Turning Produced Water into an EOR-Fluid for Carbonate Reservoirs

Summary Modified injection water, also called Smart Water, has proven to be a successful enhanced oil recovery (EOR) fluid in carbonate rocks. Wettability alteration induced by favourable crude oil-brine-rock interactions taking place by the introduction of an injection water (IW) of different composition than the formation water (FW) leads to accelerated and enhanced oil production. Seawater (SW) injection into the Ekofisk chalk field on the Norwegian Continental Shelf is one such example. The reason for the EOR-effect by SW injection is the favourable composition of SW, containing sulphate, calcium and magnesium ions, which are active in the wettability alteration process. In areas where SW is not available, the choice of water to be injected in a waterflood would naturally be aquifer brines, the FW, or a source of surface water. According to experimental laboratory research, these latter water compositions are not favourable for wettability alteration in carbonate rocks. Produced water (PW) is initially of a composition similar to that of the FW but will gradually change to a mixture of increasing IW/FW ratio over time. In the absence of SW, both PW and FW can turn into EOR-fluids if certain ions responsible for wettability alteration are added to those brine compositions. This can be easily done by dissolving a naturally occurring salt, polysulphate (PS) – containing high content of sulphate and calcium ions, into PW or FW. In this work, outcrop Stevns Klint chalk and Indiana limestone cores restored to mixed-wet conditions using a crude oil with AN 0.6 mgKOH/g were used as the carbonate material. Spontaneous imbibition oil recovery tests were performed, at 110 degC and 10 bar back pressure, to investigate the wettability alteration EOR-potential by the addition of 3 grams of PS salt to 1 litre FW of salinity 63000 ppm. By comparing the oil recovery by PS-spiked FW to that obtained by FW alone, it was clear that the addition of PS turned the FW into an EOR-fluid for both outcrop chalk and limestone at the experimental conditions used. An additional oil recovery of 3–17 %OOIP was obtained in secondary imbibition mode. Thus, in areas where SW is not available or where PW reinjection is preferred, these results indicate that Smart Water can be made by adding PS to PW, aquifer brines, or surface water.