Carbonate reservoirs commonly exhibit great morphological complexity from pore to field scale. Interpretation of laboratory waterfloods is often problematic because of unexpected sensitivity of oil recovery to flood rates comparable to field values. The circumstances under which rate sensitivity occurs need to be further identified. In this work, three outcrop limestones with distinct differences in petrophysical properties were selected for investigation of the combined effect of pore structure and wettability on residual saturations. The rocks were tested at very strongly water-wet conditions followed by preparation of mixed-wet states. A comparative study of waterflood recovery was made for mixed wettability with crude oil or mineral oil as the test oil. Mineral oil was tested after either direct displacement of crude oil or first displacing crude with an intermediate solvent to avoid surface precipitation of asphaltenes. Flooding rates ranged from below or near field rates to well above with increase in capillary number achieved by increase in flood rate. Reduction of residual oil saturation with increase in flood rate ranged from slight for a homogeneous grainstone to highly significant for both a heterogeneous grainstone and a boundstone of very high porosity and permeability.
The magnitude of the ratio of viscous to capillary forces is usually expressed as capillary number. In waterflood experiments, capillary number represents the ratio of viscous driving forces to capillary retaining forces that control the retention of residual oil in waterflood experiments. Taber listed the numerous expressions used for this ratio and many of them are equivalent. The following definition for capillary number is employed in this paper.
Early laboratory studies of rate effects, usually related to tertiary oil recovery by surfactant flooding, examined the mobilization of trapped oil blobs or ganglia from very strongly water-wet (VSWW) sandstones and unconsolidated media. The entrapment of nonwetting phase during waterflooding is caused by capillary action.