Mutual solvent driven inorganic precipitation in the pre-flush stage of squeeze treatments

Scale Inhibitor (SI) squeeze treatments are commonly used to inhibit mineral scaling in reservoirs, hence preventing formation damage and other production problems. Conventional squeeze treatments comprise of five stages, namely: pre-flush, main treatment, post-flush, shut-in and back-production stages. In the pre-flush stage, a mutual solvent (MS) is often applied to the formation either neat or in a blend of water and/or other additives. This practice is believed to offer numerous benefits including: the prevention of emulsion formation, water-blocking avoidance and enhancements to SI adsorption through oil and water displacement. In applying a MS, any additional solid deposits formed due to this pre-flush stage are generally undesirable. The nature of these possible additional deposits and their potential to cause formation damage have not been studied systematically to date. This paper aims to characterise the predominantly inorganic scales formed in mixtures of oil, brine and a mutual solvent. This characterisation is very useful in developing an understanding of the possible risks associated with mutual solvent applications and it will ultimately enable us to the design of better optimised scale inhibitor squeeze treatments including MS pre-flush stages. For a range of mutual solvents, qualitative "pseudo" ternary phase diagrams were produced at room temperature and pressure; it is denoted "pseudo" ternary since the "components" are the MS, a mineral oil and a brine (all of which are actually multicomponent in nature). Two brine chemistries were investigated including a sulphate-free formation brine and normal seawater. These investigations aided the definition of inorganic precipitation regions on the phase diagram as a function of brine chemistry. For the two brine chemistries investigated, the precipitates were collected and analysed using two methods. The first method used ESEM-XRD analysis to produce an elemental composition of the precipitates. This enabled the determination of the abundant elements in the precipitates and the compounds forming these. In the second method, the precipitates were redissolved in de-ionised water, and ICP-OES analysis was performed to determine the relative elemental ratios. Using these two approaches, a decisive characterisation of the predominant precipitates and their proportions can be made. Significant mutual solvent driven precipitation was found to occur at almost all mutual solvent concentrations in seawater, whereas mineral precipitation occurred only at near-neat mutual solvent concentrations in formation brine. This indicates that the sulphate ion may be important. Indeed, in seawater, the precipitates were found to be predominately Na₂SO₄ and CaSO₄ in approximately a 2:1 mix, respectively, with traces of other SO₄ ^2- and Cl^- precipitates. In formation brine, the precipitates comprised almost entirely of NaCl with small traces of other Cl^- salts. These findings provide practical means for preventing mutual solvent driven precipitation that can be tailored to specific squeeze treatment designs. In this regard, key considerations would be the use of sulphate-free brines in preparing the pre-flush, and the avoidance of mutual solvent and brine mixtures at near-neat mutual solvent concentrations.