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.