Effect of Gas Adsorption on the Estimation of Gas in Place (GIP) in Conventional and Unconventional Reservoirs

The estimation of the gas in place (GIP) in conventional and unconventional reservoirs is crucial for the gas industry. Further development in gas fields depends on the gas stored in the underground reservoirs. Several methods are used to determine the GIP, such as material balance calculations or decline curve analysis. If the gas adsorption effect is considered and well modeled using Langmuir isotherm, for instance, for the unconventional reservoirs, it is not the case for the conventional ones. In this study, several adsorption experiments using methane were conducted on intact conventional and unconventional rocks at high pressure and high temperature. Conventional calcite rock samples of high permeability (around 100 mD) and tight sandstone rocks of low permeability (around 0.1 mD) were used to determine the gas adsorption. Tight sandstone rocks of low permeability (around 0.1 mD) were tested for gas adsorption. Finally, unconventional shales rocks were tested for adsorption experiments as well. The adsorption isotherms were developed for all rocks, and these isotherms were used to correct the estimation of the GIP. The results of this study showed that conventional carbonate rocks can adsorb considerable amount of gas. This adsorbed gas was not taken into account in the previous estimations of the GIP. The adsorbed gas can add up to 10% of the GIP. In addition, Langmuir isotherm was not the right fit for all experiments for the different rock samples tested in this study. BET and Freundlich models were the best fit for all experiments that consider multilayer adsorption. Including BET or Freundlich models in both conventional and unconventional rocks will enhance the estimation of the GIP and further field development plans.