EXPLORATION FOP COALBED METHANE GAINS MOMENTUM IN UINTA BASIN

Robert W. Gloyn, Steven N. Sommer
Utah Geological Survey
Salt Lake City

Exploration and development of coalbed methane has occurred in several U.S. basins since the late 1970s. The industry is now recognizing the tremendous potential of Utah, which recently joined the ranks of coalbed methane producing states.

Utah's first commercially viable coalbed methane wells were completed by Cockrell Oil Corp. in 1990 in the Book Cliffs coal field in the southwestern Uinta basin (Fig. 1). The wells are producing at higher rates than comparable wells in the Black Warrior basin of Alabama and Piceance basin of Colorado.

A development program is planned, and at least three other companies are exploring for coalbed methane in the surrounding area.

Estimates have been revised by the Utah Geological Survey for the coalbed methane potential of the southern Uinta basin. They are 8 tcf to more than 10 tcf, significantly higher than the earlier estimates of 0.8-4.6 tcf.

COCKRELL'S DISCOVERY

Cockrell recently began producing coalbed methane from five wells in 11- and 14-12s-10e about 12 miles north of Price.

Coalbeds were dewatered 9 months before gas began flowing at rates of 57-118 Mcfd/well 1 (Fig. 2). Production is from the Castlegate coals in the middle part of the Cretaceous Blackhawk formation at 4,200-4,400 ft 2 (Fig. 3).

These wells have been producing for more than a year. Through July 1992 the wells had produced 201 MMcf of gas and 1. II million bbl of water (Fig. 2). More importantly, the daily gas production has been increasing and the daily water production has been decreasing.

For example, average well production in May 1992 was 121 Mcfd of gas and 318 b/d of water, compared with 92 Mcfd and 356 b/d seven months earlier.

Individual well production has ranged from 58-179 Mcfd, with the 4 and 5 Shimmin Trust wells being the most and most consistent producers. The gas composition is 89% methane, 1% ethane, and 10% Co,.3

This is a relatively high CO, content for natural gas, but a number of gas fields in the area such as Farnham Dome and the Gordon Creek fields also have high CO2 contents.

The produced water has total dissolved solids (TDS) of 5,489 mg/l and an average pH of 8.0. 3 The water is moderately saline and chemically mostly bicarbonate. It is thought to represent connate water trapped in the coal during coalification. 3

The project is currently shut-in awaiting completion of a pipeline. The water will be reinjected or possibly discharged into local stream drainages after reducing solids and salinity through reverse osmosis filtration.

The Cockrell wells are producing gas at an average flow rate greater than similar wells in the Black Warrior and Piceance basins, two major U.S. coalbed producing basins (Fig. 4).

Using a four well comparison, the Utah wells produced 12.6 MMcf during the first month of gas production compared with 3.2 MMcf and 15.4 MMcf for wells in the Black Warrior and Piceance basins, respectively.

Production from the Utah wells has increased with time, and the four Utah wells are now producing 15.7 MMcf/month, which is larger than the four-well groupings for each of the other two basins.

DRILLING PROGRAM

Due to the success of its early wells, Cockrell announced plans for a large, aggressive drilling program covering 21,450 acres in 12s-10e and 12s-11e.3 Cockrell later sold its interest to Pacific Gas & Electric Resources (PG&E), which will continue with the announced drilling program.

Cockrell later sold its interest to Pacific Gas & Electric Resources (PG&E), which initially planned to continue the announced drilling program. PG&E drilled and cased an additional 21 holes but does not plan to drill any additional holes barring unexpectedly high gas prices or a reinstatement of federal tax credits.

Wells were drilled to depths of 3,900-5,800 ft on 160 acre spacing in seven adjacent sections in 12s-10e and tested multiple coal seams in the Blackhawk formation. Well completion and field facility installation are scheduled to start in mid-1993 with marketable gas sales starting in late 1993 or early 1994. A 12 mile, 8 in. gas sales line will be constructed to connect the wells to an existing Questar pipeline.

OTHER EFFORTS

Anadarko Petroleum Corp., Houston, is also testing the coalbed methane potential of the Blackhawk formation to the east and west of PG&E (Fig. 1).

