Sulphate in Original Reservoir Chalk Rocks: Release of Sulphate by Desulphated Sea Water and Effects on Wettability

Abstract

Abstract Injection of sea water (SW) into oil reservoirs for pressure maintenance or sweep can in some cases cause reservoir souring, sulphate scaling and formation damage. Change of injection water from SW to desulphated sea water (D-SW) may reduce these problems. The objective for the presented study was to investigate the interactions between reservoir chalk rocks and D-SW, and to determine the effects of low sulphate concentration in formation water (FW) on oil production from reservoir chalk. FW, SW, D-SW (synthetic) and water from a sulphate removal plant (SRPW) were injected to reservoir chalk plugs. Effluent samples were analysed for sulphate, pH and elements to investigate the interactions between brines and minerals. During the brine injections, the potential for formation damage was also evaluated by measuring differential pressure across the core plugs. Reservoir chalk plugs were prepared using FW without sulphate and FW with sulphate concentration as in real FW, to investigate the effect of initial sulphate on spontaneous imbibition and viscous flooding. No pressure build-up was observed when FW, SW, D-SW and SRPW were injected to the reservoir chalk plugs. Sulphate effluent peaks were observed during injection of FW, D-SW and SRPW to the reservoir chalk. The effluent pH was higher in the D-SW and SRPW injections than in the FW injections. The compositions of effluent samples confirmed the interactions between the reservoir chalk and these brines. Both spontaneous imbibition and viscous flooding with SW showed that the reservoir chalk with initial sulphate was more water-wet than the same chalk without initial sulphate. Injection of D-SW will reduce the amount of sulphate produced in the oil fields, but the study has shown that sulphate will be produced due to the release of sulphate from the original reservoir chalks. Since SW has been reported to improve the oil recovery, it is important to compare the oil recovery potential for D-SW and SW before the type of injection brine is selected. It will then be important to prepare the reservoir rock with the correct amount of initial sulphate.