Laboratory Investigation of Shale Permeability

Abstract

Abstract Matrix permeability of shales is an elusive but important parameter in characterizing shale gas reservoirs. Permeability is typically measured using steady-state flow tests or the more timely transient methods such as pulse-decay. Due to the low permeability nature of shale rocks, slip flow regime is observed to be dominant in pore scale. As a result, permeability changes with pore pressure. Traditionally, permeability is measured at various mean pore pressures and the data is used to extract the Klinkenberg or absolute permeability. However, it has been shown recently that Klinkenberg permeability significantly overestimates the absolute (liquid) permeability of shale rocks. Additionally, several studies have shown different methods of permeability measurement can lead to significantly different results at similar pressures. In this work, steady-state laboratory gas flow experiments have been conducted on four shale samples using methane and nitrogen as flowing fluid. At each stage, mean pressure and mean effective stress is held constant and permeability is measured at various gas flow rates. Mean pressure is then raised and the tests are repeated at the next stage. The tests are designed to study the influence of mean pressure as well as flow rate on gas permeability. Subsequently, the samples are sheared in a triaxial cell and similar permeability tests are repeated to compare the permeability behavior before and after shearing. Finally, the samples are saturated with water to measure water permeability in order to analyze the gas permeability observations using theory. The results indicate strong rate sensitivity in shale permeability measurements. Measured shale permeability is observed to rise as flow rate increases and reaches a constant value at higher rates. Permeability of sheared samples shows a similar trend although with higher permeability values. This phenomenon casts a shadow of doubt around the common non-steady-state permeability measurement methods and has never been discussed previously with respect to shale gas reservoirs. The basic definition of gas permeability needs to be revisited in order to set up new standards (concerning testing pressure, rate, and stress state) in order to obtain meaningful and comparable permeability measurements.