To the west, one well has been completed at 4,000 ft and four others are planned in 12s-9e. To the east, an additional well is planned in 5-12s-12e. Cockrell recently announced plans to test the Blackhawk formation in 25- 12s-7e and 30-12s-8e.

Two companies are testing the coalbed methane potential of the slightly older Cretaceous Ferron sandstone member of the Mancos shale (Fig. 1).

West of Cockrell's original discovery, PG&E has staked a location in 2-12s-7e. This location is about 14 miles north of Clear Creek gas field, which has produced more than 133 bcf of gas from sandstone reservoirs in the Ferron sandstone. The source of this gas is thought to be coals within the Ferron sandstone. 5

Three miles southwest of Price, River Gas Co. of Utah, a subsidiary of River Gas of Alabama, is exploring the northern Emery coal field and has drilled a well in the Ferron sandstone in 25-14s-9e. This well produced more than 2 MMcfd of gas during preliminary tests. 6

Four more wells have been completed and 14 are in progress of an estimated total of 20-30 wells. The wells are being drilled in 25-, 26-, and 36-14s-9e and 19-, 30-, and 31-14s-10e, with projected depths of 2,000 ft.

The proposed wells are within 21/2 miles of a shut-in Ferron gas discovery made by Texaco in 1987 . 2

UGS COALBED DATA

The Utah Geological Survey has maintained an interest in coalbed methane for many years, particularly the coalbed methane potential of the Book Cliffs area (Fig. 5).

Since 1979, UGS has provided a field gas desorption service to exploration companies in the coal fields. As a result, the UGS database now contains 332 gas desorption analyses for seven coal fields. 7 8

Most of the gas desorption work has been done on coal from Book Cliffs field. The methane content of 14 individual coal seams in the Blackhawk formation has been evaluated using 113 separate analyses from drill core.

Methane desorption values (lost and desorbed gas) range from 0 to 11.0 cc/gm or 352 cu ft/ton with the highest value from the Kennilworth coal seam at 3,177 ft.7 8 Although individual seam data are limited and generally restricted to within five miles of the outcrop, gas values show three interesting relationships:

1. Recoverable gas content (lost and desorbed) was compared to total gas content (lost, desorbed, and residual) for 75 samples. A significant change was found in the ratio of recoverable gas to total gas content at a total methane content of 4.8 cc/ (equivalent to 154 cu ft/ton).

   Above the 4.8 cc/g value, 90% of the total methane could be recovered under ideal conditions. Below this 4.8 cc/g value, only 50% or less of the gas could be recovered (Fig. 6).

2. Based on samples for individual seams, methane values generally improve away from the outcrop and with depth. Methane contour lines commonly but not invariably parallel the outcrop pattern, and methane values double to triple every 2-3 miles away from the outcrop (Fig. 7). We suspect the increase eventually flattens out but do not know at what methane content or depth that might occur.

3. Plots of gas content vs. depth for individual seams fall into two groups (Fig. 8). Although both groups show increasing gas content with depth, for one group this increase is much greater. For this group, gas content increases rapidly with depth (0.313 cc/g or 10.0 cu ft/ton/ 100 ft).

There is a fairly good correlation between gas content and depth for individual seams in this group, and the slopes of the gas content vs. depth plots for the four beds are nearly the same. This group contains Subseam 1, Castlegate B, Castlegate C, and Kennilworth coal beds (Fig. 3).

The gas content for the second group increases much less rapidly with depth (0. 136 cc/g or 4.4 cu f t/ ton/100 ft). This group has significantly more scatter in the data points and the correlation between gas content and depth is much poorer.

As with the first group, the slopes of the gas content vs. depth plots for three of the four beds within this group are roughly parallel. This group contains the Castlegate D, Gilson, Rock Canyon, and Sunnyside coal beds (Fig. 3).

The explanation for the above relationships is not yet known, but the "rapid increase beds" all occur in the lower part of the formation.

