ABSTRACT Experimental measurements of capillary pressure, resistivity index and relative permeability display hysteresis manifested through the dependence of these properties on the saturation path and saturation history whenever fluid saturations undergo cyclic processes. At the pore scale, hysteresis is typically influenced by contact-angle hysteresis, trapping of one phase by another and wettability changes.
A laboratory study was conducted to investigate hysteresis effects measured on reservoir core plugs for a major carbonate hydrocarbon reservoir in the Middle East. Representative core samples covering reservoir rock types (RRT) were selected based on whole-core and plug X-ray CT, high-pressure mercury injection, porosity, permeability and thin-section analyses.
Primary drainage and imbibition capillary pressure and resistivity index (PcRI) were measured by the porous-plate method using stock tank oil and simulated formation brine at reservoir temperature and overburden conditions. Large hysteresis effects were obtained between primary drainage and imbibition for both Pc and RI curves. Low residual oil saturations (Sor) were measured at the end of forced imbibition indicating oil-wet to mixed-wet characteristics. Nonlinear RI curves were found during imbibition which could not be described by conventional Archie equation. Water-oil relative permeability curves were measured on similar reservoir core samples by the steady-state technique using live fluids at full reservoir conditions with in-situ saturation monitoring. Hysteresis effects on both oil and water curves were observed between primary drainage and imbibition, and appear to be influenced by the sample rock type involved (i.e. wettability and pore structure).
The strong hysteresis in RI was explained by a fluid invasion behavior at the pore level, and was attributed to varying displacement mechanisms between primary drainage and imbibition. Conventional assumption of Archie behavior is therefore not always valid for such carbonate rock types. This RI hysteresis, together with the variation of K hysteresis trends with different rock types, may help improve the current hysteresis models and provide better understanding of the hysteresis phenomena in natural porous media.