Deformation of the void space of pores and fractures of carbonates: Comprehensive analysis of core and field data

Many new and developed oilfields are confined to carbonate reservoirs with a complex void space structure. In this study, the behavior of carbonates as a function of decreased reservoir pressure was examined. The study was based on actual production data from wells rather than just on model data or results from computational experiments. Well-test data (170 well tests) and laboratory core samples (45 samples) results were used in the analysis. The problem of reservoir deformation has been comprehensively addressed at the micro and macro levels. The effect of reservoir permeability throughout the entire reservoir volume on reservoir pressure was determined. With a decline in reservoir pressure, the permeability of fractures and pores in carbonates decreases. The deformation coefficients, which characterize the actual rate of decrease in permeability with decreasing reservoir pressure, were also calculated. The deformation coefficients and data from core studies using μ-CT and SEM were compared. Based on micro-CT data, the effective diameters of the voids were calculated to be 93 μm, 109 μm, and 140 μm for the Vinnikovskoe, Sofinskoe, and Sukharev fields, respectively. Based on μ-CT and SEM data, the predominant geometric shapes of the voids were assessed. The deformation coefficient of voids was found to depend on their size and geometric shape. The study's findings revealed deformation coefficients of 0.172, 0.205, and 0.748 for the Vinnikovskoe, Sofinskoe, and Sukharev fields, respectively. Maximum deformation coefficients are typical for large voids with a predominant slot-like shape. However, even with the predominance of primary spherical voids in carbonates, the permeability of voids decreases with decreasing reservoir pressure.