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.

Water drive gas reservoirs are frequently abandoned at high pressures which results in low recovery of the initial gas in place.The gas remaining in these reservoirs is trapped as residual gas saturation.The techniques to remobilize a portion of this gas that has been trapped by encroaching water are presented along with a method to evaluate the incremental reserve potential of such reservoirs. The remobilizat~on of the trapped gas requires the production of large volumes of water to lower the reservoir pressure which in turn allows the gas to expand and flow.A new use of the p/z vs cumulative production chart is presented which gives the engineer a quick method to evaluate the feasibility of recovering a portion of the gas that has been trapped by water influx.A field example is presented which illustrates the concepts and techniques. Presented is field data References and illus. at end of paper. 375that demonstrates the remobilization of gas that had been trapped by encroaching water.Significant increases in recovery have been experienced at the demons tra tion field, and ul timate recovery is estimated to be about 8% of initial gas in place greater than was recovered using conventional practices. Also given are actual operating costs and revenue calculations that show the process to be economic at 1990 market prices.

This paperwas preparedfor the 46thAnnualFall Meetingof the Societyof PetroleumEngineers of AIME, to be held in New Orleans,La.,Oct. 3-6,1971. Permission to copy is restricted to an abstractof not more than 300 words. Illustrations may not be copied. The abstractshouldcontain conspicuous acknowledgment of whereand by whom the paper is presented.Publication elsewhere after publication in the JOURNALOF PETROLEUM TECHNOLOGY or the SOCIETYOF PETROLEUM ENGINEERS JOURNALis usuallygrantedupon requestto the Editorof the appropriate journalprovidedagreement to give propercreditis made. Discussion of thispaper is invited. Threecopiesof any discussion shouldbe sentto the :~ociety of Petroleum Engineers office. Such discussion may be presentedat the abovemeetingand, with the paper,may be considered for publication in one of the two SPE magazines.

This paper presents the results of a simulation study of the producing and storage mechanisms of the Devonian Shales at Cottageville Field in West Virginia. The study compares the performance of a system in which the gas is stored in a macrofracture system which is also the flow channel to wells with that of a system in which the gas is stored primarily in an adsorbed state in a micropore structure and diffuses into a macrofracture system of low pore volume through which it migrates to wells. The results show that within the range of measured parameters, either system will adequately describe the twenty-six year production history at Cottageville Field. Further it production history at Cottageville Field. Further it is virtually impossible to discern between the two mechanisms even though the system postulating adsorbtion as the storage mechanism predicts a much larger resource base. Introduction The Devonian Shale has been producing natural gas for over a century. This resource is characterized by extremely long life with low individual well deliverability. Because of the widespread occurrence of the shale throughout the Appalachian Region, it has been identified as a potential source of increased domestic natural gas production. Because of its potential, the U. S. Department of Energy and its predecessors initiated its Eastern Gas Shales Project. As a part of this project, a study of the producing and storage mechanisms was commissioned. producing and storage mechanisms was commissioned. Several mechanisms have been postulated. It was the objective of this study to investigate the most plausible of these postulations to see if any could plausible of these postulations to see if any could explain the long-term behavior of this type of natural gas production. There are two widely held theories for the occurrence of the gas in the Devonian Shales. The first theory is that the shales are a conventional gas reservoir and that the porosity is created by a macrofracture system which is also the flow channel to the wells. Under this theory the gas is stored by compression only. This is known as a single porosity system. The second theory is that the shales contain the gas as an adsorbed state in a micropore structure. The adsorbed gas diffuses into a macrofracture system of low pore volume and then migrates to the well in response to a pressure differential. The second theory is referred to as a "dual" porosity system. A reservoir simulator which incorporates the adsorbed gas concept with conventional Darcy flow in porous medium was used to determine the properties of porous medium was used to determine the properties of the dual porosity system. An actual field example was used to test the validity of both theories. SIMULATOR DESCRIPTION The production of gas from a dual porosity system is believed to be dependent upon two distinctly different physical processes,diffusion from the interior of a solid particle to a crack or macropore in the rock, andDarcy flow through the fracture or macropore structure to a production well. Diffusion of methane through solid particles of coal is thought to be a much slower process than the fracture flow. Depending on particle size, it may or may not be the controlling factor in production. Diffusivities have typically been measured by grinding rock particles to a uniformly small size and comparing particles to a uniformly small size and comparing rates of desorption to analytical solutions for diffusion in a sphere of comparable diameter. The differential mass balance describing diffusional transport in a sphere is as follows: ............(1) The concentration of methane, C, is expressed as moles/unit volume of rock. The boundary conditions for this equation are as follows: ......................(2) P. 263

Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract This paper represents an effort to apply simulation techniques to low permeability gas reservoirs. Problems encountered are illustrated by an actual field example. Techniques normally employed for gas reservoirs are not easily extended to the low permeability case. The results show that it is permeability case. The results show that it is possible to rigorously calculate the possible to rigorously calculate the performance of such reservoirs, but to do performance of such reservoirs, but to do so requires data that make application to large fields impractical. However, certain adjustments can be made that approximate performance of such reservoirs, making large performance of such reservoirs, making large applications possible under some restricted conditions. Introduction Natural gas transmission companies must invest millions of dollars in surface pipeline systems and compression facilities in pipeline systems and compression facilities in order to produce natural gas and transport it thousands of miles to the market area. These surface systems are designed to fit the specific reservoirs with which they are connected. In the mid-continent gas supply area, and specifically the Anadarko Basin, many gas reservoirs are characterized by low permeability. Reasonable gas rates that permeability. Reasonable gas rates that satisfy both the transmission company's market requirements and the producer's economics result in large pressure transients existing over extended periods of time. While the pipeline company is basically interested in stabilized well performance, the individual well rates are often determined by short term transient well tests. This makes it necessary to calculate both highly transient and stabilized flow. Numerical simulation techniques would appear to offer the best solution to such a problem. This paper deals with experience gained from simulating a low permeability reservoir and is illustrated with an actual field example. THE PROBLEM AND DATA AVAILABLE The simulation of gas reservoirs in the Anadarko Basin requires consideration of low permeability. The extension of techniques permeability. The extension of techniques used for high permeability fields has proved to be not entirely adequate. This study represents an effort to calculate the complete performance of a reservoir that is characterized performance of a reservoir that is characterized by low permeability, then to develop a technique to evaluate such reservoirs in a routine manner.