A Parametric Study of the Effects of Coal Seam Properties on Gas Drainage Efficiency

Abstract

Summary A parametric study is conducted to investigate the effects of reservoir properties on gas drainage efficiency. It is found that when a coal seam is opened to production, the gas desorption and production rates increase to a maximum value and then decline. The magnitude of the early desorption peak was found to be a function of (1) the ability of the micropore matrix to supply gas to the macropore system, and (2) the coal seam's conductivity to water. The desorbing gas was observed to create a localized, high-gas-saturation bank in the area enclosed by the pressure transient. The gas bank provided an internal pressure maintenance to the reservoir, while it decreased the relative permeability to brine. This created a competing effect with respect to water production. Because water removal strongly influences the pressure decline and, consequently, the desorption rate, a unique production mechanism was observed. The study explored the interference effects on gas and water flow in multiple-well systems. It was found that the pressure drawdown caused by the multiple wells enhanced the desorption of gas into the macropore system and caused a positive interference effect on the gas flow rate. The water rate, however, encountered the more conventional negative interference effect. Introduction During the metamorphosis of organic material to coal, vast quantities of methane gas are produced and retained by the coal. It has been estimated that during the formation of 1 ton [0.9 Mg] of coal, up to 46 Mscf [1303 std m3] of gas is produced. In a study comparing the anthracitic coalbeds of northern Pennsylvania with the bituminous coalbeds of southern Illinois, Darton found that most mature coal contains between 20 and 100 scf [0.566 and 2.832 std m3] of methane per ton. With the onset of energy conservation, much attention has been given to unconventional gas resources. Unconventional natural gas can be defined as gas produced from resources other than those historically exploited by the oil and gas industry. Unconventional gas resources include tight gas formations, eastern gas shales, coal seams, and geopressured aquifers. Estimates of technically recoverable gas contained in domestic coalbeds range from 300 × 10 12 to 800 × 10 12 Scf [8495 × 10 9 to 22 653 × 109 M3]. This gas is high-quality and requires little or no processing before transmission. Reservoir characteristics of coalbeds are quite complicated, which makes mathematical modeling a challenge. The coal matrix is heterogeneous and characterized by two distinct porosity systems: macropores and micropores. The macropores (commonly known as cleat in the coal industry) constitute the cracks and fissures inherent in all coals. The cleat system is composed of two major components: the face cleat and butt cleat. The face cleat is continuous throughout the reservoir and is capable of draining large areas, the butt cleat, on the other hand, is discontinuous usually terminating at an intersection with the face cleat (see Fig. 1). Butt cleats contact a much smaller area of the reservoir and thus are limited in their drainage capacities. The micropore system consists of the primary-porosity matrix that exists between the cleat. This primary-porosity matrix that exists between the cleat. This system is not accessible to water. Gas flow does occur, however, but is considered restricted to diffusional flow. The major portion of gas stored in coal exists in an adsorbed state, rather than in a free state. When the system is in equilibrium, the free gas saturation is negligible. As water is removed from the macropores, the reservoir pressure is lowered, causing gas to desorb from the pressure is lowered, causing gas to desorb from the micropore surfaces and to diffuse into the macropores. The free gas saturation in the macropore system increases as the desorption process continues until the critical saturation for flow is reached. On reaching this critical value, the gas becomes mobile and is subject to transport in the fractures of the macropore system (Fig. 2). The methane drainage process of coal seams is analogous to the two-porosity system described by Warren and Root. The Warren and Root model applies to fractured hydrocarbon reservoirs that contain both a low-permeability, high-storage, primary-porosity system and a high-permeability, low-storage, secondary-porosity system. Flow can occur only between the primary- and secondary-porosity systems but cannot take place through the primary-porosity elements.