By: Richard B. Wells, Consulting Geologist,
Reprinted with permission from the August 1999 issue of The National Drillers Buyers Guide
One of the hottest new gas plays in the United States (which any shallow-well driller with an idle rig should be aware of) is coal gas (methane). The wells are shallow, they need lots of them, and they are drilled with water well rigs like the Gardner Denver 1500. The idea of hundreds of new shallow wells should rekindle the old entrepreneurial spirit in any drillers heart. These fields cannot be found just anywhere, but they are in enough states to make quite a difference in the national shallow rig count. The main plays are in New Mexico, Wyoming, Kansas, Iowa, Alabama, and along the Appalachians from Pennsylvania to Virginia.
In any coal bed methane prospect, the key parameters are the thickness of the coal, the gas content and permeability of the coal. A fairly large number of pilot wells are needed before you can predict the productivity of the reservoir in terms of recoverable reserves for the average well and for the field as a whole.
Coal bed methane field development should be done according to a well-designed plan in order to maximize total gas production, field life, and profitability. First, the geology of the reservoir (and the interbedded formations) should be studied in considerable detail so that the lateral extent, coal thickness, and degree of fracturing are known. With this information, the approximate volume of the reservoir can be calculated. The gas content and temperature in the reservoir should be determined as the field is developed through an extensive coring and core analysis program, including pressure cores. Consistent and accurate well testing must be conducted, and the results stored in a database. This database can be used to map the reservoir, with the maps updated as each new well is drilled. Carbonaceous shales interbedded with the coals should also be cored and analyzed for their gas content.
Cavitation tests are run to determine how production rates can be enhanced by hydraulic fracturing. The orientation of stress fields in the reservoir should be determined to aid in predicting fracture directions, and if faulting is suspected, downhole imaging tools can be run to find their location, orientation, and permeability characteristics.
Some coal bed methane fields are divided into separate reservoir compartments by faults, coal bed pinch-outs, or permeability variations. Detailed structural and lithofacies mapping are needed to get an understanding of these reservoirs. Different reservoir compartments can have different reservoir pressures, water levels, and permeabilities.
Regional stratigraphic studies are useful for understanding sedimentary sequences, sequence boundaries and unconformities in the section, important information for predicting coal cutouts and oxidized zones. Thermal modelling of the field is needed for understanding the burial history, coal maturity, gas content, and formation pressure. Mud logs and wireline logs should be run in all the pilot holes as well as the production wells. The hydrogeology of these fields is also important, because the water strongly influences reservoir pressure, the gas saturation, and the ability to de-water the coals. Fluctuations in groundwater levels and the piezometric surface can also indicate the connectivity of coal seams.
Those are some of the things to consider when prospecting for coal bed methane gas. Now, let's look at some of the fields that have been found...San Juan Basin of New Mexico has about three thousand coal bed methane wells, producing from coals between 30 and 80 feet thick at depths between 1,600 and 3,300 feet. Gas content ranges from 300 to 500 cubic feet per ton, and well spacing is generally 320 acres, or two wells per square mile. One of the biggest operators in the basin is Meridian Oil, with more than four hundred shallow methane wells.
A new coal bed methane development project has begun in the Raton Basin, Colfax County, New Mexico by Pennzenergy and Sonat Exploration, in a leasehold covering 700,000 acres. The coals are in the Raton Formation (between 960 and 1,740 feet) and in the Vermejo Formation (1,470 to 2,500 feet). They plan to drill six hundred wells over the next five years to tap a gas reserve estimated to hold a trillion cubic feet of gas.
The coals in the Powder River Basin of Wyoming are quite different from those of the major coal bed methane basins. They have a high water content, low gas content, low ash, and high permeability. Because of these characteristics, it was thought that commercial coal bed methane production would not be possible here. However, during the past six years several shallow coal bed methane fields have been developed, proving that gas can be released from the coal matrix even where the gas content is low and permeability is high.
Coal bed methane fields in the Powder River Basin lie near the basin margins, where the coals typically lie between 250 and 1,000 feet. The wells can be drilled and completed for about $50,000 and average 100 thousand cubic feet per day or more. The wells are drilled with air, air-mist, or water, with 7-inch casing set at the top of the producing seam. Wells are completed open-hole, from a single coal seam in each well. Submersible electric pumps are used to pump the water to the surface where it is discharged on the surface. Water disposal is not a problem here, as the water quality is good enough to be used by local ranchers for tending livestock. Combined production from these wells was 28 million cubic feet per day at the end of 1997, with another 35 million cubic feet per day in wells already drilled, tested, and waiting for pipeline connections. The reservoirs range from 40 to 90 feet thick and lie at 150 to 600 feet below the grass roots.
A major research effort on Powder River Basin coals by the Gas Research Institute, has resulted in detailed guidelines for well testing and formation evaluation involving a combination of coring, openhole logging, and flow testing. The objectives are to determine the reservoir thickness, gas content, pressure and storage capacity of the reservoir, and the permeability of the natural fracture system. Fracturing with a moderate to high rate of water injection without the use of any proppants (chemicals used to improve yield,) generally improves production rates significantly. A detailed summary of the GRI study appeared in the Oil and Gas Journal for April 26, 1999.
The Forest City Basin of Kansas and Iowa has several thick Pennsylvanian coal beds that are very gassy. They formed from peat deposits in an old river delta that once crossed the area. The coal beds are about 4 feet thick and contain from 50 to 435 standard cubic feet of methane per ton. They have high sulfur and low ash content. Four exploration companies, including Duncan Energy Company, have formed a consortium to exploit the gas in northeastern Kansas, where they have leased some 250,000 acres. These wells produce from 10 to 300 thousand cubic feet per day and around two hundred barrels of water.
There are around three thousand two hundred coal bed methane wells in the Black Warrior Basin of Alabama, producing from coals between 15 and 25 feet thick at depths between 500 and 3,000 feet. Gas content ranges from 250 to 500 cubic feet per ton, and well spacing is typically 80 acres. River Gas Corporation developed a coal bed methane field northeast of Tuscaloosa with some 535 wells on a leasehold of 32,480 acres. Typical wells are drilled with a 7 and 7/8-inch bit and 6-inch air hammer to a depth of 2,200 feet for around $190,000. Median production per well is 82 thousand cubic feet per day.
The Appalachian Basin has about five hundred coal bed methane wells, producing from coals between 10 and 20 feet thick at depths between 500 and 2,800 feet. Gas content ranges from 300 to 600 cubic feet per ton, and well spacing is generally 80 acres, or eight wells per square mile.