Mixing of injected, connate and aquifer brines in waterflooding and its relevance to oilfield scaling

Waterflooding is one of the most common methods of oil recovery although it does lead to certain production problems after water breakthrough, e.g. co rrosion, scaling, etc. The issue of concern in this paper is mineral scale formation by brine mixing as occurs in barium sulphate (barite, BaSO4) scal ing. Barite formation in the production well and tubulars occurs in many oi lfields when sulphate-rich injection water (IW) (often seawater (SW)) mixes with barium-rich formation water (FW) close to or in the wellbore. However , when a brine is injected into the reservoir, it may mix to some extent wi th the formation (or connate) brine deep within the system. Such in situ mi xing of barium-rich and sulphate-rich brines would certainly result in bari te deposition deep within the reservoir due to the low solubility and rapid kinetics of this precipitation process. Conversely, in order to estimate h ow much of this type of in situ precipitation might occur in reservoirs, we must be able to model the appropriate displacement processes incorporating the correct level of dispersive brine mixing in the reservoir formation. I n this paper, all of the principal mechanisms of brine mixing in waterflood displacements are considered and modelled, Mixing between the IW, the oil leg connate water (CW) and the aquifer water (AQW) is analysed starting fro m a one-dimensional (1D) frontal displacement, extended Buckley-Leverett (B L) analysis. This particular mechanism occurs in all other types of displac ement and reservoir mixing process including those in both heterogeneous la yered systems and in areal flooding situations. Of vital importance to brin e mixing is the level of reservoir sandbody dispersivity, and field values of this quantity are estimated. Results from the numerical modelling of oil displacement and IW/FW mixing are presented to illustrate various points w hich arise in the discussion. These calculations show that quite complex pa tterns of mixing of connate, aquifer and injection brines can occur in rela tively simple two-dimensional (2D) systems. The significance of in situ bri ne mixing to barite scaling is discussed in some detail. (C) 2000 Elsevier Science B.V. All rights reserved.