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Coal as a Conventional Source of Methane: A Review and Analysis of 50 Wells in Two Production Areas in the Black Warrior Basin of Alabama
Abstract
This paper presents a review of the actual production, sales, and economic data from two production areas with 52 wells developed by a joint coal industry' gas industry effort owned equally by Jim Walter Resources, Inc. (JWR), a subsidiary of Jim Walter Corporation of Tampa, Florida and Enhanced Energy Resources, Inc. (EER), a subsidiary of Kaneb Services, Inc. of Houston, Texas. The unique reservoir characteristics of the coal environment are described in brief, a comparison of actual methane production from coal with computer model predictions is presented, and the capital and operating costs are discussed with specific emphasis on the economic results.
This information differs from similar previous work in that economic vitality is now apparent whereas previous inquiries were essentially restricted to the technical reservoir engineering characteristics and the physical capability of coal to desorb (produce) methane. There are a number of published papers on this important technical aspect several of which are references for this presentation.
Production Area I (31 well production area) has been generating an operating profit for the past 21 months. Profits have increased substantially in the past year as a result of the completion of an 8" transmission line and reduced operating costs. Initial production commenced in late 19/9. A five well pilot project was evaluated for approximately two years before commercial development commenced in late 1981. A total of 31 wells were drilled by mid-1982. First sales commenced in February of 1982. Production Area II drilling commenced in January of 1983 with initial sales in March of 1983.
The economic viability is demonstrated based on actual operating profits over the past twenty-one months and current experience with respect to improvements in operational techniques and costs. These data are applied to the computer forecasts of long term dellverabilities for projections of expected economic performance.
Coalbed gas, which mainly consists of methane, has remained a major hazard affecting safety and productivity in underground coal mines for more than 100 years. Coalbed gas emissions have resulted in outbursts and explosions where ignited by open lights, smoking or improper use of black blasting powder, and machinery operations. Investigations of coal gas outbursts and explosions during the past century were aimed at predicting and preventing this mine hazard. During this time, gas emissions were diluted with ventilation by airways (e.g., tunnels, vertical and horizontal drillholes, shafts) and by drainage boreholes. The 1970's `energy crisis' led to studies of the feasibility of producing the gas for commercial use. Subsequent research on the origin, accumulation, distribution, availability, and recoverability has been pursued vigorously during the past two decades. Since the 1970's research investigations on the causes and effects of coal mine outbursts and gas emissions have led to major advances towards the recovery and development of coalbed methane for commercial use. Thus, coalbed methane as a mining hazard was harnessed as a conventional gas resource.
A sorption apparatus for measuring the equilibrium sorption isotherm and the rate of diffusion of methane from fine-sized coal was designed and constructed. Preliminary work with Pittsburgh and Pocahontas No. 3 coal shows that the diffusion coefficient varies with pressure and that the fracture spacing in these two coals is substantially different.
American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the SPE 4th Annual Eastern Regional Meeting to be held in Pittsburgh, Pa., Nov. 2–3, 1967. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.
Abstract
Gas occurs in coalbeds in an adsorbed and a free gas state. Adsorbed gas is stored in the micropore structure and its transport is governed by Fick's law. The free gas occurs in the fracture system and flows according to Darcy's law. These two modes of mass transport are interdependent. Production decline curves are of the constant percentage decline type and, thus, show no indications of flow characteristics peculiar to coal-gas reservoirs. The effectiveness of surface boreholes as a degasification scheme depends upon both good fracture permeability and a high fracture density. Conventional methods of reservoir engineering analysis are not applicable to coalbeds.
Introduction
Mining of deep coalbeds (1,500 to 2,000 ft.) and production of the associated gas is analogous to a gas well with an expanding well bore radius. Gas production of 10 to 15 MMscfd is not uncommon from deep mines. State and Federal codes require that each unit vol. of gas be diluted with approximately 100 vol. of air. Methane-air mixtures in the range 5 to 15 pct. methane are explosive.
Mining deeper coalbeds requires smaller mine openings and these mines tend to be more gassy. Thus, the use of ventilation (dilution) as a means of controlling methane concentrations at active face areas and in the returns is limited severely. Degasification of coalbeds prior to mining and methods of controlling the flow during mining are necessary. Before effective methods of control and degasification can be developed, the laws governing mass transport through coalbeds must be understood.
The purpose of this paper is to present the fundamental concepts governing the transport of gases through a coalbed, and to define one of the environmental problems related to rapid, economical, and safe mining of coal deposits.
MICROPORE STRUCTURE OF COAL
Coal is formed from plant substances which mere preserved in various states of degradation in a favorable environment, and later altered by chemical and physical processes. However, there is no universal agreement on what chemical and physical changes take place in the transformation of plant substances to coal.
This paper presents an analysis of the coalbed degasification process. The theoretical and experimental basis of the degasification process are discussed and a simulation model which incorporates all aspects of this process is described. The simulator is demonstrated using actual field data developed by a joint industry/government demonstration project funded by the DOE and U. S. Steel. The basic reservoir description is discussed in detail, including variations of important description parameters with location.
Initial and boundary conditions are demonstrated and analyzed. Initially, the coalbed was saturated with water. With water production, reservoir pressure is lowered, causing gas to desorb from the coal creating a mobile gas saturation. Subsequently, interwell interference effects are demonstrated and the need for such effects explained.
Finally, the long term gas deliverability of the pattern is forecast. This forecast shows that about 45% of the gas within the pattern can be removed if the pattern is in operation six years ahead of mining.
Determines rates for adsorption and desorption of methane as a function of particle size and temperature, using coals from the Pocahontas No. 3 seam (lvb) from the Bishop mine of western Virginia, and the Pittsburgh seam (hvab) from the Pursglove No. 15 mine of northern West Virginia. For a given coal sample and temperature, the rate curves for adsorption and desorption are essentially the same when expressed on a fractional approach to equilibrium basis. The rates of adsorption and desorption increased eightfold as the particle size was decreased from 6 to 8 to 270 to 325 mesh per inch. The adsorption process appears to be diffusion controlled. Rate of sorption processes increased with increasing temperature; however, the amount adsorbed at equilibrium decreased.
The Composition of Coalbed Gas
- Kim
Degasificatior Parameters and Well Completion Procedures for the Mary Lee
- K L Ancell
New Advances in Coalbed Methane
- H S Price
Contribution to the Geochemistry of Mine Gas in the Carboniferous Coal Field of the Saar Region, Germany
- Kneuper