Measured permeability of an organic-rich shale sample varies significantly with applied laboratory conditions, such as the confining pressure, temperature, and the measurement fluid type. This indicates that the measured quantity is influenced by several mechanisms that add complexity to the measurement. The complexity is mainly caused by stress dependence of the matrix permeability. Also, it is because organic-rich shale holds significant volumes of fluids in sorbed (adsorbed, dissolved) states; sorption can also influence the permeability through its own storage and transport mechanisms. The stress-dependence and sorption effects on permeability could develop under the reservoir conditions and influence the production, although we currently do not have a predictive permeability model that considers their coexistence.
In this work, this is accomplished by considering that the shale matrix consists of multiple continua with organic and inorganic pores. Stress dependency of the permeability comes along with slit-shaped pores, whereas the sorption effects are associated with nanoscale organic capillaries. A simple conceptual flow model with an apparent shale permeability is developed that couples the molecular-transport effects of the sorbed phase with the stress dependence of the slit-shaped pores. The simulation results show the impact of the permeability model on the production.
Sensitivity analysis on the new permeability model shows that the stress dependence of the overall transport is significant at high pore pressure, when the effective stress is relatively low. Diffusive molecular transport of the sorbed phase becomes important as the stress gets larger and, hence, the slit-shaped pores close. The constructed apparent-permeability vs. pore-pressure curves show the dominance of the molecular transport as an increase in permeability characterized by appearance of a minimum permeability value at the intermediate values of the pressure. One can use the new permeability model easily in history matching a well performance and optimizing its production.