Depth variation of wettability alteration during oil-base mud-filtrate invasion and corresponding effects on resistivity logs

摘要

Resistivity logs acquired in hydrocarbon-bearing formations invaded by oil-base mud (OBM) often indicate abnormally high values of mobile water saturation. It is not possible to explain such abnormally high values of water saturation with saturation-height analysis. The common explanation invokes rock wettability alterations due to surfactants included in oil-base mud-filtrate (OBMF). A quantitative study is needed to explain whether the interaction of OBMF surfactants with water-wetted grains can cause a sufficiently large increase in mobile water saturation in the near-wellbore region to affect resistivity logs. In this paper, we use a near wellbore simulator to model the processes of mud-filtrate invasion and ensuing wettability alteration once emulsifiers included in OBMF make contact with grain surfaces. We assume a wettability alteration model in which the degree and type of alteration are governed by the pore-volume concentration of emulsifier in OBMF within the invaded formation. Results indicate surfactants included in OBMF may change the rock's surface wettability from a water-wet to a neutral or oil wet condition. This behavior causes a fraction of the originally residual pore volume of connate water to become moveable. The radial displacement of movable water by OBMF can give rise to an annulus of water bank, which in turn causes the resistivity annulus. We perform simulations of OBMF invasion into oil and gas saturated formations. Simulated apparent resistivity logs across layers which exhibited shallow invasion showed a reverse OBM effect where deep resistivity was larger than shallow resistivity. (C) 2016 Elsevier B.V. All rights reserved.