The first stage of a squeeze treatment entails a pre-flush, usually containing a mutual solvent that prepares the surface of the rock and allows greater retention of scale inhibitor that is injected during the main part of the treatment. We use experimental studies to tackle the issue of mutual solvent propagation and its impact on oil displacement and inhibitor retention. Clashach cores were used in this study, and the propagation of mutual solvent and subsequent scale inhibitors through the core were studied, along with any influences on rock properties such as permeability and on residual phase saturations. It was evident that during waterflooding a high residual oil saturation affects squeeze lifetime, and that mutual solvent can partially compensate for this, although the effect could not be definitively identified. Thus, further corefloods were conducted to understand how long it takes to remove residual mutual solvent from the core plug during the main treatment injection stage, and what is the influence of mutual solvent concentration in the pre-flush on this process.
The corefloods conducted were thus: i. Three floods with different concentrations (100%, 75% and 50%) of mutual solvent, using the injection of tracers at various stages, to identify how the residual phase saturation changes with main treatment aqueous phase volume throughput, and ii. Three scale inhibitor floods at residual oil saturation (using Multipar H). One with no mutual solvent pre-flush and two with a mutual solvent pre-flush (100% EGMBE). The difference between the latter two mutual solvent pre-flush floods was the amount of residual oil in the core prior to mutual solvent injection.
The results are discussed in detail, to understand the effect of the mutual solvent during a squeeze treatment under typical field conditions. According to the results it is proposed that using mutual solvent as a pre-flush enhances scale inhibitor squeeze lifetime, by increasing scale inhibitor adsorption/retention, due to a wettability change towards more water-wet – as evidenced by the reduction in residual oil saturation. Furthermore, more pore volumes of aqueous phase main treatment (up to seven) were required to displace the residual phase when lower mutual solvent concentrations were used in the coreflood pre-flush stage.
The results of this work indicate that mutual solvent concentration, and the ratio of main treatment volume to pre-flush volume should be carefully considered when designing squeeze treatments, since the contact between the aqueous scale inhibitor solution and the rock will be affected by the ability of the mutual solvent to displace oil, and by the volume of main treatment required to displace the mutual solvent rich pre-flush. Finally, modelling work was undertaken using the isotherms generated from the corefloods to demonstrate the impact on squeeze life of various mutual solvent applications.