Effect of gas adsorption on pulse-decay permeability measurements

Pulse-decay method is a technique used to estimate cores permeability using gas injection based on Darcy's flow model. Helium and nitrogen are generally used when the target formation is an oil formation or liquid permeability is sought. However, new approaches suggest using CH4 to avoid correcting for properties difference between the testing fluid and reservoir fluid. This study focused on the latter approach to quantify the correction to gas adsorption. The Cui et. al, model based on Langmuir adsorption isotherm is modified to Cui-Freundlich isotherm which has been proven adequate to fit the adsorption on intact reservoir rocks. The model is developed using gas and rock properties and the approach is generalized by using reduced pressure and temperature to account for the gas compressibility. The results of the modified model showed that the new model can capture effective porosity of adsorption used to correct the pulse-decay storage capacity parameters a and b. This correction in the storage volume and rock porosity values will enhance the permeability estimation in the ultra-tight rock samples.