The role of mutual solvents in scale inhibitor squeeze treatments

Abstract

The role of mutual solvents in scale inhibitor (SI) squeeze treatments is to improve contact surface area between the SI and the rock to increase SI retention and thereby extend squeeze lifetime. For this to occur the mutual solvent is applied as a pre-flush before the main SI injection stage. This is important because squeeze treatments are used to avoid the formation of mineral scale in oil and gas wells, and maximising squeeze lifetime is essential to extending well protection and minimising cost, logistical complexity, and footprint of repeated squeeze treatments. The method used in this study to address the question of how mutual solvents interact with the rock substrate was to perform different coreflood tests, and to use two analytical techniques, UV-Vis and ICP-OES, for tracing the mutual solvents (traced with trans-Stilbene) and brines (traced with I for UV-Vis or Li for ICP-OES). This gave information on time of breakthrough of the various components, and hence on degree of interaction with the in-situ fluids and mineral surfaces. Additionally, pressure differentials across the core were measured to assess the risk of formation damage or the improvement in fluid mobility. Effluent samples were analysed visually for evidence of changes in phase behaviour. An early finding was that Ethanol creates cloudiness when it contacts sulphate free seawater in a Clashach core. Furthermore, this work identified that longer Mild Miscible cleaning needs to be undertaken in these types of Clashach cores for such experimental tests. Furthermore, EGMBE creates striations when applied in similar coreflood experiments. Also, we could not solve the problem of UV-Vis interference using trans-Stilbene for both Ethanol and EGMBE. The experiments demonstrated that EGMBE increases the SI (in this case, DETPMP) retention (adsorption), by removing synthetic mineral oil (Multipar-H) from the Clashach core, allowing for greater, prolonged squeeze lifetime. Then, a higher residual oil saturation was achieved by reducing the pump rate during Multipar-H injection. Moreover, the higher the EGMBE concentration, the better the removal of Multipar H from the Clashach core. In addition, the MS concentration is more important than volume when trying to remove residual oil saturation. 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. Additionally, 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.