A Model for Wettability Alteration in Fractured Reservoirs

Abstract

Many of the world's oil reservoirs are naturally fractured. Injected water will flow into this highly permeable network and not sweep the matrix containing most of the oil. Spontaneous imbibition can drive oil into the fractures by capillary forces. However the wettability of the reservoir is critical for the ultimate potential of oil recovery. Experimental results indicate that exposing chalk cores to seawater or similar brines may alter the wettability towards a more water-wet state. In a fractured reservoir this exposure will happen by molecular diffusion and by water imbibing into the matrix. The injected brine is not in chemical equilibrium with the in-situ rock, water and oil so that chemical reactions occur. Experiments have shown that for chalk rocks the relevant processes are adsorption/desorption of ions on the mineral surface, precipitation/dissolution of minerals or ion exchange. We have developed a model where a reservoir is defined by a fracture surrounded by matrix on both sides. The injected brine contains a component that can adsorb on the matrix surface. These changes of the mineral surface are assumed to alter the wettability towards a more water-wet state. The wettability alteration is described by shifting curves for relative permeability and capillary pressure from curves representing oil-wet conditions towards curves representing more water-wet conditions. The model can make predictions regarding the effect of matrix properties such as wettability and external parameters such as schedule of brine compositions and injection rate. The understanding of the coupling between wettability alteration controlled by water-rock chemistry and fracture-matrix flow is highly relevant for gaining more insight into the observed high recovery in the North Sea chalk field Ekofisk. TX 75083-3836, U.S.A., fax +1-972-952-9435

Modeling of relative permeabilities including dynamic wettability transition zones

“…The simulated data was used to formulate and quantify a interpolationbased capillary pressure model at the Darcy scale. The new dynamic model resolves the existing interpolation models used in the studies of reservoir simulation [28,25,3,27,62] by including the dynamics in time and quantifying the pore-scale WA process to the interpolation model in a systematic manner.…”

“…Other alternatives are models designed to handle the instantaneous WA process in the relative permeabilities. The first class of these models involves a heuristic approach that interpolates between the initial and final wetting states in which the WA effect is captured as a coefficient function [25,3,27,28,62]. Interpolation models are conceptually simple, while the initial and final wetting states are characterized by standard functions, e.g.…”

Numerical Investigation of Non-Darcy Flow Regime Transitions in Shale Gas Production

“…The transport system described by Eqs. (41) and (42) is solved numerically using an operator splitting approach like that presented in Andersen et al (2014Andersen et al ( , 2015, Berawala et al (2019) and Berawala and Andersen, (2020). The coupled system is split into two subsystems, one for the flow in y-direction (fracture diffusion) and second, for the flow in x-direction (fracture/matrix diffusion).…”

“…This extends work presented in Berawala et al (2019) focusing on flow regime characterization to now also consider and focus on non-Darcy flow mechanisms. Similar to the works by Mainguy and Ulm (2001) and Andersen et al (2014Andersen et al ( , 2015 the model consists of a high-permeable fracture (length L y ) with width 2b. This depicts a typical hydraulic fracture in a real-field scenario.…”

Numerical Validation of Chemical Compositional Model for Wettability Alteration Processes

“…The results introduce a new method for monitoring the wettability change of porous media during dynamic injection processes. Andersen et al (2015) presented a mathematical model for wettability alteration in fractured reservoirs [18]. The wettability alteration is described by shifting curves for relative permeability and capillary pressure from curves representing preferentially oil-wet conditions toward curves representing more-water-wet conditions.…”