Pressure transient testing in coalbed methane reservoirs

Pressure transient testing in coalbed methane reservoirs differs significantly from well testing in conventional gas reservoirs. Coal's storage and production mechanisms are the primary cause for the disparity in the analysis for the two types of reservoirs. A modification of conventional analysis methods is required, at most times, to adequately evaluate a coal reservoir.

This study characterizes a dewatered coal gas reservoir at a specific test site through evaluation of pressure responses from the coal wells in the area. Three different analysis techniques, namely, single-well test analysis, multi-well test analysis and reservoir simulation, were employed to estimate the reservoir properties for the formation at the site. Additionally, the intent of this study was to identify the well testing model representative of the pressure response from a coal reservoir.

The real gas pseudopressure method was employed for single- and multiwell test analysis using PIE, a well testing software. Some multi-well test analysis was also performed using Papadopulos-Ramey method, which is specifically designed for anisotropic formations. The reservoir simulation portion of the study, for evaluation of multi-well test analysis results, was performed on ECLIPSE-200, a coalbed methane simulator.

Single- and multi-well test analysis of the pressure transient tests run on the coal wells at the site suggested a homogeneous and radial reservoir with boundary effects. This conclusion was reached despite the fact that coal is a dual porosity and dual permeability reservoir. Single- and multi-well test analysis also suggested scale dependent permeability and partially sealing no-flow boundaries at the formation site. Additionally, an average geometric mean permeability was determined for the site from multi-well test analysis.

Reservoir simulation gave an estimate for the directional orientation of the face and butt cleat systems, and a measure of the anisotropic permeability ratio between the face and butt cleats for the tested site.

Future work needs to be done to determine the exact location of the noflow boundaries and the orientation of the face and butt cleat systems.