Low Salinity Water Flooding Possible Mechanisms and the impact of Injected Sulphate variation on Oil Recovery in Carbonate Reservoirs: Compositional Modelling Approach

摘要

Carbonate reservoirs store more than 50% of the earth hydrocarbon reserves with mostly mixed wettability. Water injection in these reservoirs likely results in low recovery due to uneven water movement. In the past years, it has been noticed that Low Salinity Water Flooding (LSWF) regularly led to incremental recovery. Some of the possible Enhanced Oil Recovery LSWF mechanisms on the relation of the injected fluid composition and orginal reservoir fluids.In this study the effects of altering the concentration of the injected Sulphate ions on pH, CO2, calcite dissolution and its impact on oil recovery in carbonates are investigated. Ca2+, Mg2+ and SO-4 2are determining ions to the carbonate surface which impact the surface charge of the rock (Zhang, 2006). The adsorption of the determining ions having conflicting charges Ca2+ and SO4 -2 on the carbonate rock surface is dependent on the relative concentration and equilibrium. An increase in pH causes a reaction with the acidic components of the oil which improves reservoir in-situ surfactants. This study describes LSWF five spots simulation model with 4 producers and one injector. A 3D homogeneous reservoir was built using CMG simulators specifically the compositional simulator GEM. The basic data used to build the model like PVT data, crude composition, injected and connate water geochemical compositions were collected from several case studies conducted in a carbonate reservoir. From the simulation results, pH was found to increase during the interaction between low salinity brine and reservoir rocks. The results show that dissolution of carbonate minerals can occur and this will alter the composition of the injected low salinity brine, the concentration of divalent ions on rock surface and hence the potential of low salinity to improve recovery. Calcite dissolution and indirectly Anhydrite Precipitation is being accelerated and controlled by the CO2 within the residual oil. The study indicates strong bond between pH and CO2 and their effect on oil recovery while preforming LSWF. Enhancements on the injected water composition on a LSWF project based on the recommended chemical reaction controlling factors will enhance the understating of the driving mechanism of the LSWF and improve the oil recovery.