(159e) Measurements and Modeling of High-Pressure Gas Adsorption on Shales

In the coming years, recovery of natural gas from shale reservoirs is expected to increase significantly due to technological advancement in drilling and horizontal fracturing. Shale formation is categorized as unconventional gas reserve where a large amount of gas is adsorbed within the shale matrix. Therefore, knowledge of adsorption behavior is important in predicting reservoir gas content. However, availability of adsorption data on shales is very limited, especially at higher pressures, which hinders development of generalized predictions offered by theory-based adsorption models.

To enrich the gas adsorption data on shales, adsorption isotherms of methane, nitrogen and CO₂ were measured on several shale samples. The measurements were made at 328 K and pressures up to 12.4 MPa. The expected experimental uncertainties were estimated by propagating errors in the experimentally measured quantities. Further, the experimental design was re-optimized to minimize the expected experimental errors by minimizing the void space in the adsorption cell and injecting gas at pressures that reduced the uncertainty in the density.

A theory-based model such as the simplified local-density (SLD) model was applied to describe the adsorption data on diverse shales. The SLD model was found capable of representing the adsorption capacities on these shales with satisfactory precision. Further, several case studies were undertaken to investigate the predictive capability of the SLD model when applied to shales.