Abstract
Low-salinity waterflooding has been recognized as a method of enhancing oil recovery in low-permeability reservoirs. This method is relatively inexpensive and can be easily implemented in the field. Various mechanisms of low-salinity flooding have been proposed including interfacial tension reduction, wettability alteration (cation exchange), change in pH (increase), emulsion formation, and clay migration. Hydraulic fracturing has been known as a technique of stimulating hydrocarbon flow from low-permeability matrix into wellbores by creating high-conductivity fractures. The objective of this study is to evaluate the effectiveness of sequential low-salinity brine flooding process to enhance oil recovery in low-permeability fractured and non-fractured chalky limestone core samples. The low-salinity waterflooding tests were conducted with synthetic brines of five different salinity concentrations, namely, 157,662, 72,927, 62,522, 6252, and 1250 ppm. The properties of these brines have been thoroughly investigated in the laboratory. The crude oil and chalky limestone core samples, permeability range between 0.01 and 1.2 millidarcy, were gathered from a selected oil field in United Arab Emirates. When used as an opening move in a three-stage sequential brine flooding (SW/10→SW/50→SW 6xSO4− 2), sea water diluted ten times at 6252 ppm (SW/10) has been found to yield the highest oil recovery in fractured and non-fractured tests at the prevailing reservoir conditions, of 82.64 and 76% of OOIP, respectively. In all sequential brine flooding scenarios tested, sea water with sulfate concentration spiked six times (SW 6 × SO4− 2) only slightly increased oil recovery. The highest observed incremental recovery with sulfate spiking was 2.083% of OOIP. The effectiveness of oil displacement by sequential brine flooding has been attributed to mineral dissolution and fines migration which resulted in a favorable wettability alteration. This postulation of flow mechanism is confirmed by introducing a “flow resistance index” concept and measurements of key properties of the injected and effluent brines of each stage of the attempted sequential brine flooding scenarios. Results of this study could be consulted when selecting most efficient EOR method to develop tight carbonate oil reservoirs in the UAE and worldwide.
Highlights
The world energy demand is not static and it is forecasted to increase rapidly
We have investigated the possibility of using low-salinity, sequential low-salinity waterflooding, fracturing, and combination of sequential low-salinity flooding with hydraulic fracturing to improve oil recovery from lowpermeability carbonate reservoirs
The results of one sequential brine flooding scenario of the non-fractured core ZK-454-3 are presented in Table 7 and Fig. 4
Summary
The world energy demand is not static and it is forecasted to increase rapidly. The world is rapidly moving in the direction of using a mix of energy, it is expected that 53% of the world’s energy needs will be met by oil and gas in 2040 (Ban et al 2016). We can safely say that oil will be a major player for the energy market for the 30 years at least. Low-permeability carbonate reservoirs have been considered uneconomical to develop because of their low flow rates and longer pay out times. Oil in lowpermeability oil reservoirs is classified as unconventional reserves. Unconventional reserve is defined by the oil industry as ‘hard to recover’ oil. The definition ‘hard to recover’ reserve includes reservoirs having low-permeability and/or porosity.
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