Summary The bulk “apparent-adsorption” behavior (Γapp vs. Cf) of two polymeric scale inhibitors (SIs), polyphosphino carboxylic acid (PPCA) and phosphorus-functionalized copolymer (PFC), onto carbonate mineral substrates has been studied for initial solution pH values of 2, 4, and 6. The two carbonate minerals used, calcite and dolomite, are much more chemically reactive than sandstone minerals (such as quartz, feldspars, and clays), which have already been studied extensively. In nearly all cases, precipitates formed at higher SI concentrations were caused by the formation of sparingly soluble SI/calcium (Ca) complexes. A systematic study has been performed on the SI/Ca precipitates formed by applying both environmental scanning electron microscopy energy-dispersive X-ray (ESEM-EDX) analysis and particle-size analysis (PSA), and this identifies the morphology and the approximate composition of the precipitates. For PPCA, at all initial solution pH values, regions of pure adsorption (Γ) (PPCA < 100 ppm) and coupled adsorption/precipitation (Γ/Π) are clearly observed for both calcite and dolomite. PFC at pH values of 4 and 6 also showed very similar behavior, with a region of pure adsorption (Γ) for PFC < 500 ppm and a region of coupled adsorption/precipitation (Γ/Π) above this level. However, the PFC/calcite case at pH = 2 showed only pure adsorption, whereas the PFC/dolomite case at pH = 2 again showed coupled adsorption/precipitation at higher PFC concentrations. For the SIs on both carbonate substrates, precipitation is the more dominant mechanism for SI retention than adsorption above a minimum concentration of approximately 100 to 500 ppm SI. The actual amount of precipitate formed varies from case to case, depending on the specific SI, the substrate (calcite/dolomite), and the initial pH (pH = 2, 4, and 6). Although the qualitative behaviors of both PPCA and PFC were similar on both carbonate substrates, the apparent adsorption of PPCA was higher on calcite than on dolomite, and the apparent adsorption of PFC was higher on dolomite than on calcite. We discuss here how these observations are related to the reactivity of the different carbonate minerals, the resulting final pH (which affects the dissociation of the SI), the Ca-SI binding, and the solubility of the resulting complex.

Summary The development of effective scale-inhibitor (SI) squeeze treatments remains a challenge for carbonate reservoirs because of their substantial chemical reactivity with the SI. This in turn might potentially lead to uncontrolled SI precipitation and induced formation damage. This work takes a systematic approach to understanding the retention mechanisms of SI in carbonate formations with respect to the detailed carbonate-formation mineralogy, type of SI, and reservoir conditions in the absence of oil. Static adsorption/compatibility experiments, described previously as apparent adsorption tests (Kahrwad 2008), were performed to evaluate the areas of different retention mechanisms [pure adsorption (Γ) and coupled adsorption/precipitation (Γ/Π)] of different SI species in brine. Experiments were conducted for five SIs at various conditions: initial pH values, mineralogical compositions (calcite, limestone, and dolomite), and temperatures. The SI species used in this study included a phosphonate [di-ethylene tetra-amine penta (DETPMP)], a phosphate ester [polyhydric alcohol phosphate ester (PAPE)], and three polymeric SIs [polyphosphino carboxylic acid (PPCA), P-functionalized copolymer (PFC), and sulfonated polyacrylic acid copolymer (VS-Co)]. All precipitates were studied using environmental scanning electron microscopy/energy dispersive X-ray (ESEM/EDX) and particle-size analysis (PSA). The overall results from these coupled Γ/Π experiments are as follows: For the polymeric SIs (PPCA, PFC, and VS-Co), the highest retention levels were observed at low pH for all carbonate substrates, because of the increase in divalent cations calcium and magnesium (Ca2+ and Mg2+, respectively) available from rock dissolution for SI–M2+ ions (divalent cations) precipitation. For DETPMP and PAPE SIs, the retention level was greatest at higher pH values, because the SI functional groups were more dissociated and, hence, available for complexation with M2+ ions. The polymeric VS-Co predominantly showed pure adsorption with only a low amount of precipitation (Γapp ≈ 1.2 mg/g) in contact with the dolomite substrate. This is because of the presence of sulfonate groups (low pKa). For polymeric inhibitors, the retention level (Γapp) was highest on calcite (highest relative calcium content), followed by limestone and dolomite. DETPMP and PAPE 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, Π) was observed at elevated temperatures. At lower temperatures, an extended region of pure adsorption was observed for all SIs. The information presented in this study will be helpful in SI product selection on the basis of mineralogy and reservoir conditions. As a consequence, longer squeeze lifetimes and improved efficiency of SI deployment in carbonate reservoirs can be achieved. In addition, this study provides valuable data for validating models of the SI/carbonate/Ca/Mg system that can be incorporated into squeeze design simulations.

