The Effect of Swelling/Shrinkage on Gas Transfer Rates within Intact Cores of Powder River Basin Coals

Pribyl, Richard D. ¹ (1) University of Wyoming, Laramie, WY

Coal bed natural gas (CBNG) is natural gas, primarily methane, trapped within coal under hydrostatic pressure. Conventional methods of methane extraction require partial dewatering of the coal seams to reduce formation pressure. The volume of the extracted (produced) water is substantial. In April 2004, the Colorado Energy Research Institute (CERI) at the Colorado School of Mines assembled a group of scientists and engineers to address the management and beneficial use of CBNG produced water. Alternative recovery methods such as stripping (reducing the partial pressure of the methane) and desorption (replacing the adsorbed methane with another compound) can potentially decrease the volume of produced water. Pilot studies have shown that methane recovery may be enhanced through carbon dioxide (CO₂) injection. However, CO₂ injectivity may also be reduced by CO₂-induced swelling within the coal matrix. Coal tends to exhibit differential swelling, as the extent of swelling/shrinkage varies with the species of gas present. CO₂ has been shown to swell coal substantially more than methane (CH₄) and nitrogen (N₂). The University of Wyoming is investigating the sorption and gas transfer kinetics of CO₂, CH₄, and N₂ through intact coal core samples from the Powder River Basin. Intact cores preserve both the coal matrix and fracture system, increasing the accuracy in representing downhole conditions. Currently, an experimental device is under development to quantify the gas transfer rates within saturated coal. Strain gages will be utilized to measure coal swelling during injection. Correlations between coal microporosity, coal swelling, and gas flux will also be investigated.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana