The present invention is directed to methods of increasing the rate of production of methane from a subterranean coal seam, and more particularly, to such methods that use the injection and production of a gas which causes the coal to swell and shrink near the wellbore.
2. SETTING OF THE INVENTION
Subterranean coal seams contain substantial quantities of natural gas, primarily in the form of methane. The methane is sorbed onto the coal and various techniques have been developed to enhance the production of the methane from the coal seam. These various techniques all attempt to increase the near wellbore permeability of the coal, which will permit an increase in the rate of production of methane from the coal seam. One technique is to hydraulically fracture the coal by the injection of liquids or gels with proppant into the coal seam. Although hydraulic fracturing of coal seams is most often effective in increasing the near wellbore permeability of the coal, it is not always economical if the thickness of the coal seam is thin, e.g., less than about five feet. Furthermore, hydraulic fracturing of the coal is not environmentally desirable when there is an active aquifer immediately adjacent to the coal seam because the created fractures may extend into the aquifer which will then permit unwanted water to invade the coal seam and the wellbore. Further, some laboratory evidence suggests that fracturing fluids can lead to long term loss in coal permeability due to sorption of the fracturing fluids in the coal matrix causing swelling, and due to the plugging of the coal cleat or natural fracture system by unrecovered fracturing fluids.
Another technique to stimulate coalbed methane production from a wellbore is to inject a gas, such as air, ammonia or carbon dioxide, into the coal seam to fracture the coal seam. This technique has been utilized primarily to degassify coal mines for safety reasons. U.S. Pat. No. 3,384,416 discloses such a technique where a refrigerant fluid with proppant is injected into the coal seam to fracture the coal. The injected refrigerant fluid and methane are permitted to escape from a borehole under its own pressure or the fluid and methane may be removed with the help of pumps.
U.S. Pat. No. 4,083,395 discloses a technique for recovering methane from a coal seam where a carbon dioxide-containing fluid is introduced into the coal deposit through an injection well and held therein for a period sufficient to enable a substantial amount of methane to be desorbed from the surfaces of the coal deposit Following the hold period, the injected carbon dioxide-containing fluid and desorbed methane are recovered through a recovery well or wells spaced from the injection well. The process is repeated until sufficient methane has been removed to enable safe mining of the coal deposit.