A Method to Measure Ultralow Permeabilities of Shale Core in Multiple Directions Using Pressure-Pulse Decay Technique

摘要

Permeability is a very critical tight reservoir parameter to characterize the process of oil/gas storage and production.In particular,distinct anisotropy of the shale reservoir,demands a practical method to measure permeabilities in multiple directions using the same core.In this work,a systematic and practical pressure-pulse decay(PPD)technique is given to determine ultralow permeabilities of shale reservoir core in a cell with finite volume.The test gas can be non-adsorption gas such as helium or adsorption gas such as methane.As described herein,given appropriate assumptions,the mathematical models based on the corresponding designed experiments(non-adsorption gas or adsorption gas)are formulated,and the exact solutions for the corresponding mathematical models are proposed using Laplace transform.Approximation techniques are further given to analyze experimental data to obtain radial and axial permeabilities.It is found that there are distinct anisotropy of shale reservoir and the permeability may be smaller when adsorption gas is used instead of non-adsorption gas.Compared with traditional two-chamber PPD techniques,its advantages mainly lie in three points: only the pressure in one chamber is measured,which can lead to fewer pressure transducers and easier experimental operation.Meanwhile,the center of shale core needs not be drilled when the radial permeability is measured.What's more,the whole measurement is based on the same core and more information of the core can be obtained.However,it should also be noted that there is about 1 MPa difference of effective stress between our axial experiments and radial experiments.The effect of this difference on permeability is typically less than 5%,so the modified PPD can detect anisotropy efficiently.