Impact of methane adsorption on tight rock permeability measurements using pulse-decay

Abstract

The permeability of the rock is usually measured by the injection of gas using Darcy's flow model (pulse-decay). For oil formations, helium and nitrogen are the most common gases used to measure the permeability of the rock. However, recent approaches are based on the use of methane as it minimizes the properties difference between the testing fluid and reservoir fluid. This work focused on the latter approach to compute the correction of gas adsorption. The most widely used model is Cui et al. model that is based on Langmuir adsorption isotherm. In this work, we introduced a modified model that is based on Freundlich isotherm. This model also includes the correction for gas adsorption such as Freundlich isotherms proved to be more appropriate for the adsorption on intact reservoir rock. The model is based on gas and rock properties and reduced pressure and temperature were used to accommodate the gas compressibility. The modified model can also capture effective porosity of adsorption (ϕa) that can correct the pulse-decay storage capacity parameters a and b. The permeability estimation of ultra-tight samples using the modified approach is enhanced owing to the correction in the storage volume and rock porosity. Including the proper adsorption isotherm enhanced the porosity estimation because Langmuir isotherm yielded 11% porosity and Freundlich isotherm yielded 12% porosity. Similar results were obtained in the permeability estimation, Langmuir isotherm resulted in a 1.5% error compared to zero error in the Freundlich isotherm estimation. Keywords: Methane adsorption, Pulsed decay, Tight rocks, Permeability measurements