The Effects of Pore Geometry and Wettability on Flow Properties in an Arab Reservoir in Bahrain

Abstract Core measurements are critical for reservoir modelling and optimizing long-term production. In carbonate reservoirs, heterogeneity complicates the task of reservoir description and thus necessitates the establishment of accurate understanding of the geology and its impact on petrophysics and reservoir engineering. The objective of this paper is to evaluate wettability and its effects on capillary pressure and relative permeability curves for a wide range of rock types in an Arab reservoir in Bahrain. Representative dolomite and limestone reservoir rock types were identified based on down-hole logs in combination with dual-energy core CT imaging, porosity-permeability data, thin-section descriptions and high-pressure mercury injection capillary pressure curves. Core samples’ porosity values ranged from 0.15frac to 0.35frac, while absolute permeability varied between 1 mD and 2000 mD. Modified Amott-USBM measurements provided insight into rock wettability states and advancing contact angles. Primary drainage-imbibition relative permeability curves were measured at reservoir conditions using steady-state fractional flow technique with in-situ saturation monitoring. The different rock types gave similar initial oil saturations (i.e., ~ 0.90 fraction) and similar endpoint oil relative permeability (i.e., 0.70 to 0.90). Residual oil saturation (Sor) ranged from 0.10frac to 0.20frac, and the endpoint water relative permeability (Krw(Sor)) varied between 0.30 to 0.60. The dolomite samples showed a tendency towards more oil-wetness, but did show a major impact on flow properties. Low permeability limestone samples tend to show less oil wet conditions, which can have an impact on endpoint saturations and relative permeability. Drainage relative permeability curves showed a complex behavior that could not be modelled by a simple exponential function like Corey. However, the Corey function gave a reasonable match to the experimental imbibition relative permeability data. Nevertheless, all the relative permeability samples gave similar drainage-imbibition hysteresis trends indicating oil wet to mixed wet conditions. Amott-Harvey Wettability Index ranged from +0.40 to -0.96 with corresponding advancing contact angles varying from 105 degrees to 180 degrees. This study provided detailed classification of rock types in a heterogeneous Arab reservoir in Bahrain. Consistent results were obtained from multi-speed oil-water centrifuge and steady-state oil-water core flood relative permeability experiments. Variations in saturation and flow properties were understood based on geological textures, pore types and wettability. The study examined wide range of porosity and permeability values and provided unique experimental uncertainty boundaries that can be used in similar rock types, which are usually lacking for proper reservoir simulation modeling.