Illinois Basin Coalbed Gas: Is there a play?
Moore, Thomas R., Demir, Ilham, and Morse, David G.
Illinois State Geological Survey
Coalbed gas plays often follow a pattern from scattered reconnaissance wells to minimal pilot tests, often dragging on for years. Then, a threshold is crossed. A technological advance, a fortuitous break, enough wells drilled, or a determined operator finally leads to a success. Then, the number of wells drilled will increase dramatically and the play blossoms. Is the Illinois Basin coalbed gas play at such a point? Drilling has been accelerating recently.
Early investigations suggested that the Illinois Basin suffered from thin, low gas content, tight coals. Recent mapping, coring, and testing have shown that view to be pessimistic, however. The average aggregate thickness of seams >1.5 ft. is more than 15 ft., and multiple seams are consistently 3–5 ft. thick over wide areas. Measured gas contents typically range from 60–115 scf/t, and as high as 175 scf/t (dmmf). Recent pressure transient tests indicate permeability from 3 to 200 md. Minor thermogenic and dominant secondary biogenic methane, and appreciable nitrogen fractions are present in the sorbed gas, suggesting a complex geologic history.
The “right” stimulation design for these coals has not yet been found. Most wells have used off-the-shelf stimulations. Air drilling, nitrogen fracs, radial pulse fracs, or other innovative techniques have yet to be tested. Horizontal or multi-lateral wells may lead to commercial success. The nature of these coals suggests that gas production rates and recoveries will likely be moderate at best, so synergistic development with other opportunities may be necessary for economic success.