Digital Rock Core Simulation of Waterflooding, Showing the Impact of Rock Heterogeneity on Oil Production

Abstract

Digital rock special core analysis has been used in recent times as an alternative to laboratory special core analysis (SCAL) but has failed to deliver the level of accuracy required by service companies. Current research suggests the dominant role of capillary pressure heterogeneity in flow characterization compared to viscous and gravity forces. However, the irreversible changes in relative permeability (hysteresis) during strategies for hydrocarbon recovery, have not been integrated. Hence, capillary heterogeneity and hysteresis were incorporated in numerical corefloods of primary waterflooding. Heterogeneity in the Bentheimer and Berea cores used for this study, were defined by a 3D spatial variation in capillary entry pressure obtained from experiments. Steady state Decane drainage preceded waterflooding. Capillary numbers were obtained across a range of rates depicting capillary to viscous dominated flow regimes, controlled by rock heterogeneity and distance from the well. Simulation results showed a dominance of heterogeneity in lowering oil production especially in the Berea-characterized by increased capillary strength-than the corresponding Bentheimer case. Hysteresis accelerated but decreased ultimate oil recovery, with greater impact in viscous (higher rates) than capillary dominated conditions (lower rates). Capillary number increased nonlinearly with flow rate. Also, residual oil saturation increased from low to high rates until a considerable decline ensued beyond threshold rates for which viscous pressure drop exceeded 60–3 times the highest capillary entry pressure in the Bentheimer and Berea cores respectively. Thus, the significant influence of capillary heterogeneity on oil production and the use of digital cores to estimate the irreversible oil trapping effects of hysteresis within heterogeneous rock sections is highlighted. This gives insight into effective enhanced oil recovery strategy to capture capillary trapped oil across core to field scales.