One of the defining characteristics of coalbed-methane plays is that they refuse to behave as a group. Coal seams in one area will produce methane at commercial rates from openhole completions, while those in another area will need extensive fracture stimulations. Some make prodigious volumes of potable water; others produce formation waters that must be injected into disposal wells. What has been a common thread, however, is that commercial CBM plays are all quite shallow. Now, there's even an exception to that rule. In northwestern Colorado's Piceance Basin, Tom Brown Inc. is developing a CBM accumulation at White River Dome Field that is both deep and prolific. For the last two years, the Denver-based independent has held the unusual play close to its vest. "Up until now, we have kept the play quiet because we were still leasing," says Harry Held, senior geologist. With its position solidified, TBI is ready to highlight its success. White River Dome Field, discovered in 1896, lies in the north end of the Piceance Basin in Rio Blanco County. The structure is a dome at the surface, evident from the younger beds that dip away from the sagebrush flats covering its apex. At the turn of the 20th century, the field produced natural gas from very shallow wells in the Tertiary Wasatch, and supported a carbon black plant in nearby White River City. In the 1950s, a subsequent round of drilling rejuvenated the field. The sedimentary section down through the Upper Pennsylvanian Weber sandstone was drilled and tested by several wells, and various Cretaceous reservoirs were developed. In 1959 Northwest Pipeline built a pipeline to the field. In its next incarnation, the White River Dome structure was the target of CBM prospecting. In the late 1980s, Fuelco, a subsidiary of Public Service Co. of Colorado, purchased the field. Inspired by the CBM federal tax credits, the company and its 50% partner Texaco drilled wells to produce the coal seams that occur in the bottom 600 to 800 feet of the Williams Fork formation, a part of the Cretaceous Mesaverde group. Throughout the Piceance, this interval is called the Cameo coal, although technically the Cameo is a discreet coal seam that overlies the Rollins formation, notes Held. The results of the CBM wells were largely disappointing, however, and after the government phased out the tax credits the field lay fallow again. Tom Brown entered the picture in 1996, when it acquired a 25% interest in White River Field as part of a larger deal. The company bought 22 producing wells, of which 15 were completed in both coals and sands and seven were just sand completions. TBI was interested in the conventional sandstone reservoirs that draped across the dome, and it decided to up its interest in the properties to 100% through additional leasing and acquisitions. In 1998, the company began a development program. It drilled six wells that year, targeting Mesaverde sands. Results were mixed. The following year, it drilled just two wells, one of which had no sand. While discouraging at the time, this test fortuitously focused the company's attention on the coals. TBI completed the well in the Cameo coals and was pleasantly surprised by its strong production. After further study and additional testing, TBI concluded that the coals were actually the primary reservoirs in the field. Certainly, the coals in question exhibited some fine reservoir characteristics: the gas content of the Cameo ranged from 650 to 750 standard cubic feet per ton, a level on par with the premier CBM seams in the San Juan Basin. And, pressure build-up tests on sand-only wells versus coal-only wells showed that the permeabilities of the coal seams were 10 times that of the sand intervals. "We have perms of 0.1 to 1 millidarcy in the coals, which is pretty astounding at these depths," says Charles Hager, reservoir engineering manager. Conventional wisdom held that commercial CBM production was limited to depths above about 5,000 feet, notes Hager. The paradigm was rooted in the characteristics of San Juan Basin coals, in which permeability drops off dramatically with depth. "But, at White River Dome we are producing coals at depths from 6,500 to 7,500 feet. We think that we have the world's deepest economic coal play." Another unique aspect of the accumulation is that it also exhibits characteristics of a basin-center gas deposit. Although White River Dome lies on the flank of the basin, the bottom 1,000 to 1,100 feet of the Williams Fork is a basin-center accumulation, says Held. "Any porous and permeable sands in the interval are water-free and gas-bearing." The Cretaceous sandstones, while no longer the main objectives, are still vital contributors to the wells' rates and reserves. Full-scale development To efficiently develop both the coals and sands, TBI formed the Ant Hill Federal Unit over its area of interest, which basically encompasses the eastern half of the dome. The company has acquired 39,000 acres in the play, almost all of which it owns 100%. In 2000, it