Understanding the mechanisms of scale inhibitor (SI) retention in carbonate formations is key to designing efficient SI "squeeze" treatments in oil reservoirs. By performing "apparent adsorption" experiments, this paper demonstrates that a coupled adsorption/precipitation (Γ/Π) retention mechanism dominates in calcite and limestone for two widely applied scale inhibitors, DETPMP and PPCA. Precipitation was a more dominant retention mechanism at both initial pH values (pH 0 4 and 6) and T = 80 and 95 °C for both SIs. At 95 °C, the pure adsorption (Γ) region only extends up to [SI] ∼ 100 ppm, above which precipitation (Π) dominates. At lower temperatures (T = 80 °C), the solubility of the SI−M 2+ complex increases, resulting in less precipitation. The apparent adsorption results are supplemented by measuring the corresponding solution [Ca 2+ ], pH values in solution, and environmental scanning electron microscopy/energy dispersive X-ray analysis (ESEM/EDX) and particle size analysis (PSA), which give us a full mechanistic explanation of our results. For DETPMP, the retention increased as the solution pH increased, while retention of PPCA increased as the test pH decreased. Moreover, DETPMP was retained more than PPCA due to their differences in chemistry. Furthermore, the retention of both SIs was greater for the limestone sample due to Fe 2+ traces enhancing the precipitate of SI−M 2+ .

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 SI_Ca n complex, coupled to the carbonate system, is proposed to qualitatively explain these experimental findings.

Studying the interaction between scale inhibitors (SIs) and chemically reactive carbonate minerals is crucial for determining SI retention in “squeeze” treatments. This study investigated the retention of the environmentally friendly SI, polyhydric alcohol phosphate ester (PAPE), on calcite and dolomite substrates. Elemental analysis of the supernatant solution as well as pH measurement and environmental scanning electron microscopy (ESEM) with energy dispersive X-ray analysis (EDX) were all used to investigate SI retention and to identify the morphology/composition of the resultant SI–Ca precipitates. Results revealed that PAPE was retained by calcite via pure adsorption at an initial test pH (pH0) of 4 and then precipitated at pH0 6. In contrast, the PAPE/dolomite system was found to be effectively pH-independent, with precipitation dominating at both pH0 values. Any temperature effect was negligible for dolomite/PAPE retention, whereas with calcite, retention was smaller at lower temperature, which is attributed to the temperature-dependence of the substrate solubility. Overall, the final pH of the system and the resulting degree of SI dissociation contributed more to PAPE retention than did the final calcium concentration. EDX analysis confirmed scale-inhibitor phosphorus in the deposited solids, indicating coupled adsorption/precipitation. This phosphorus increased with the amount of precipitation and with the temperature, confirming the corresponding static adsorption test results.

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.

Summary Preformed particle gel (PPG) is an appropriate solution for conformance control and water management in low permeability reservoirs. In this paper, the network parameters of PPGs are evaluated through swelling tests and rheology and by determining their role in maintaining structural strength. Several PPG hydrogels were prepared by varying the concentrations of polyacrylamide and chromium triacetate [Cr(OAc)3] as copolymer and crosslinker, respectively, using a central composite design. The characterization of these hydrogels was performed using a scanning electron microscope (SEM), electron dispersion X-ray (EDX), and environmental scanning electron microscope (ESEM). The correlation between reaction conditions and the network parameters of polymer networks, such as the molecular weight of the polymer chain between two neighboring crosslinks (Mc¯), the crosslink density, and the size fraction, have been determined. In addition, swelling of the hydrogels took place through the Fickian diffusion mechanism. Structural states of Laponite dispersions strongly depend on concentration and ionic strength. Based on the rheological tests, the dynamic modulus of the PPG was strongly dependent on the initial concentration and resulting network parameters of the hydrogel. The results showed an effective interaction between Mc¯ and the elastic modulus of the gel network. Through the optimization of the network parameters, the appropriate composition was presented on the basis of the factors of strength (complex modulus of 4×104 Pa in the plateau region), the formation of a 3D network, and the preservation of the viscoelastic structure in the presence of Na+, Ca2+, and Mg2+, at a salt sensitivity of 0.5. In addition, the optimum sample structure was confirmed on the basis of microscopic images. Based on the coreflooding data, the optimal PPG showed a disproportionate permeability reduction (DPR) index of 17.01 and indicated the dual performance of these materials against water and oil. Also, the permeability diagrams of the core showed wettability of the oil-wet core could shift to more water-wet after PPG injection. To summarize this research, we present the determination and analyses of the network parameters as a novel technique for predicting the performance of hydrogels in porous media, and for investigating their strength under harsh reservoir conditions. In other words, determination of the network parameters can be used to ensure the success of the gel performance in porous media.

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