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

Abstract

Waterflooding is one of the most common methods of oil recovery although it does lead to certain production problems after water breakthrough, e.g. corrosion, scaling, etc. The issue of concern in this paper is mineral scale formation by brine mixing as occurs in barium sulphate (barite, BaSO 4) scaling. Barite formation in the production well and tubulars occurs in many oilfields 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 with the formation (or connate) brine deep within the system. Such in situ mixing of barium-rich and sulphate-rich brines would certainly result in barite deposition deep within the reservoir due to the low solubility and rapid kinetics of this precipitation process. Conversely, in order to estimate how 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. In 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 from a one-dimensional (1D) frontal displacement, extended Buckley–Leverett (BL) analysis. This particular mechanism occurs in all other types of displacement and reservoir mixing process including those in both heterogeneous layered systems and in areal flooding situations. Of vital importance to brine 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 which arise in the discussion. These calculations show that quite complex patterns of mixing of connate, aquifer and injection brines can occur in relatively simple two-dimensional (2D) systems. The significance of in situ brine mixing to barite scaling is discussed in some detail.