Surface Chemistry of Phosphonate Scale Inhibitor Retention Mechanisms in Carbonate Reservoirs

Building a fundamental understanding of the reactions between scale inhibitor (SI) and formation minerals is essential for effectively designing SI “squeeze” treatments. Results of bulk “apparent adsorption” (Γapp) experiments are presented for a widely used phosphonate SI, DETPMP, on calcite and dolomite mineral substrates. The apparent adsorption results are supported by (i) measuring the corresponding solution Ca2+ and pH values in solution, (ii) studying the surface chemistry of the resulting SI/Ca precipitates using environmental scanning electron microscopy–energy-dispersive X-ray (ESEM-EDX) analysis to identify the morphology/composition of the SI/Ca precipitates, and (iii) a detailed mass balance analysis, indicating the fate of the Ca2+ and the SI. Results revealed that DETPMP was dominantly retained by both calcite and dolomite via a precipitation mechanism (actually coupled adsorption/precipitation) for all initial pH values (pH0 2, 4, and 6) and T = 95 °C, although a small region of pure adsorption (Γ) was observed at DETPMP < 100 ppm. Moreover, higher Γapp occurred on dolomite than on calcite for all initial pH0. This result is counterintuitive, because it is well-known that calcite is much more reactive than dolomite. However, final equilibrium pH values are higher for dolomite, compared to calcite. Thus, a higher pHfinal led to a more dissociated DETPMP and this effect had a greater effect on SI/Ca precipitation than the higher Ca2+ by rock dissolution. EDX analysis confirmed scale-inhibitor phosphorus in the deposited solids, indicating coupled adsorption/precipitation. Supporting mass balance calculations correlated very well with our experimental observations, showing higher generated calcium in calcite than dolomite and less calcium generation at higher initial pH0 (lower rock dissolution). Finally, an equilibrium mechanistic model describing the inhibitor dissociation, Ca-binding to the dissociated SI species, and precipitation of the SICan complex, coupled to the carbonate system, is proposed to qualitatively explain these experimental findings.