Reservoir Engineering in Coal Seams: Part 1-The Physical Process of Gas Storage and Movement in Coal Seams

Abstract

Reservoir Engineering in Coal Seams: Part 1-The Physical Process of Gas Storage and Process of Gas Storage and Movement in Coal Seams Gray, Ian, SPE Summary This is the first of two papers concerning the movement of gas in coal seams. It deals directly with the physical behavior of the coal seam as a reservoir. Coal seams show considerable differences in behavior from normal porous gas reservoirs in, both the mode of gas storage and permeability characteristics. Most of the storage of gas in coal is by permeability characteristics. Most of the storage of gas in coal is by sorption into the coal structure, while the coal permeability is cleat- (fracture-) or joint-controlled and may vary over a wide range during production. This permeability fluctuation is not solely a phase relative production. This permeability fluctuation is not solely a phase relative permeability effect, but is rather a result of the opposing effects of permeability effect, but is rather a result of the opposing effects of effective stress increase with fluid pressure reduction and shrinkage of the coal. Reducing fluid pressure tends to close the cleats, reducing permeability, while shrinkage tends to open them. permeability, while shrinkage tends to open them. Introduction The two papers in this series evolved out of 3 1/2 years of work (1979–82) on the problems of outbursting in Australian underaround coal mines. This phenomenon invalves the expulsion of gas and coal from the working face with resultant danger to the mining crew. Because energy release studies showed the outbursts to be primarily gas-driven, efforts concentrated on studying gas drainage to alleviate the problem. Because underground mining methods were being used, most of the experimental work was conducted horizontally in-seam rather than from surface-drilled holes. The mines studied are situated in the Bowen basin area, which extends from central to north Queensland. Australia. The seams with high gas contents are Permian age (260 million years) and bituminous, with rank determined by vitrinite reflectance lying between 1.00 and 1.30. The working depths were typically 380 m 11,247 ft] for Leichhardt Colliery, Blackwater; 135 m [443 ft] for Moura No. 4 mine, Moura, and 250 to 350 m [820 to 1, 148 ft] for Bowen No. 2 Colliery, Collinsville. The first two mines contained a seam gas consisting predominantly of methane, while the Bowen No. 2 seam gas was mostly CO2 of igneous origin. In all three mines, seam thickness was about 6 m [20 ft], with seam slope lying between horizontal and 10 degrees [0.17 rad] above horizontal. Observations of flow and pressure variations in the seam caused by drainage into boreholes or mine openings showed that conventional oil and gas analysis approaches to describe the reservoir were inappropriate, particularly with respect to the apparent large increases in permeability with drainage. Attempts at simulation with a permeability with drainage. Attempts at simulation with a conventional simulator incorporating phase relative permeability effects could not explain large flow increases permeability effects could not explain large flow increases measured during drainage. Conversely, some Japanese coal mines showed the opposite effect with apparently "self-seating" coals. The theory presented in this paper helps to explain the variations of permeability in coal associated with drainage. Gas Storage in Coal Gas is stored primarily by sorption into the coal. This typically accounts for 98 % of the gas within a coal seam, depending on the pressure at which the gas is sorbed. In addition, gas is stored in the pore or cleat space either free or in solution. As Fig. 1, a typical sorption isotherm, shows, the amount of gas sorbed per unit increase in pressure decreases with increasing sorption pressure. Because pressure decreases with increasing sorption pressure. Because gas drainage is often conducted under vacuum, the sorption isotherm has been extended below atmospheric pressure. Such tests for sorption may be conducted pressure. Such tests for sorption may be conducted volumetrically 4 or indirectly by weighing pressurized coal samples at a measured equilibrium pressure. -@ Because seams are often water-saturated. in some cases it is possible that the water pressure exceeds the pressure at which all gas becomes sorbed into coal solids or into solution gas. This is the coal-seam equivalent of the bubble-point of an oil/gas system and is referred to in this paper as the equivalent sorption pressure. paper as the equivalent sorption pressure. Darcy Flow and Diffusional Movement A number of references indicate that the movement of gas in coal is caused by Darcy flow down a pressure gradient or diffusion along a concentration gradient. The literature provides no clear answers as to which type of behavior is taking place or which type of transport governs the rate of gas production. One concept is that diffusive flow from the solids between cleats and Darcy flow along the cleat structure take place. P. 28