The Effect of Blending Polymeric and Phosphonate Scale Inhibitors on Fluid/Fluid and Rock/Fluid Interactions: A Comprehensive Experimental and Theoretical Study
Authors: Talebi Sina , Riahi Siavash , Rostami Behzad 
Published: June 2022

Summary
Using seawater or engineered water to inject into oil formations can cause inorganic scaling, such as calcium sulfate, barium sulfate, and strontium sulfate. These scales may clog pore throats and limit production. Scale inhibitor (SI) squeeze treatment reduces inorganic scaling and improves oil recovery. Chemical compounds called SIs suppress or delay mineral scaling. SIs fall into two categories, each with its own mechanism. Most polymeric inhibitors impede nucleation, whereas phosphonate SIs deform crystal formation. The oil and gas industry now uses a fixed mixture of these inhibitors to maximize all inhibition mechanisms. However, the impact of blended SIs on fluid/fluid and rock/fluid interactions has not been addressed.
This study aims to evaluate the efficiency of blended SIs when dealing with three well-known mineral scales all together. In this study, first, the process of mineral scale formation was simulated using PHREEQC (pH-REdox-EQuilibrium), and the impact of temperature and mixing ratios was investigated. Then, by choosing a proper temperature and mixing ratio, the effect of polymeric and phosphonate SIs blending on barium, calcium, and strontium sulfate inhibition is examined. Additionally, fluid/fluid interactions and rock/fluid interactions were studied via interfacial tension and contact angle measurements.
The influence of seawater mixing ratio on calcium, strontium, and barium sulfate saturation indices is medium, low, and very low, respectively, but the effect on the amount of scale precipitation is very high, high, and low for the scales indicated. This adds to the fact that barium takes precedence in scale formation, and its presence may affect the other two scales, but calcium ions have barely any effect on barite formation. Moreover, the results show a positive synergistic effect of SIs blend on sulfate scale mitigation. However, this positive figure completely depends on the concentration of phosphonate SI for calcium and strontium sulfate, while the positive synergy exists at all concentrations of phosphonate SIs for barium sulfate. Moreover, SIs could lower the water/oil/rock contact angle by 10° on average and make it more water wet. Same enhanced results have been achieved for interfacial tension by adding SIs, reducing by 8 dynes/cm on average. It is worth noting that the interfacial tension and contact angle measurements are unaffected by the synergistic action of SIs mixing. The results of coreflooding experiments substantiate the effectiveness of SIs and show 8 and 45% permeability reduction for injection of seawater with and without SIs on two different core samples, respectively. For the first time in a sulfate scaling system, this work investigates the copresence of barite, celestite, and anhydrite against a wide range of blending SI concentrations. Based on the results, it is derived that the presence of all three sulfate scales undoubtedly affect the quantity of each scale’s precipitation and the efficacy of SIs.