Implications to Conventional Gas Reserve Growth of Coalbed Methane Desorption Analyses from the Beaver Creek and Kenai Gas Fields, Kenai Peninsula, Alaska

By

T.A. Dallegge (University of Alaska, Fairbanks) and C.E. Barker (U.S. Geological Survey)

Canister desorption of coal drill cuttings from two mature conventional natural gas fields, the Beaver Creek Field and Kenai Gas Field, near Kenai, Alaska indicate that significant gas resources are contained in the ubiquitous coals of the Sterling and Beluga Formation where they are apparently isolated from production zones. In contrast, adsorption analyses of coals within the upper regions of the conventional gas production zone indicate that the coals are grossly undersaturated. Yet desorption values from deeper levels have gas contents closer to the expected values of similar quality coals. Further, the gas composition suggests that the conventional gas has been locally generated from the coalbeds themselves and that they were saturated before conventional gas production commenced.

Extended conventional production has depressurized these fields and we believe much of the original gas-in-place within the coalbeds has been co-produced with the conventional gas. At present, original reservoir pressures have been reduced to near the ultimate recovery level, especially in the Kenai Gas Field. This reduction in pressure is far below the critical desorption pressure indicated from the coal adsorption isotherm data and implies that significant quantities of CBM has apparently been desorbed and produced through the conventional reservoir sands. The co-production of coalbed gas would account for the local undersaturation in some coals in the reservoir interval as well as unexpected reserve growth that has been noted in these fields. Our contention is that co-produced coalbed gas accounts for reserve growth in conventional gas fields with abundant interbedded coals.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.