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Abstract

This paper, in its first part, reports the estimation of permeability and skin factor by simulating the slug test using a commercial coalbed methane simulator (COALGAS). Sensitivity of permeability and skin on the pressure transient profiles are also discussed along with parametric multiplicity. The second part of this paper reports analysis of data from packer tests performed at one of the BHP Steel Colliery sites. Two patterns of pressure build-up profiles are observed during the packer tests with one of the patterns having an unusual S-shape. Single and two phase models are constructed to estimate the in-situ permeability of coal using data from these packer tests and COALOAS. Influence of fluid leakage through the packer system is also discussed and incorporated in the estimation of permeability.

Introduction

The in-situ permeability of a coal seam is one of the important reservoir properties required to plan stimulation and production activities to drain methane from the seam. Although permeability can be determined from laboratory tests on cores, other methods (field tests) of estimating this permeability including well testing, and history matching of production from drainage holes are frequently used. This is due to the concern that the cores may not represent the subsurface heterogeneity and anisotropy of the coal seam.

Among the field tests, slug test and pressure build-up packer test are the two widely used methods to estimate in-situ coal permeability in a water saturated coal seam. The slug test begins With an instantaneous withdrawal of a slug of water from a well. The pressure build-up is then measured until the bottom-hole pressure reaches its `pre-slug' value. Slug tests can also involve injection of a slug of water and recording of pressure fall-off to the `pre-slug' value. Withdrawal of fluid during a slug test may cause desorption of methane from coal. In the pressure build-up packer test, sections of an in-seam hole are isolated by inflatable packers and the pressure build-up measured till the pressure is stabilised. The schematics of slug tests and packer tests as well as their typical pressure profiles are shown in Fig. 1.