Mechanistic Modeling of Modified Salinity Waterflooding in Carbonate Reservoirs
Authors: Korrani Aboulghasem Kazemi , Fu Wensi , Sanaei Alireza , Sepehrnoori Kamy 
Published: September 2015

Abstract
Low primary recovery factor in carbonate reservoirs creates a tremendous potential for enhanced oil recovery (EOR). However, reservoir conditions limit the number of applicable EOR methods in these reservoirs. Recently, several investigations have focused on tuning ion compositions of the injected water to improve oil recovery. While this new process is supported by extensive promising laboratory and single-well chemical tracerk tests, the underlying mechanism of this process is still a subject of ardent debate.
UTCOMP-IPhreeqc is a coupled multi-phase reactive-transport simulator developed in The University of Texas at Austin to comprehensively model the modified salinity waterflooding. This simulator combines the geochemical modeling power of IPhreeqc, the USGS geochemical package, with the important aspects of hydrocarbon flow of a compositional reservoir simulator, UTCOMP. Thermodynamic constrains are used to explicitly include the effect of the water-soluble hydrocarbon components (carbon dioxide, methane, etc.) in this coupled simulator. Hence, this combines the geochemical power of a comprehensive geochemical package with the important aspects of hydrocarbon flow and compositional effects to produce a robust, flexible, and accurate integrated tool capable of including the reactions needed to mechanistically model modified salinity waterflooding. Consistent with several laboratory observations, we believe the underlying mechanism in carbonates during modified salinity waterflooding is combination of calcite dissolution and surface reactions. Calcite dissolution is explicitly introduced in our model whereas surface reactions are implicitly included. In the model implemented, calcite is dissolved and that liberates the adsorbed oil from the surface and hence, fresh surface with the wettability towards more water-wet is created. Moreover, calcium, magnesium, and sulfate ions can go through surface reactions. Therefore, we model wettability to be dynamically altered as a function of calcite dissolution and surface reactions in our integrated simulator. We then apply the tool to model not only the oil recovery but also the entire produced ion histories of a recently published coreflood. Surface reactions are mandatory to model produced ion histories and improved oil recovery. This confirms the extensive discussion in the literature on the importance of calcium, magnesium, and sulfate surface reactions in carbonates during modified salinity waterflooding. Moreover, if we assume surface reaction to be exothermic, our model predicts the right trend of increased oil recovery with increasing temperature. To study the effect of modified salinity waterflooding in large scales, we scale up the coreflood modeled to a synthetic field. Our simulation results of the synthetic field demonstrate that modified salinity waterflooding also shows promising results in large scales. No improved oil recovery due to modified salinity waterflooding is observed in large scales when surface reactions are excluded from the model. The reason is that: if the underlying mechanism depends solely on the calcite dissolution then the injected water becomes saturated for ions after penetrating only small distance into the reservoir. Hence, no more dissolution and subsequently no more wettability alteration occur afterwards.