Building a Fundamental Understanding of Scale Inhibitor Retention in Carbonate Formations

Abstract

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.

Surface Chemistry of Phosphonate Scale Inhibitor Retention Mechanisms in Carbonate Reservoirs

“…Once substrates (calcite and dolomite) are added to the DETPMP solution, the pH rises from their initial values (pH 0 2, 4, and 6) to pH ∼8, because of dissolution of the calcite/dolomite substrate in the SI solution. At DETPMP concentrations of ∼100 ppm, the retention regime changes from pure adsorption (Γ) to coupled adsorption/precipitation (Γ/Π), and we observe that the pH decreases sharply. , This is obviously associated with the precipitation process and this will be explained below. The detailed results for pH f in Figures a, b, and c show that the final pH values of the solutions in the calcite/DETPMP system are less than the final pH values in the dolomite/DETPMP system solutions .…”

“…At DETPMP concentrations of ∼100 ppm, the retention regime changes from pure adsorption (Γ) to coupled adsorption/precipitation (Γ/Π), and we observe that the pH decreases sharply. , This is obviously associated with the precipitation process and this will be explained below. The detailed results for pH f in Figures a, b, and c show that the final pH values of the solutions in the calcite/DETPMP system are less than the final pH values in the dolomite/DETPMP system solutions . Then, for the DETPMP/calcite system, for pH 0 2, the pH goes from pH f 7.5 (brine only) down to pH f ∼5 as the DETPMP concentration increases; the corresponding result for pH 0 4 are pH f ∼7.5 (brine only) and pH f ∼5.5 (high DETPMP concentration); for pH 0 6, these values are pH f ∼7.5–8 (brine only) and pH f ∼6 (high DETPMP concentration).…”

“…This is actually quite surprising, since it is well-known that calcite is rather more chemically reactive than dolomite. This result has been repeated at our laboratory and has been reproduced several times on the Moroccan calcite and Skye dolomite samples used in this work. , To explain this finding, we must first present the additional final pH and [Ca 2+ ] data for each of the above experiments.…”

“…For example, calcium carbonate (CaCO 3 ) and/or barium sulfate (BaSO 4 ) may form, and this constitutes a severe problem in the oil and gas industry . Scale formation can be initiated by the mixing of incompatible waters (e.g., barium sulfate, BaSO 4 ) or by changes in temperature and pressure due to production from the well (e.g., calcium carbonate, CaCO 3 ). − If a scale management program is not properly implemented, serious problems such as near-well formation damage and production system blockages can be encountered, leading to reduced oil production and considerable additional operating expenditure. − …”

“…Thus, in the case of pure adsorption (Γ), then the apparent adsorption (Γ app ) is not a function of the m / V ratio. However, if above a certain SI concentration, a precipitation mechanism occurs, thus giving us coupled adsorption precipitation (Γ/Π) …”