Necessity of porosity correction before simulation and re-understanding of the effects of gas adsorption on production in shale gas reservoirs

Abstract

Shale gas contains free gas and adsorbed gas in matrix pores. The size of matrix pore is on the order of nanometer, large specific surface area makes the adsorbed gas occupy a large part of the total void volume. However, most widely used simulators ignore the volume reduction of free gas due to adsorbed gas, resulting in the OGIP errors. In order to make the simulation results be more reasonable and accurate, firstly, we derived the correction formulas of bulk porosity, free gas saturation and connate water saturation used for converting measured data into simulation model. Then, the matrix porosity, gas saturation and connate water saturation are corrected to match the original gas in place (OGIP) and the production data of Barnett Shale. Finally, the effects of gas adsorption on gas production in Barnett Shale with porosity correction are studied. The results show that porosity correction is significantly necessary before inputting the measured parameters into a simulator. In Barnett Shale, the adsorbed gas takes up 1/4 of the total void volume, if this partial volume is ignored, the free gas storage capacity and OGIP will be overestimated by 52.49% and 25.5%, respectively. The cumulative gas production will be much higher without porosity correction due to the artificial increasing of the free gas volume in the model which should be occupied by adsorbed gas in reality. The adsorption effect of shale gas is adverse to the development of shale gas. Only a little fraction of the adsorbed gas contributes to gas production in the late-stage. For Barnett Shale, the recovery factor of free gas is about three times of adsorbed gas; the gas recovery factor in the near-wellbore region is about two times of the gas recovery factor in the whole reservoir.