ABSTRACT:

Reservoir compaction and surface subsidence are significant issues for shallow coal seam gas plays, and regulatory authorities can mandate subsidence predictions as a License-To-Operate requirement. It is therefore important to accurately determine the compaction behavior of the coal under the expected in-situ conditions as the reservoir depletes. This paper reports the results of methane pore pressure depletion tests under approximate uniaxial-strain and in-situ stress conditions on whole core coal samples from a coal seam gas reservoir. Test results showed that there is a substantial increase in the compressibility of the core material when the effects of methane desorption are included. The effective uniaxial-strain compaction coefficient Cm show a clear increase in compressibility with decreasing pore pressure, and an apparent acceleration of the increase below 1.2 MPa pore pressure. For the tested samples the Cm values began at 1.5 to 4.4x10-4/MPa and increased to peak values of 5.1 to 11x10-4/MPa. The compressibility excluding the desorption effects was generally below 2x10-4/MPa and constant or decreasing with decreasing pore pressure. These data agree with previously reported results on coal from other locations and clearly show the importance of including methane desorption effects in reservoir compaction and subsidence calculations for coal seam gas fields.

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