Building a Fundamental Understanding of Scale Inhibitor Retention in Carbonate Formations

Scale inhibitor (SI) squeeze treatments in carbonate reservoirs are often affected by the chemical reactivity between the SI and the carbonate mineral substrate. This chemical interaction may lead to a controlled precipitation of the SI through the formation of a sparingly soluble Ca/SI complex which can lead to an extended squeeze lifetime. However, the same interaction may in some cases lead to uncontrolled SI precipitation causing near-well formation damage in the treated zone. This paper presents a detailed study of the various retention mechanisms of SI in carbonate formations, considering system variables such as the (carbonate) formation mineralogy, the type of SI and the system conditions. Apparent adsorption (Γapp) experiments, described previously (Kahrwad et al. 2008), are used to show when the SI/substrate interaction is pure adsorption (Γ) or coupled adsorption (Γ)/precipitation (∏). Experiments were performed for different SIs at various operational conditions, i.e. initial pH values, minerologies - calcite, limestone and dolomite - and temperatures; the overall results from these coupled Γ/∏ experiments are summarised in Table 3. The SI species used in this study included 1 phosphonate (DETPMP), 1 phosphate ester (PAPE) and 3 polymeric scale inhibitors (PPCA, PFC, VS-Co); the full names of these SIs are given in the paper. All precipitates were studied using Environmental Scanning Electron Microscopy/Energy Dispersive X-Ray (ESEM/EDX) and Particle Size Analysis (PSA). These measurements confirmed that when precipitation occurred, it was mainly in the bulk solution and not on the rock surface. For all SIs, both adsorption (Γ) and precipitation (∏) retention mechanisms were observed, with the dominant mechanism depending on SI chemistry, temperature and mineralogy. Differences were observed between the "apparent adsorption" (Γapp) levels of polymeric, phosphonate and phosphate ester scale inhibitors, as follows: For the polymeric SIs (PPCA, PFC and VS-Co), the highest retention levels were observed at low pH for all carbonate substrates, due to the increase in divalent cations (Ca2+ and Mg2+) available from rock dissolution for SI-M2+ precipitation. For phosphonate (DETPMP) and phosphate ester (PAPE) SIs, the retention level was greatest at higher pH values, as the SI functional groups were more dissociated and hence available for complexation with M2+ ions.The polymeric VS-Co showed the lowest amount of precipitation (Γapp ~ 1.2 mg/g) in contact with dolomite substrate due to the presence of sulphonate groups (low pKa); indeed this showed low Γapp which was predominantly pure adsorption. However, a small amount of precipitate was observed by ESEM/EDX and PSA.For polymeric inhibitors, the retention level (Γapp) was highest on calcite (highest relative calcium content), followed by limestone and then dolomite. Phosphonate and phosphate ester SIs showed the highest retention levels on dolomite (higher final solution pH and more SI dissociated), followed by limestone and calcite.For all SI species, higher retention (more precipitation, ∏) was observed at elevated temperature. At lower temperatures, a more extended region of pure adsorption was observed for all SIs. The information presented in this study will help us in SI product selection for application of squeeze treatments with longer squeeze lifetimes in carbonate reservoir based on mineralogy and reservoir conditions. In addition, this study provides valuable data for validating models of the SI/Carbonate/Ca/Mg system which can be incorporated in squeeze design simulations.