Growing interest in gas reservoirs in nonconventional environments includes coal bed methane, which constitutes a potential important source of hydrocarbon gas in the UK but has been traditionally eclipsed by the conventional offshore oil and gas production. The current coal bed methane projects exist in different parts of the UK mainland. Drilling of these targets requires the use of minimum overbalance and nondamaging fluids.
The evaluation of coal bed methane reservoirs has its technical challenges. One of the challenges is to estimate the coal permeability by using both injection and production tests to ascertain coal cleat behavior. Although normal testing techniques can be used, interval testing using wireline can be more efficiently deployed to test individual coal bed seams. Traditionally, either an injection test or production test was applied to coal bed seams. In this paper we present a new approach that combines injection and buildup testing.
The methodology deploys a straddle packer string with a downhole pump on wireline. The packer spacing can be adjusted prior to deployment to suit the expected height of the coal bed to be tested. The tool is capable of both injection into and producing from the coal bed interval with the minimum storage volume. Downhole packer interval pressure is monitored on surface. Both the injection pressure and the drawdown pressure must be limited to avoid putting excessive force on the coal seam while maintaining single-phase flow.
Data from a recent application of the technique is presented for two different tested intervals. Obtaining permeability data from both falloff and buildup helps insure consistency between the tool data sets although some difference in the response between the two techniques results from the nature of the coal bed seam at each particular interval.