Transition Zone Behaviour: The Measurement of Bounding and Scanning Relative Permeability and Capillary Pressure Curves at Reservoir Conditions for a Giant Carbonate Reservoir

Abstract

Abstract Significant reserves are held in the transition zone of an Abu Dhabi carbonate reservoir, but uncertainty exists in how relative permeability (kr), capillary pressure (Pc) and residual saturations vary with initial water saturation throughout the column. A study has been undertaken to provide an improved understanding of the water flood recovery and remaining oil saturation distribution within the transition zone by better definition of the scanning kr and Pc curves. Primary drainage, and imbibition from irreducible water saturation were measured for both kr and Pc to provide the "bounding" data. Imbibition kr and Pc curves were then measured at three higher initial water saturations up to 0.60PV. The semi dynamic method was used for Pc measurements and steady state method for kr. All tests were performed with live reservoir fluids at full reservoir conditions with in-situ saturation monitoring (ISSM). Drainage curves are often measured in the laboratory with refined oil and then aged with live crude oil at Swirr. This may not truly reflect the reservoir wetting conditions and so in this study both the primary drainage kr and Pc measurements were performed with live reservoir crude oil. During the drainage tests an ageing period was allowed at each progressively higher oil saturation as new regions of the pore space were exposed to crude oil. Transition zones are typically modelled using curves that "scan" from the drainage curve origin to the imbibition curve as water saturation increases. Models typically show a distinct separation between the drainage and imbibition bounding curves, within which the scanning curves are located. The shape of the scanning curve is dependent on the model used and typically retains the characteristic shape of the bounding curve. Results from this study showed: little spread in the bounding curves with minimal "hysteresis space" for both the kr and Pc datasets. the water imbibition bounding curve to be of lower relative permeability than the water drainage bounding curve, contrary to many models. no clear relationship of remaining oil saturation to initial water saturation. These results may have a significant impact on how the transition zone modelling is implemented for this carbonate reservoir. This study adds to the literature data set on a topic where few full reservoir condition laboratory measurements have been published.