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An experimental investigation is presented that shows oil recovery and wettability changes accompanying alteration of injection water salinity. The study aims to provide a better understanding of the impact of salinity leading to greater oil recovery and the mechanisms that take place for carbonate rocks. A series of forced imbibition experiments was conducted using crude-oil at different total salinity and ionic composition. In the first test set, three brines of different salinity were injected sequentially into reservoir cores 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. The injection of synthetic 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. Injection of twice-diluted seawater of 27 kppm, during the tertiary stage, yielded an incremental oil recovery of 0.9%. The effect of reducing the total salinity was evaluated in a second test set using outcrop limestone cores and another crude-oil. An incremental oil recovery increase of 6.5% was observed when twice-diluted seawater (29 kppm) was injected during the tertiary stage following seawater injection. Wettability alteration was assessed using classic crude-oil contact-angle tests on smooth calcite surfaces. Contact-angle measurements suggest that the release of oil is caused by a wettability shift toward water wetness. The static water contact-angle was reduced in first test set from 92.7° to 55.4° when the brine was switched from seawater to the magnesium-rich brine. Similar reduction was observed in the second test set. 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 the rock/brine/oil interactions 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, using the measured zeta potentials rationalized observations of oil recovery in the case of low water salinity. The finding of this work provides a new approach for utilizing salinity alteration to achieve greater oil recovery. The impact of water salinity is usually correlated with low water salinity or with increasing the concentration of the key divalent ions. This study combines both salinity modification schemes to increase tertiary oil recovery.

The common wisdom is that gravity methods have limited application in the oil industry although they have long been available. The main use of gravity has been for exploration purposes. 4D microgravity monitoring is another new promising gravity application to monitor changes of fluid contacts. Some successful 4D monitoring surveys have been conducted in the industry revealing that this technique is a proven technology in monitoring of gas-water contacts.This paper studies the ability of microgravity to capture movement of the injected water in a giant carbonate field. The oilwater case is more difficult due to the significantly lower density contrast as compared to the gas-water case. Monitoring water floodfront in the field is a key factor in applying successful reservoir management practices to maximize recovery and prolong the field life. The monitoring of inter-well fluids would characterize any pre-mature water breakthrough to allow planning and design of appropriate remedial well interventions. The current applied monitoring tools such as carbon-oxygen and resistivity logs can only detect fluids near to the wellbore due to their shallow radius of investigation. For the study field, 4D seismic cannot be used for fluid movement detection due to issues related to formation acoustics impedance and data quality.The study has shown that surface microgravity monitoring could successfully detect the inter-well fluid changes due to water injection with a high precision tool (0.01 microgal). It also shows that microgravity monitoring can capture water bodies located hundreds of meters away from the location of the 4D measurement.

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