A Systematic Investigation into the Effect of Mutual Solvent Pre-Flushes on Scale Inhibitor Retention and Return

Abstract

Abstract Mutual solvents (MS) have been extensively applied prior to the main scale inhibitor (SI) treatment slug of squeeze treatments. Evidence from the field indicates that MS pre-flushes are beneficial, however, no systematic evaluation of their role has been performed. A series of core floods have been undertaken to elucidate the predominant mechanism that leads to these beneficial effects. Factors investigated included; presence/absence of an oil phase, type of oil phase present (mineral/crude oil), and core wettability (oil/water-wet). The objective was to investigate the impact of a specific mutual solvent (EGMBE) on the retention of DETPMP in cores that had been pre-flushed with MS prior to the main treatment and compare them to the corresponding core flood where no MS pre-flush had been applied ("No MS" case). In all the core floods, scale inhibitor injection and return profiles were evaluated and compared. Saturation changes throughout each flood were also monitored. In all the water-wet cases, the return profiles for the flood with MS as a pre-flush were seen to be comparable to the "No MS" case; the mutual solvent showed no obvious improvement in SI adsorption and its return in the long post flush tail. However, it was found that applying a MS pre-flush to an oil-wet core significantly enhanced squeeze lifetime in comparison to its "No MS" counterpart. The mutual solvent phase behaviour was also consistent with that for the water-wet cases, with it breaking through earlier than expected due to the presence of a residual mutual solvent phase that is stripped from the core throughout the injection stage. A repeat experiment using the same materials did not show the expected improvement in SI return, however, examination of the crude oil used, indicated that it had changed significantly from the first experiments such that comparison to the initial "No MS" case was invalid – a different wetting state was achieved. It is still our belief that the predominant mechanism by which MS beneficially enhances Squeeze lifetimes is through a wetting change from oil-wet to a more water-wet state, however further work is required to confirm this hypothesis.