DGMK/ÖGEW-Frühjahrstagung, Fachbereich Aufsuchung und Gewinnung Celle, 18./19. April 2013

Use of Black Oil Simulator for Coal Bed Methane Reservoir Model R.Sonwa, C. Enachescu, S. Rohs Golder Associates GmbH, Celle

Abstract Coalbeds are naturally fractured, low pressure, water saturated gas reservoirs. While some free gas may exist in a coal deposit, the majority of the gas is absorbed on the surface of the Coal matrix. When water is removed from the natural fractures of the coal, the pressure is reduced and the gas is released from the matrix into the fractures. Once in the fractures, the gas flows to the wellbore. So the coal degasification is i) a two-phase flow: water and gas ii) a two-step process: desorption of gas from the Coal matrix followed by flow through the fractures. Usually a conventional dual-porosity simulator is used to model a system such as a fractured carbonate reservoir where a low permeability matrix is coupled to a high-permeability fracture network. Each system has his own permeability and porosity and a matrix/fracture transfer term generally called SIGMA-FACTORgoverns the fluid flow from the matrix into the fractures. In coal bed methane (CBM) model, the fracture network represents the coal cleatssystem and the matrix of the system has no effective permeability and porosity and is used only as a gas storage source with gas flow controlled by gas content vs. pressure. While it is common to refer to CBM models as dual porosity models, they are in fact single porosity modelswith a pressure dependent source coupled to the reservoir. According to several authors, the slowest factor of these two processes mentioned above will control the rate of gas production from a coal reservoir. This means that i) if the rate of gas desorption from the matrix is very slow compared to the rate of fluid transport in the fractures, diffusion equations need to be incorporated into a conventional simulator to describe the gas production and ii)If the coal is well cleated, it can be assumed - for engineering purposes - that the gas desorbs instantaneously from the Matrix into the cleat when the pressure decreases. Based on this assumptionthe adsorption of gas on the surface of the coal can be modeled as gas dissolved in immobile oil. Therefore conventional reservoir simulators can then be used with only minor changes in the input data. The current paper describes firstly a simple model generated using the black oil simulator tNAVIGATORto compare the results of existing CBM simulators like COMETPC. The aim of these simulations is to validate the capabilities of tNAVIGATOR. Secondly, a DFN model taking into account the cleats densities, distributions, orientations and dimensions will be generated using FracMAN. Based on the DFN model the flow mechanism in a CBM reservoir can be evaluated. A material balance calculation will give us a first comparison between the modeled and the calculated gas content as a simulator initialization check.