Sulfate scaling in North Sea oil fields

A common problem in offshore petroleum production is that sulfate scale may form when seawater is injected into the formation during waterflooding operations. The scale forms when seawater, which is rich in sulfate but relatively poor in Ca++ and nearly depleted in Sr++ and Ba++, mixes with formation fluids, many of which contain bivalent cations in relative abundance but little sulfate. The mixing causes minerals such as gypsum (CaSC 4 2H2O), anhydrite (CaSC 4), celestite (SrSOzO, and barite (BaS04, an almost insoluble salt) to become saturated and precipitate as scale. 

Sulfate scaling poses a special problem in oil fields of the North Sea (e.g., Todd and Yuan, 1990, 1992 Yuan et al., 1994), where formation fluids are notably rich in barium and strontium. The scale can reduce permeability in the formation, clog the wellbore and production tubing, and cause safety equipment (such as pressure release valves) to malfunction. To try to prevent scale from forming, reservoir engineers use chemical inhibitors such as phosphonate (a family of organic phosphorus compounds) in squeeze treatments, as described in the introduction to this chapter. 

To start the simulation, we equilibrate seawater (as we did in Chapter 6), using REACT to carry it to formation temperature, and then pick up the resulting fluid as a reactant in the mixing calculation. The procedure (taking the temperature of 121 °C reported for the Miller field) is 

We then equilibrate the formation fluid, using data from Table 30.1. Since pH measurements from saline solutions are not reliable, we assume that pH in the reservoir is controlled by equilibrium with the most saturated carbonate mineral, which turns out to be witherite (BaCC U) or, for the Amethyst field, strontianite (SrC03). Using the Miller analysis, the procedure for completing the calculation is 

To model the case in which scale is prevented from forming, we repeat the calculation 

TABLE 22.1 Compositions of formation fluids from three North Sea oil fields (Edward Warren, personal communication) and seawater 

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