A series of pore and bulk compressibility measurements were made on a coal from the Bowen Basin, Queensland, Australia. They were conducted under hydrostatic compression and uniaxial strain conditions. It is found that when a hydrostatic stress is applied the pore and bulk compressibilities of the coal are highly stress dependent. By comparison, the compressibilities of the coal measured under a uniaxial strain condition are smaller in magnitude and much less sensitive to effective stress. This justifies the use of a constant pore compressibility in coalbed methane reservoir simulation. The results with different pore pressures show that the conventional effective stress concept of soil mechanics is valid for the compressibilities of the coal.
Most coalbed methane reservoirs contain adsorbed methane gas and water. Before the methane gas can migrate to a well and be produced, a coal seam must be de-watered to lower the reservoir pressure and allow desorption of the adsorbed gas from micropore surfaces of the coal. A decline in reservoir pressure induces changes in pore and coal matrix volumes. These changes are characterised by various coal compressibilities.
The product of porosity and pore volume compressibility of coal characterises water storage capacity of a coalbed methane reservoir and is an important input parameter in forecasting gas and water production (Young, et al. 1992). Unlike the pore volume compressibility of source rocks of conventional oil and gas reservoirs, which have been studied extensively (e.g., Hall 1953, Geertsma 1957, Teeuw, 1971, Lachance & Anderson 1983, Zimmerman 1991, Rhett & Teufel 1992, Zheng 1993), the behaviour of pore volume compressibility of coals, particularly the effects of stress and stress paths, has been far less investigated, as evidenced by very limited numbers of publications (Koenig 1991, Zheng et al., 1992).
Reported in this paper is a series of laboratory measurements of pore and bulk compressibilities, made on a coal from the Bowen Basin, Queensland, Australia. The study was initiated with aims, i) to characterise the coal in terms of the pore and bulk compressibilities, and ii) to study the dependence of the pore and bulk compressibilities of the coal on stress and stress path.
The tests were conducted following four different stress paths, including hydrostatic and uniaxial strain compression. The pore pressure was either kept constant, or decreased at a constant rate to simulate de-watering process. It has been found when a hydrostatic stress is applied, the compressibilities of the coal are highly stress dependent. The dependence is most significant at low effective hydrostatic stresses. The comparison of the compressibilities obtained at different pore pressure levels demonstrates the validity of conventional effective stress concept of soil mechanics for coal compressibilities. The compressibilities measured under uniaxial strain conditions are significantly different from those measured under hydrostatic compression. They are smaller in magnitude and much less sensitive to effective overburden stress than those measured under hydrostaticompression. The results obtained from this study highlights the importance in realistically simulating reservoir stress and stress path conditions.