Numerical Simulation of the Transient Behavior of Coal-Seam Degasification Wells

Abstract

Summary This paper describes the mathematical and numerical developments for a series of finite-difference models that simulate the simultaneous flow of water and gas through dual-porosity coal seams during the degasification process. Models for unstimulated and hydraulically stimulated degasification wells are included in this series. The hydraulically stimulated wells are assumed to be intercepted by a single infinite- or finite-conductivity vertical fracture. Introduction Unconventional natural gas has been defined as pipeline- quality (high-Btu-content) gas produced from resources other than those historically exploited by the oil and gas industry. These unconventional gas resources include geopressured aquifers, tight sands (koo less than 0.1 md), Devonian shales, and coal seams. The potential of unconventional gas, broken down by each resource, is presented in Table 1. In addition to the pricing incentives associated with unconventional natural gas (unconventional gas prices are unregulated under Sec. 107 of the 1978 Natural Gas Policy Act), several geographic and economic factors make the future of gas production from coal seams quite promising.1.Many producible coal seams are in the eastern U.S., close to established pipelines and markets. 2.Most major domestic coal seams are thought to have been discovered before or during the industrial revolution (see Fig. 1). These seams are well characterized; therefore, exploration costs would be minimal. 3.Many major domestic coal seams are shallow (depths less dm 1,000 ft [300 ml). Therefore, drilling costs would be minimal. 4.Drilling, completion, and stimulation technology borrowed from the natural gas industry have been well developed. 5.The gas from coal seams is generally sweet, requiring only dewatering, metering, and compression facilities at the surface. In addition, there are other incentives for producing gas from coal seams when the seam in question is minable:mining safety can be increased;mining rates can be increased; andmining costs, especially for ventilation systems, can be reduced. Coal Gas Coal gas is a byproduct of the physical and chemical reactions associated with the coalification process (the process by which vegetable matter is converted to coal). process by which vegetable matter is converted to coal). Consequently, coal seams are different from conventional gas reservoirs because the coal acts as both the source rock and the reservoir rock for the gas. Approximately 46 Mscf [1300 std m ] of gas are evolved during the formation of 1 ton [0. 907 Mg] of coal. Coal gas is composed primarily of methane and CO2, with trace amounts of higher-molecular-weight hydrocarbons and other gases-such as oxygen, nitrogen, and helium. Table 2 lists the compositions of gases from several domestic coal seams. Samples of gases from virgin coal seams yield calorific values that range from 900 to 1075 Btu/scf [34 x 10 to 40 x 10 kj/M ]; this makes these gases commercially profitable with little processing. Gas from gob (previously mined areas) may contain 25 to 60 vol % air and generally needs processing to upgrade it to commercial quality. Coal Seams as Natural Gas Reservoirs Pore Structures. Coal seams are characterized by a Pore Structures. Coal seams are characterized by a dualporosity nature: they contain both a micropore (primary porosity) and macropore (secondary porosity) system. The porosity) and macropore (secondary porosity) system. The micropores have a diameter ranging from 5 to 10 k [0.5 to 1.0 mn] and exist in the coal matrix between the seam's cleat (uniformly spaced natural fractures). Because of the dimensions of the micropores, the micropore system is inaccessible to water, The macropore system is made up of the volume occupied by the cleat. The fracture spacing is very uniform and ranges from a fraction of an inch to several inches. Two types of cleat are present in coal: the face and butt cleat. The face cleat is continuous throughout the seam while the butt cleat in many cases is discontinuous, ending at an intersection with the face cleat. Generally, the face and butt cleats intersect at right angles. The dimensions of the macropores may vary from aperture widths on the order of angstroms to microns. There do not appear to be any transitory pores between the two systems. SPEFE p. 165