Digital core analysis: change of filtration-capacity properties of UGS reservoir after geomechanical impact

In this paper we present the results of a series of comprehensive studies of residual changes in the filtration-capacitance properties of reservoir rocks of an underground gas storage facility caused by the impact of a non-uniform stress field during modeling of controlled pressure changes at the bottomhole. As part of digital studies of core material, multi-scale CT-images were obtained, on the basis of which three-dimensional models of rock pore space were created, homogeneity of rock structure and composition was analyzed, followed by geometric and statistical analysis of pore space, as well as numerical modeling of filtration processes in the rock to estimate permeability before and after mechanical tests. Measurement of porosity, geodesic tortuosity of pore space, estimation of pore size distribution, and analysis of rock filtration properties were performed. Spatial distributions of filtration flow velocities and pressure gradient changes were plotted. Within the framework of geomechanical tests, the study of rock deformation and filtration processes under the action of real stresses occurring in wells under controlled change of pressure in the well was carried out. The tendency to insignificant decrease of values of open porosity, permeability and pore geometry after non-uniform triaxial loading is shown. Conclusions about homogeneity and high connectivity of the pore space of the rocks under consideration are made. The absence of significant irreversible changes in the filtration-capacitance properties of rocks is shown. The characteristics obtained as a result of numerical analysis showed very good agreement with the results of field and laboratory measurements. The results of the work and the applied techniques can be used in the creation and adjustment of reservoir models, as well as in the development of an operational model of UGS. The presented technique of CT-data analysis allows to simplify data acquisition on core material characteristics, as well as to expand the results of laboratory and field studies of reservoir properties.