Analysis of Gas Storage and Transport in Shale Using Pressure Pulse Decay Measurements with He, Kr and Co2

Abstract

To predict the recovery of natural gas resources or to estimate the carbon sequestration potential in shales accurately, it is critical to represent the relevant transport and storage mechanisms, that include viscous flow, slip flow, transition flow, Knudsen diffusion, and sorption in multi-porosity (fracture/matrix) systems. Several efforts have been published in the literature aiming to improve our knowledge of how fluids are transported and stored in shale, however, a comprehensive evaluation of shale samples continues to pose a significant challenge.In this work, we investigate gas transport and sorption in shale using inert (helium -He) and adsorbing (krypton - Kr and carbon dioxide - CO2) gases by performing pressure pulse-decay measurements on an Eagle Ford shale core sample. Pressure pulse-decay measurements with He (nonsorbing gas) are used to assess the overall porosity. The overall porosity of the sample consists of natural fractures, microcracks, mesopores, and micropores. We present a modified analytical approach for evaluating the mass transfer rate in microcracks and mesopores. A previously established triple-porosity model (TPM) is then utilized to estimate the remaining parameters: the effective gas permeability in the fracture, the mass transfer rates in micropores, and the volume split between mesopores and micropores, by matching the pressure decay data of two He pulse experiments.Based on the model parameters obtained, the pressure decay behavior of a 3rd He pulse experiment (at higher pressure) is accurately predicted. For adsorbing gases (Kr and CO2), excess adsorption isotherms at varying bulk pressures, were evaluated by comparing the equilibrium pressure at each pressure stage to the pore volume estimated from He. The adsorption isotherms were then utilized in the TPM to predict gas transport and storage for Kr and CO2: The gas transport behaviors and storage were predicted with transport coefficients translated from the He. Predictions for Kr and CO2 are demonstrated to be in excellent agreement with experimental observations, indicating that the modified analytical approach can effectively characterize mass transfer in shales and provide direct input to a TPM representation of the shale core. We furthermore demonstrate and discuss the importance of the density model for the adsorbed phase in the interpretation of transport and storage. A key contribution of the presented workflow is the demonstration that a TPM approach is effective for interpretation and prediction of gas transport/storage during pressure pulse-decay measurements on shale cores without a need to discretize the shale matrix.