Design and Interpretation of Injection/Falloff Tests for Coalbed Methane Wells

Abstract

SPE Members Abstract This paper presents the results of a study undertaken to investigate the reasons why data from injection/falloff tests of coalbed methane wells were not interpretable using conventional well test analysis techniques and the results of a field study to investigate completion effectiveness during injection tests. The pressure transient test data obtained using the old injection/fallout test design were found to be interpretable only by history matching analysis. A modified test design is presented. A field example is presented which illustrates how application of the modified test design resulted in easily interpretable data using conventional well test analysis techniques. Spinner surveys run during injection tests showed that breakdown/fallout treatments are required prior to simultaneous testing of multiple coals in cased-hole completions to ensure adequate communication between the well and all target coal seams. Introduction Recent work has shown that permeability is the reservoir parameter that primarily controls methane recovery from coals. In water saturated coal seams, permeability controls the initial rate at which water can be removed from vertical degasification wells, Therefore, permeability controls the rate at which the reservoir can be dewatered and the producing time required to achieve maximum gas production from the coal reservoir. Consequently, it is extremely Important for engineers to have accurate measurements of reservoir permeability to design field installations (well completions) and optimally manage reservoir performance. Several recent papers show evidence of the importance of reservoir permeability in determining optimal well spacing and stimulation design. Well tests are used to determine the reservoir permeability in coals. Most coal seams are saturated with water at virgin reservoir conditions. The best opportunity to test coal seams is prior to well production when the reservoir is 100% water production when the reservoir is 100% water saturated. Interpretation of tests run on coal wells after pressure drawdown, when two-phase flow conditions are established in the reservoir, is difficult at best. Historically, slug tests and water injection/falloff tests have been used to determine permeability in water saturated coals. These tests are generally preferred over classical drawdown/buildup tests because they test the reservoir under single-phase flow conditions. This paper is based primarily on the experiences of one operator, Taurus Exploration, Inc. (Taurus). Taurus operates hundreds of coalbed methane wells in the Black Warrior Basin, covering a wide range of geologic conditions (permeabilities). Both slug tests and injection/falloff tests have been used by Taurus to determine the permeability of coal seams in the Black Warrior permeability of coal seams in the Black Warrior Basin. P. 425