Gas desorption is one of the major gas transport mechanisms in shale gas reservoirs. However, its actual contribution to gas production is often masked by the indiscriminate use of adsorption-derived parameters for desorbed gas volumes during gas production calculations at reservoir conditions. Traditionally, it is believed that gas adsorption is fully reversible at the high-pressure, high-temperature conditions found in shale gas reservoirs.
In this paper, we studied methane adsorption and desorption isotherms using three shale samples at a reservoir temperature of 80oC. The resulting isotherms were modelled using the Langmuir model, following the conversion of measured excess amounts to absolute values. Lastly, a compositional 3D dual-porosity model was developed with CMG-GEM to test the effect of sorption hysteresis on gas production from a shale rock. For each sample, a base scenario, equivalent to a "zero-sorption" case, was compared against two other scenarios representing the two sorption processes.
For each sample, significant hysteresis was observed between the adsorption and desorption isotherms, with the desorption isotherms resulting in lower Langmuir parameters than the corresponding adsorption isotherms. For each process, Langmuir volumes showed a positive correlation with total organic carbon (TOC) contents. Also, the simulation results showed that gas production was lowest for the base case and highest for the adsorption case for each sample. This implies that neglecting the contribution of gas desorption can result in under-prediction of the gas production performances. On the other hand, using adsorption parameters to simulate desorbed gas volume could result in over-estimation of gas production performances.