An optimized transient technique and flow modeling for laboratory permeability measurements of unconventional gas reservoirs with tight structure

Considering the significance of pressure-dependent-permeability in characterizing fluid flow and production from gas reservoirs and the inaccuracy of the conventional transient technique in laboratory permeability measurements of core samples from unconventional reservoirs with tight structure and strong sorption potential, the conventional transient technique is optimized by using two equal-sized gas reservoirs mounted in the up and downstream sides of the experimental setup and creating same magnitude of pressure pulses in each reservoir concurrently. Applicability of the optimized transient technique is examined through both numerical simulation and laboratory tests. A mathematical model is firstly developed to numerically investigate fluid flow behavior in time and space domain. The results from numerical simulation showed that the optimized technique can greatly improve the efficiency and accuracy of the measurement, because a steady flow state as a function of time can be achieved much faster in comparison with the conventional transient technique when the same magnitude of initial pressure difference is established between the two ends of the sample. Given the difference in experimental design and procedures between the conventional and the optimized technique, it is questionable whether currently used analytical solutions developed for the conventional technique for permeability calculation can be applied for the optimized technique. Therefore, an analytical solution, corresponding to the optimized transient technique, has been then theoretically derived. It is finally experimentally verified that accurate permeability results can be obtained based on late-time pressure responses by the derived analytical solution. The optimized experimental method is capable of extending the application of the transient technique into permeability measurements of rocks with tight structure and strong gas sorption potential, providing a more reliable approach for laboratory permeability tests.