Improvement of Relative Permeability and Wetting Condition of Sandstone by Low Salinity Waterflooding

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Waterflooding is generally performed to maintain reservoir pressure and as a consequence, production period is extended. However, conventional waterflooding may be not suitable for every reservoir. Low Salinity Waterflooding (LSW) which is recently the most mentioned technique is therefore, considered. Although clear explanation of oil recovery mechanism is not available, it is believed that the key mechanism is Multi-component Ion Exchange (MIE). This study aims to assess improvement of relative permeability and wetting condition of sandstone formation from oilfield in Thailand, through the MIE mechanism. The study is divided into two major parts. The first part is laboratory study including core and fluid preparations, imbibition test, coreflooding test and ion-exchange detection. After laboratory data including fluid production rate and pressure difference versus time are obtained, core simulation using reservoir simulation program called STAR® commercialized by Computer Modelling Group (CMG) is performed to study the change of wetting condition through the shifting of relative permeability. Obtaining results would help verifying suitability of the implementation of this technique in oilfield. Results from imbibition test showed that LSW yields higher oil recovery factor compared to formation water. Diluted formation water at 5,000 ppm which is approximately one-third of formation water, is the best water formulation, yielding the fastest rate of imbibition in this study. From ion-exchange detection test, results showed the variation of divalent ion concentrations compared to injected water and moreover, presence of calcium and magnesium ions in oil phase supports the occurrence of MIE during the displacement mechanism. The theoretical model matching with physical data using reservoir simulation program indicates that LSW slightly affects the original wetting condition. Water wetness is lessened as can be observed from the increment of relative permeability to water at the end point saturation. Moreover, LSW reduces residual oil saturation and at the same time, increases relative permeability to oil. From the detection of MIE together with the observation of wetting condition through core simulation, it can be concluded that LSW is a promising technique for oilfield in Thailand. As produced water must be reinjected back into reservoir based on zero discharge regulation of Thai government, this low-cost technique of diluting water salinity prior to reinjection would fulfil the requirement of government and at the same time helps increasing oil recovery of the total field.