Waterflooding in Chalk: Relationship between Oil Recovery, New Wettability Index, Brine Composition and Cationic Wettability Modifier

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Abstract Naturally fractured carbonate reservoirs are usually neutral to preferential oil-wet, which prevents water from being an efficient IOR-fluid. Spontaneous imbibition of water into the matrix blocks is believed to be the key mechanism for improved oil recovery by waterflooding. In previous study, it was documented that the wetting condition of carbonates was mainly dictated by the acid number, AN, of the crude oil. In the present study, series of long-term spontaneous imbibition tests were conducted at 70°C on chalk cores representing different wetting conditions. The oil recovery is discussed in terms of a new wetting index and the property/composition of the imbibing fluids. Synthetic seawater was used as the base imbibing fluid, and model Ekofisk formation water was used as initial water present in the cores. The cores were aged in crude oils with different AN and at a variety of temperatures ranging from 40-120°C. Major observations are: (1) Good relationship between the new wetting index and oil recovery was established. (2) The water-wet fraction of the chalk surface must be above 0.6 for spontaneous imbibition to occur.(3) Sulfate in the imbibing fluid appeared to have a wettability modifying effect, especially at low water-wet conditions. (4) Cationic surfactant of the type [R-N(CH3)3]+ dissolved in the imbibing fluid improved the oil recovery significantly, especially close to neutral wetting conditions. The effects of sulfate on the oil recovery is discussed in terms of potential determining ions towards chalk, as well as its potential as wettability modifying agent.