A SPATIOTEMPORAL CHARACTERIZATION METHOD FOR THE MECHANICAL CONTRIBUTION INTENSITY IN WATERFLOODING OF LOW-PERMEABILITY RESERVOIRS

After long-term waterflooding of low-permeability reservoirs, areal, interlayer and intralayer injection–production conflicts become pronounced, and the residual oil is distributed in a highly heterogeneous manner. Conventional regulation strategies are based directly on the residual oil distribution; the residual oil, however, is merely the final static manifestation of the flow process, and although numerical simulation can delineate where it accumulates, it cannot reveal the mechanical origin of that accumulation, leaving the choice of regulation measures without an a priori mechanical criterion. To this end, a numerical model coupling the nonlinear flow mechanisms of low-permeability reservoirs is established. Starting from the oil–water two-phase flow equations, the viscous, capillary and gravitational actions are cast into a unified dimensional form, the concept of force contribution intensity is proposed, and a spatiotemporal characterization method for the macroscopic force contributions during waterflooding is thereby developed, through which the control exerted by reservoir heterogeneity and development conditions on the spatiotemporal evolution of the force contributions is systematically revealed. The results show that the viscous-dominated zone is concentrated along the injector–producer connecting line, whereas the capillary-dominated zone lies immediately adjacent to the waterflood front; high-permeability streaks induce a directional extension of the viscous-dominated zone along the streaks; and the mechanical coupling between gravity and the high-permeability channel acts in opposite senses for different depositional rhythms—in fining-upward reservoirs the two are superimposed in the same direction and aggravate basal water channelling, whereas in coarsening-upward reservoirs they act in opposing directions and suppress breakthrough. Such force patterns essentially determine where the residual oil is retained, and the dominant force type can be altered by adjusting the production regime, which provides a mechanical criterion for the regulation of waterflood fields. Application to the SPE 10 model demonstrates that the mechanics-dominated zones delineated by the relative magnitudes of the force contributions effectively indicate both the residual oil distribution and its mechanical origin; after an additional injector was introduced in the capillary-dominated zone to strengthen the viscous driving force, the dominant force type in that zone shifted from capillary to viscous and the cumulative oil production increased substantially. This study provides a theoretical basis for selecting regulation strategies in the late stage of waterflood development in low-permeability reservoirs.