How to Correct the Petro-Physical Properties for Simulating Shale Gas Production Using Current Commercial Simulators?

Abstract

AbstractNumerical simulation is important to understand and predict the development of oil and gas reservoirs. Existing commercial simulators, such as CMG, ECLIPSE and VIP, have been widely used in the past several decades for their robust performance in computing and scaling. In these reservoir simulators, the fluid flow models are based on Darcy's law or its extended form; the volume of the adsorbed phase or component is overlooked as well. While this is acceptable for conventional oil/gas reservoirs or chemical flooding reservoirs, the gas flow regimes such as slippage flow, transition flow, and molecular-free flow significantly is deviated from Darcy flow for shale gas reservoirs. Besides, a large portion of the gas is stored in the pore in the form of adsorbed gas. If the volume of the adsorbed gas is still overlooked, the volume of free gas and original gas in place (OGIP) will be seriously overestimated. For the above reasons, it is commonly thought that existing commercial simulators could not be ideally used to simulate the development of shale gas reservoirs. Hence, it is desirable to attain a feasible approach to correct the petro-physical properties of shale gas effectively within the commercial simulators, in order that one can use them to accurately simulate the development of shale gas reservoirs. In this paper, we first derived the correction formulas for the bulk porosity, free gas saturation, and connate water saturation used for correcting the disregarded volume of adsorbed gas in commercial simulators. Then, we derived the models of permeability and porosity multipliers in matrix considering gas adsorption/desorption, geomechanics, non-Darcy flow regimes, and diffusion of adsorbed layer. Finally, the above models were used to attain the corrected petro-physical properties for simulating gas production based on the practical properties of shale gas reservoirs using commercial simulators. The validation was performed by comparing the simulation results of commercial simulator with the published mechanism simulator using gas field data. The results show that the simulation results using commercial simulator achieve good agreement with the published mechanism simulator with the corrected petro-physical properties. The corrections of bulk porosity, connate water saturation, and free gas saturation are very essential. The correction formulas for these properties can largely decreases the error of OGIP and the calculated gas production. Both permeability and porosity multipliers are the functions of gas pressure, but they are not a monotone decreasing/increasing function. The gas production may be significantly overestimated or underestimated without consideration of these characteristics of shale gas in different fields. The contributions of different mechanisms are also demonstrated using the commercial simulator. This work can evently solve the issue that existing commercial simulators cannot accurately simulate shale gas production. The researchers can easily use these commercial simulators with these corrected formulas, which is a great progress for modeling the development of shale gas reservoir.