Wettability Assessment in an Asphaltic-Shaly Sandstone Reservoir Using Relative Permeability, Capillary Pressure, and Resistivity
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Date
2021-05-25
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Abstract
Wettability is an important aspect of reservoir characterization. However, measuring in-situ reservoir wettability is challenging. The composition of the reservoir fluid can change during bottom hole sampling, core handling, and transferring. Because of the change of the polar molecules, the wettability assessment results can deviate from the actual in-situ condition. Overall, the change in fluid composition, pressure, and temperature can change the wettability. This study developed workflows for assessing wettability using relative permeability, capillary pressure, and resistivity data. Both numerical and analytical methods were applied to assess the wettability of an asphaltic-shaly sandstone reservoir.
A relative permeability numerical simulation workflow was employed to investigate the application of the water saturation-crossover point for wettability assessment. The effect of pore-body to pore-throat ratio on the microscopic displacement efficiency and residual non-wetting fluid saturation was observed in the forced imbibition simulation. The study shows that the wettability of the composite core samples varies from water-wet to a condition slightly above the borderline of intermediate-wet.
The application of conventional well logs for wettability assessment in the reservoir is limited. Moreover, log analysis is mostly an indirect approach and presents tremendous inherent uncertainties, especially in shaly laminated sandstone with some thin bed layers. The accuracy of the resistivity-based water saturation model can be decreased in a thin-bed reservoir. The saturation height model is an alternative solution using capillary pressure instead of resistivity. Two modified J function questions were proposed: (1) to estimate the wettability via a dimensionless capillary pressure and (2) to allow the utilization of laser particle data by using the specific surface area. The results also indicated a wettability variation from strong water-wet to intermediate-wet in the reservoir.
A new approach for using resistivity data is proposed. The wettability change was assessed by calculating the deviation of the resistivity index curve from the strong water-wet condition. A modified Waxman-Smits equation is proposed to distinguish the saturation exponent between the free pore and bound water. It was observed that the degree of wettability change increased proportionally with an increase in the characteristic pore radius.
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wettability, relative permeability, capillary pressure, resistivity