 1800 2000
gas content, scf/ton
pressure, psia
Vermejo Raton
Fig. 5–19. Isotherm variation between Vermejo and Raton formations—Raton Basin59
0 20 40 60 80 100 120 140
0 100 200 300 400 500 600 700 800 900 1000
gas content, scf/ton
pressure, psia
High Average Low
Fig. 5–20. High, low, and average in-situ isotherms—eastern margin, Powder River Basin60
Fig. 5–21. In-situ methane isotherms—Fruitland coal, San Juan Basin
Table 5–3. Isotherm data—San Juan Basin, Fruitland coal
Seq no. Isotherm ID VL is, scf/ton pL, psia Ash, % Moisture, % Rank Temp, °F Sec T R
1a So Ute 5-7 538.7 362.2 21.00 1.91 na 108 7 32N 11W
2b COAL 783 606 na na na 120 17 32N 11W
3c Arri, et al. 762.6 365.8 18.12 2.27 mv 115 na na na
4d Cox, et al. 640.1 310 na na na na na na na
5e H&P 371.6 299 na na na 112 na na na
6f Ham # 3 611 388.5 21.71 4.90 hvA 115 30 32N 10W
7g NEBU 756.4 309 23.49 4.80 hvA 110 9 30N 7W
8h So. Ute 36-1 623.1 641.8 8.57 7.41 mv 125 36 34N 10W
9i Huerfano-top 238 823 na na na na 36 27N 10W
10j Huerfano-mid 306 823 na na na na 36 27N 10W
11k Huerfano-bot 320 823 na na na na 36 27N 10W
na = not available
aMavor, M. J., et al. 1995. bYoung, G. B. C., Kelso, B. S., and Paul, G. W. 1994. Understanding Cavity Well Performance. Paper SPE 28579. Presented at the Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, New Orleans, Louisiana, September 25–28. cArri, L. E., et al. 1992. dCox, D. O., Young, G. B. C., and Bell, M. J., 1995, “Well Testing in Coalbed Methane (CBM) Wells: An Environmental Remediation Case history,” SPE 30578, presented at the Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Dallas, Texas, October 22–25. eHarpalani, S., and Pariti, U. M. 1993. fMavor, M. J., et al. 1990. gIbid. hIbid. iClarkson, C. R., and McGovern, J. M. 2005. Optimization of coalbed-methane reservoir exploration and development strategies through integration of simulation and economics. Society of Petroleum Engineers Reservoir Evaluation and Engineering. V. 8 (no. 6/December). p. 502. jIbid. k Ibid.