An experimental study of the impact of injection water composition on oil recovery from carbonate rocks [electronic resource]
Keywords: Energy Resources Engineering.

Evidence from laboratory studies and field tests suggests that implementing certain modifications to the ionic composition of the injection brine leads to greater oil recovery from sandstone rocks. More recent studies indicate that salinity and ionic composition impact oil recovery from carbonate rocks. The mechanisms that take place and techniques of altering the salinity may be different from those experienced in clastic systems. This work examines experimentally the factors that influence oil recovery from carbonate rocks when the salinity is altered. It also investigates mechanisms that lead to greater oil recovery. A series of forced imbibition experiments were conducted at different total salinity and ionic composition using reservoir limestone cores and crude-oil. Brines of different salinities were injected sequentially into a core with realistic initial oil and water saturation. Additional incremental oil recovery of 4.4-6.4% of the original oil in place (OOIP) was observed, during the tertiary stage, when the injection seawater, that has a salinity of 55 kppm, was replaced by a new brine (MgSO4) of similar total salinity (45 kppm) and rich in Mg2+ and SO42- ions. The effect of reducing the total salinity was evaluated using outcrop limestone cores and another crude- oil. An incremental oil recovery increase of 3.2-6.5% was observed when twice-diluted seawater (29 kppm) was injected during the tertiary stage following seawater injection. Direct measurements of crude-oil contact-angles on smooth calcite surfaces suggest that the release of oil is caused by a wettability shift toward water wetness. The static water contact-angle was reduced from 92.9 to 58.7 when the brine was switched from seawater to MgSO4 solution of similar salinity. Similar reduction was observed when measurements were conducted using the fluids of the second system. The static water contact-angle was reduced from 70.1 to 58.9 when the brine was switched from seawater to twice-diluted seawater. The contribution of each component of the rock/brine/oil system to the wettability was evaluated by measuring zeta potential of water/oil and water/solid interfaces. DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory of surface forces uses the measurements to predict disjoining pressure and contact-angle. The results rationalized observations of recovery and crude-oil adhesion to solids. They also show that Mg2+ ions play a key role in the wettability alteration process when MgSO4 brine was used and no significant contribution was observed for SO42- ions. For tests that used the twice diluted seawater, the wettability alteration was attributed to the additional Ca2+ ions that added to the brine from the rock dissolution. Conventional fluid flow simulation was able to predict the additional oil recovery that was observed in the core-flood experiments. The input relative permeabilities for each brine were generated using pore network modeling that simulated flow in a carbonate system under different wettability conditions.