Water Saturation Estimation in Oil-Wet Reservoirs
Mantilla, Andres E.
The estimation of water saturation from electrical logs in oil-wet reservoirs is non-trivial, as the relative position of water and oil molecules with respect to mineral surfaces in an oil-wet pore network is different than that of water-wet rocks, and this significantly influences electrical transport phenomena.
In this paper I refer to the Archie model, and other models for water saturation estimation from resistivity and porosity logs; elaborate on the reasons behind their limited applicability to oil-wet reservoirs; introduce two new models, specifically tailored to address the resistivity problem in partially saturated oil-wet porous media; discuss the parameterization of the models in relation to pore geometry, and rock quality; present an approach to identify oil-wet reservoirs in logs; and illustrate the approach with an example from two reservoirs from the same geographic region, depositional setting, and geologic age, one preferentially oil-wet, and the other neutrally- to water-wet.
In preferentially oil-wet rocks, water is pushed away from grain surfaces into the middle of many pores and pore throats. As water is displaced from the pore space during buoyancy-driven primary drainage (e.g., trap charging), or later as a result of wettability alteration processes, pathways through the water phase -between opposite sides of any representative elementary volume- become increasingly more tortuous, as water blobs left in the middle of pores become isolated from water molecules in adjacent pores, by oil-wet throats. At a certain critical value of water saturation the continuity of water films becomes too ruptured, and electrical conductivity starts to reduce at a much faster than the normal rate, with every subsequent reduction in water saturation. This does not happen in water or fractionally-wet media, where the continuity of water films is better kept through water-wet surfaces and networks of small, water-wet micropores. As a consequence, at any water saturation below that critical point, oil-wet rocks can exhibit electrical resistivity values that are much higher (orders of magnitude in some cases) than those of neutrally- or water-wet rocks with identical pore geometry and brine salinity. The lack of appreciation of this effect may lead to significant error in water saturation estimates.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013