Regional Low Permeability Gas Systems

Bruce R. Palmer and S.W. Burnie
Rakhit Petroleum Consulting Ltd, Calgary, AB

Producible tight gas resources for the United States have been estimated at 628 TCF. However, recent papers have questioned the availability of these resources due to the presence of water and the adverse effect that this has on relative permeability in tight reservoirs.

Regional low permeability gas systems (RLPGS) have gas as the continuous fluid on a regional scale and are bounded by aquifers laterally and vertically. Permeability in these systems ranges from 0.1 md to 0.1 µD and the reservoir, source and seal are often one and the same. The source rocks must be generating, or have generated significant volumes of gas creating tall gas columns. Pressures within the RLPGS can be the same as, or higher and lower than the regional aquifer. Moveable water is present in these systems as small local aquifers or as water saturations that exceed irreducible values and result in increased water cuts in the gas production.

A model has been developed to explain the evolution of the RLPGS and is comprised of the following four stages: Genesis, Transition, Steady State and Imbibition. Successful exploration strategies depend on the recognition of the defining characteristics and the stage of development of the RLPGS.

Three field examples are presented: a shallow, biogenic, steady state example from the Milk River Formation in Western Canada; a deep, actively-leaking, transitional stage example from the Jonah field in Wyoming; and a late-Genesis stage example from the LaBarge area in southwestern Wyoming.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005