Effect of Dispersion Coefficient and Frequency of Dead-end Pores on Bypassed Oil Recovery from a Matrix-fracture System

This study experimentally investigated the effects of dispersion coefficient and frequency of dead-end pores on bypassed oil recovery during CO2 injection in fractures at different miscibility regimes (first-contact miscibility, near-miscibility, and immiscibility); a special setup was designed and a series of CO2 injection experiments were performed using two different types of rocks. In the light of dimensionless analysis and tracer experiments the rule of different forces (viscous, gravity, and capillary), mechanisms (vaporization and swelling) and matrix properties (the effect of dispersion coefficient and frequency of dead-end pores) were studied. In all miscibility regimes, the oil recovery factor decreased as rock complexity (dispersion coefficient) and the frequency of dead-end pores increased or permeability decreased; thus, types of heterogeneity and pore space structure play significant roles in oil recovery and cause some oil to be left untouched by gas injection in remote dead-end pores. In rock types with a high frequency of dead-end pores, like carbonates, conventional oil-producing mechanisms are no longer effective; the role of the near-critical-point-wetting mechanism is thus more significant. As a result, the recovery factor values of the near-critical and the super-critical tests diverge from each other.