Experimental Analysis of Scale Inhibitors Retention in Carbonate Formations for Application in Squeeze Treatments

Abstract

In this work, static adsorption/precipitation (Γ/Π) experiments were conducted for two widely used scale inhibitors (DETPMP and VS-Co) using two different size fractions of Moroccan calcite (315–500 and 125–250 μm) to study the effect of particle size on the “apparent adsorption” of these SIs onto carbonate. The reason for performing these experiments at two particle size ranges was to determine whether the relative surface area to volume ratios (as presented as particle size) would affect whether the precipitating SI–Ca complex forms as a “skin” on the mineral surface and, if so, to determine whether this “skin” could affect the further interaction between the SI and the carbonate mineral by a “surface poisoning” effect. The results of both environmental scanning electron microscopy/energy dispersive X-ray analysis (ESEM/EDX) and direct particle size analysis (PSA) clearly showed that no (or very little), such surface deposition or coating around calcite grains occurred for either of these SIs. Essentially, the results for both particle sizes were qualitatively the same for each of the SIs. The DETPMP retention showed coupled Γ/Π behavior, which was predominantly precipitation at [DETPMP] > 100 ppm and was quantitatively almost the same for both calcite particle sizes. Likewise, the results for VS-Co were quantitatively almost identical for both particle sizes and the retention was predominantly via adsorption up to [VS-Co] ≈ 3000 ppm, with some small degree of precipitation at higher concentration observed. The rather different behavior of DETPMP and VS-Co may be ascribed to different functional groups having different pKa values and strengths of SI–Ca binding. Both sets of static adsorption/precipitation experimental results for DETPMP and VS-Co on each calcite particle size fraction were also predicted using a previously published model.