Relative permeability curves generally exhibit hysteresis between different saturation cycles. This hysteresis is mainly caused by wettability changes and fluid trapping. Different rock types may experience different hysteresis trends due to variations in pore geometry. Relative permeability curves may also be a function of the saturation height in the reservoir.
A detailed laboratory study was performed to investigate relative permeability behavior for a major carbonate hydrocarbon reservoir in the Middle East. Representative core samples covering five reservoir rock types (RRT) were selected based on whole core and plug X-ray CT, NMR T2, MICP, porosity, permeability and thin-section analyses. Primary drainage and imbibition water-oil relative permeability (bounding) curves were measured on all the five rock types by the steady state technique using live fluids at full reservoir conditions with in situ saturation monitoring. Imbibition relative permeability experiments were also conducted on the main RRT samples to assess the relative permeability (scanning) curves in the transition zone by varying connate water saturations.
Hysteresis effects were observed between primary drainage and imbibition cycles, and appeared to be influenced by the sample rock type involved (i.e. wettability and pore geometry). Variations in relative permeability within similar and different rock types were described and understood from local heterogeneities present in each individual sample. This was possible from dual energy CT scanning and high resolution imaging. Different imbibition trends from both oil and water phases were detected from the scanning curves which were explained by different pore level fluid flow scenarios. Relative permeability scanning curves to both oil and water phases increased with higher connate water. Relative permeability to oil was explained based on the occupancy of the oil phase at varying connate water saturations. The change in water relative permeability trend was addressed based on the connectivity of water at the varying connate water saturations. These results and interpretations introduced improved understanding of the hysteresis phenomena and fluid flow behavior in the transition zone of a cretaceous carbonate reservoir which can assist in the overall reservoir modeling and well planning.