Print this pageLow Flood Rate Residual Saturations in Carbonate Rocks
Carbonate reservoirs commonly exhibit great morphological complexity from pore to field scale. Interpretation of laboratory waterfloods for estimations of reserves and production strategies is often problematic because of unexpected sensitivity of oil recovery to flood rate at 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. Petrophysical observations and measurements include optical and UV reflectance, thin section analysis, SEM, BET surface areas, cation exchange capacities, mercury injection capillary pressures, and water adsorption isotherms. The rocks were tested at very strongly water-wet (VSWW) conditions followed by preparation of mixed-wet (MXW) states. A comparative study of waterflood recovery was made for mixed wetting states with crude oil (MXW) or mineral oil as the test oil. Mineral oil was tested after either direct displacement of crude oil (MWX-F-DD) or first displacing crude with an intermediate solvent to avoid surface precipitation of asphaltenes (MXW-F). Flooding rates ranged from below to well above field rates. Sensitivity of residual oil saturation to flood rate ranged from slight for a homogeneous grainstone to distinctly significant for both a heterogeneous grainstone and a boundstone of very high porosity and permeability. The mechanism by which incremental oil is produced with increase in flood rate was investigated by using a tracer test technique to track the production of connate water.