Application of hydrology to evaluation of coalbed methane reservoirs. Topical report, January 1987-December 1988

Coal seams have unique properties that make conventional production methods for gas-sand reservoirs ineffective. A systematic approach to selecting and developing a site using hydrologic concepts and well-testing techniques is presented. The most significant factors influencing methane production include adsorption, dewatering, gas desorption, two-phase flow of gas and water, and optimal wellfield patterns. New techniques are described for situations outside the realm of conventional reservoir engineering methods. The principal methods have been incorporated in PC software. The three stages of fluid flow from a coal seam to a well-bore are analyzed as 'windows' for determining critical reservoir properties. The most important parameters controlling production in a coal seam with adequate gas content are gas desorption pressure, permeability-thickness, and static reservoir pressure. Gas content and seam thickness are readily derived from drilling and core samples, and desorption pressure can be derived simply from a controlled drawdown test. A low-cost, easily applied procedure called a STEP Test was devised to determine permeability and static-reservoir pressure along with the condition of the wellbore.