Evaluation of residual oil saturation in a large carbonate field was undertaken to reduce the uncertainty of reservoir properties for STOIIP and remaining reserves booking. Horizontal and vertical sweep in the field has been excellent (RF now exceeds 50%), large variations in the residual oil saturation after waterflood (Sorw) have been observed (logs, PNC) and are beginning to impact heavily on remaining moveable oil. The large variations (15% – 35%) are severely exacerbated by uncertainty in Archie parameters.

Research into available methodologies showed that sponge coring, coupled with SCAL experiments, would provide the best Sorw estimates. The primary objective of SCAL was to determine Sorw directly from the sponge core from a vertical well with a waterflooded oil-leg. In this way the results of open and cased-hole logging can be better understood. Wettability evaluation in this SCAL program was performed using USBM and Amott-Harvey techniques. Rock-fluid interaction data were used to understand and explain the reservoir wettability.

Twenty two residual oil saturation (Sorw) values were measured through three different lab tests: centrifuge imbibition capillary pressure, centrifuge oil relative permeability and steady-state oil-water relative permeability. Sorw values were grouped based on the lab test and carefully studied. An average residual oil saturation of 22% was calculated. Field observations of interpreted pulsed-neutron-capture logs (PNC) support the average residual oil saturation of 22%. Both core and PNC data suggest a strong correlation of Sorw to Porosity.

The field, in the Shuaiba Formation has been successfully waterflooded since 1972, some 3 years after production commenced. Initial development of the waterflood pattern was based upon a 1km2 inverted 9 spot pattern, with intra-oil-zone injection. Later re-completion campaigns converted the waterflood from a lateral drive to provide more basal water drive. Pore volumes injected by 2006 constitute approximately 90% of STOIIP.

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