Coalbed methane (CBM) reservoirs are a growing source of relatively clean energy in many parts of the world. CBM reservoirs are fundamentally different from traditional hydrocarbon reservoirs. Gas is adsorbed on the surface of the coal cleats and not stored in pores. Still, estimation of the permeability of the coal cleats is important in judging the potential producibility of a given coal seam.Traditionally, CBM reservoirs are surface tested using injection or production techniques to access reservoir permeability (Clarkson and Bustin 2011). Recently, pressure buildup and falloff tests using the straddle packer module of a wireline formation tester have been used in CBM reservoirs to assess reservoir permeability successfully. However, low permeability, limited station time, or both have, in some cases, reduced the quality of the interpretation results.Deconvolution techniques have been available for some time; however, few practical examples are available in the literature. The use of deconvolution will generally allow extracting more of the same data. The derivative response uncertainty is normally due to errors in estimating the reservoir pressure and the flow rate. Generally, wireline formation testers provide reliable measurements of the reservoir pressure and flow rate.We applied deconvolution to pressure buildup and falloff for the first time on data acquired in a CBM environment with a straddle packer. The use of deconvolution has improved the permeability estimation from the different tests. We were also able to identify the limitations of the technique and the uncertainties in the analysis results.

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.

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