The UGS has recalculated potential methane resources for the four Uinta basin coal fields (Emery, Wasatch Plateau, Book Cliffs, and Sego coal fields), where gas desorption values are available (Fig. 5). Resources were calculated by extrapolating known coal resources to a depth of 9,000 ft and multiplying this value by an average or projected gas content determined or expected at depths of 1,500-3,000 ft.

The projected total coal resources ranged from 1.5-3 times the published coal resources" 1 depending on local geology or expected limits of the coal field.

The "average" gas contents were estimated using actual values from the deeper tests, or where there was good gas content vs. depth correlation, by using the extrapolated value at 1,500 or 3,000 ft. Deeper projected gas values were not used because we did not feel confident extrapolating the plots more than twice the depth of the actual data control.

The potential source for the four fields is estimated at 8.3-10.6 tcf and the resource for other minor fields at 0.50.7 tcf (Table 1). These figures are significantly higher than those previously estimated by Adams and Kirr 12 of 0.8-4.6 tcf.

The new estimates are conservative since they do not include gas contained in seams less than 4 ft thick and use average gas values at 1,500, 2,000, or 3,000 ft rather than an average from 4,500 ft which would be gassier.

In summary, Utah and particularly the Uinta basin has substantial coalbed methane resources. Exploration and development of these resources have only just begun, and results to date are very encouraging and point to a promising future.

ACKNOWLEDGMENTS

This article was an outgrowth of research on coalbed methane in Utah prepared for the "Atlas of Major Rocky Mountain Gas Reservoirs" funded by the Gas Research Institute, GRI Contract No. 5091-212-2120. The authors thank M. Lee Allison, David Tabet, and Thomas C. Chidsey Jr. of the Utah Geological Survey for their careful review of the manuscript and their constructive comments.

REFERENCES

1. Utah Division of Oil, Gas & Mining, 1992, 1991, and 1990 production reports, Utah Department of Natural Resources.

2. Petroleum Information, Rocky Mountain Region Report, Four Corners edition, Utah report, Vol. 65, No. 40, 1992, p. 4.

3. Bureau of Land Management, Environmental impact statement: Castlegate coalbed methane project, Carbon County, Utah, 1991, 162 p.

4. Petroleum Information Rocky Mountain Region Report: Four Corners edition, Utah report, Vol. 66, No. 68, 1993, p. 4.

5. Walton, P.T., Wasatch Plateau gas fields, Utah, AAPG Bull., Vol. 39, No. 4, 1955, pp. 385-421.

6. Lyle, Don, First well in Utah coal-gas program strikes pay on 92,000 acre Texaco farmout to River Gas, Western Oil World, Vol. 47, No. 8, 1991, p. 8.

7. Doelling, H.H., Smith, A.D., and Davis, F.D., Coal studies, methane content of Utah coals: Utah Geological Survey Special Studies 49, 1979, 43 p.

8. Smith, A.D., Methane content of Utah coals-progress report 1979-1980, Utah Geological & Mineral Survey Open File Report 28, 1981, 9 p.

9. Keith, A.C., Hand, J.S., and Smith, A.D., Coalbed methane resource map, Castlegate A bed, Book Cliffs coal field, Utah Geological Survey Open File Report 176A, 1990.

10. Doelling, H.H., Central Utah coal fields: Sevier-Sanpete, Wasatch Plateau, Book Cliffs, and Emery: Utah Geological & Mineralogical Survey Monograph Series No. 3, 1972, 571 p.

11. Doelling, H.H., and Graham, R.L., Eastern and northern Utah coal fields: Vernal, Henry Mountains, Sego, LaSal-San Juan, Tabby Mountain, Coalville, Henrys Fork, Goose Creek, and Lost Creek: Utah Geological & Mineralogical Survey Monograph Series No.2, 1972, 411 p.

12. Adams, A.M., and Kirr, J.N., Geologic overview, coal deposits, and potential for methane recovery from coal beds of the Uinta basin - Utah and Colorado, in Rightmire, C.T., Eddy, G.E., and Kirr, J.N., eds., Coalbed methane resources of the U.S., AAPG Studies in Geology Series 17, 1994, pp. 253-269.

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