drilled nine wells, targeting both coals and sandstones. All were successful. The resulting production levels were far stronger than the company had anticipated. One of the challenges of the field is its CO2 content: although the shallow Wasatch gas can be shipped directly to the pipeline, the gas from both the coal seams and the Cretaceous sandstones requires processing. The percentage of CO2 ranges from 15% to 45%, varying with depth. At the same time, the gas is so rich in liquids that it can have a Btu (British thermal unit) content of 1,000 even at 40% CO2. The high liquids content is another uncommon twist for a CBM play. During Fuelco's tenure as operator, Western Slope Gas, another subsidiary of Public Service Co. of Colorado, built a small membrane plant that could handle about 6 million cubic feet per day. TBI's new wells quickly overwhelmed that capacity. Clearly, the field infrastructure had to be upgraded, and TBI entered into an arrangement with South Tex, a midstream company based in Texas, to construct a facility. The first phase was completed in 2000 and allowed the company to process up to 14 million cubic feet of additional gas per day. In 2001, TBI drilled 30 wells in the field, most of which targeted the coal. Included in that total were a couple of shallow Wasatch wells and a water disposal well. The new producers significantly exceeded the company's expectations, and a second expansion phase was required to raise the plant capacity again. Full field production from some 70 wells now stands at 33.5 million cubic feet per day, says Doug Harris, vice president of operations, and the second expansion is already fully loaded. "We planned our surface facilities for rates of around 1- to 1.5 million per day per well, but we have wells that can produce up to 4 million per day." TBI is not yet seeing any production declines, although due to their prolific volumes the wells are only producing at about 50% drawdown, he says. Certainly, the stimulation techniques have been crucial to the phenomenal well results. Intervals in the early wells were fractured together, and coals had a tendency to screen out. Now, TBI separates the sands and coals, treating only one reservoir type in each stage. For instance, the company uses different proppant pump schedules for coal versus sand stages, and lower pump rates in the coals. "We are seeing half-lengths of 60 to 100 feet in the coals, compared with other plays where effective half-lengths are 10 to 30 feet," says Hager. TBI has also changed its designs from huge fracs over large intervals with heavy crosslinked borate gels to smaller-stage fracs across smaller intervals with lighter, cleaner frac fluids. The company routinely performs five to six individual fracs in a well; its average completion time is about eight days per well. Another choice feature of this field is that the wells initially produce gas upon completion and don't have the long dewatering phases that are typical of most CBM producers. The Williams Fork wells begin producing water at rates of about 100 barrels per day, and may only produce a total of 50,000 barrels each. This year, TBI plans to drill 13 wells, of which nine will be development wells and four will be stepouts. The development wells are designed to keep the plant working at capacity, while the stepouts will help delineate the extent of the field. The company is also debating when to kick off yet another plant expansion, jumping capacity up to 60 million cubic feet per day. "We don't yet know how big the field is," says Held. Certain boundaries are defined: to the south, a large fault terminates the play, and to the west, the Cameo coals thin. "Given the coal thickness, gas contents and our calculated gas-in-place, we believe that we hit the current economic limit at 35 net feet of Cameo coals." In the heart of the TBI acreage, those coals reach net thicknesses of 80 feet, he adds. The downdip limit remains the mystery: at some point, the permeabilities of the coals will likely decrease. Additionally, as the CO2 content increases with depth, the percentage of hydrocarbon could limit the field as well. The perennial question of economics? Recoverable reserves are still a challenge, given the extreme complexity of the gas composition. Currently, the company estimates it will produce at least 1.5 billion cubic feet of gas per 40-acre location, the spacing that it believes is the most economic. Last year, drilling and completion costs for a typical 7,000-foot well came in at $800,000. This year, as service costs drop and it tweaks its well designs, TBI expects to shave about 20% off that per-well price. Lifting costs should decline as well, dropping from last year's 52 to 54 cents per thousand cubic feet to about 45 cents per thousand for 2002. And, given TBI's extensive acreage position, it enjoys a plethora of additional locations. However large it eventually becomes, the play is certainly a keeper. "It's a world-class coal project at a depth never heard of before," says Harris.