Highly Efficient and Simplified Method for Measuring the Permeability of Ultra-low Permeability Rocks Based on the Pulse-Decay Technique

Permeability is one of the most important parameters for the evaluation and stimulation of unconventional gas reservoirs. Although several methods are used to measure the permeability of rocks, most approaches are not practical for measuring the permeability of rocks with ultra-low permeability, such as most gas shales or tight sandstones, because highly accurate devices and long test durations are necessary to determine the permeability. However, the testing efficiency can be easily improved by decreasing the flow-through trace and enlarging the flow-through area simultaneously with the simplified pulse-decay method on hollow cylindrical samples (SPMHCS). A simplified and valid mathematical model was established based on the assumption that seepage flow of an ideal gas through a hollow specimen is governed by Darcy’s law, and the compressibility of the upstream and downstream reservoirs, pipes and rock and temperature changes are ignored. Subsequently, the permeability of shale and tight sandstone was measured and discussed based on SPMHCS in the laboratory. The results show that the developed method is practical and accurate for measuring the permeability of ultra-low permeability rocks. Moreover, the new method requires much less time than traditional approaches. By discussing the sensitivity of parameters, some helpful and beneficial suggestions can be summarized: (1) ensuring that the upstream and downstream reservoirs have the same volumes is very important for determining the radial permeability; and (2) an appropriate test time is needed for the dimensionless pressure difference across the hollow sample (ΔP_D) to reach a specific value based on the ratio of the sample pore volume to the reservoir volume to ensure that the system error is less than 10%.