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

摘要

Injection of sea water(SW)into oil reservoirs for pressure maintenance or sweep can in some casescause reservoir souring,sulphate scaling and formation damage.Change of injection water from SW todesulphated sea water(D-SW)may reduce these problems.The objective for the presented study was toinvestigate the interactions between reservoir chalk rocks and D-SW,and to determine the effects of lowsulphate 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 reservoirchalk plugs.Effluent samples were analysed for sulphate,pH and elements to investigate the interactionsbetween brines and minerals.During the brine injections,the potential for formation damage was alsoevaluated by measuring differential pressure across the core plugs.Reservoir chalk plugs were preparedusing FW without sulphate and FW with sulphate concentration as in real FW,to investigate the effect ofinitial 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 chalkplugs.Sulphate effluent peaks were observed during injection of FW,D-SW and SRPW to the reservoirchalk.The effluent pH was higher in the D-SW and SRPW injections than in the FW injections.Thecompositions 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 initialsulphate 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 shownthat sulphate will be produced due to the release of sulphate from the original reservoir chalks.Since SWhas been reported to improve the oil recovery,it is important to compare the oil recovery potential for DSWand SW before the type of injection brine is selected.It will then be important to prepare the reservoirrock with the correct amount of initial sulphate.