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.
Unlike other types of inorganic scales, carbonate and sulphide scales are directly correlated to the in-situ concentration of acid gases such as CO2 and H2S, which influence the local pH and availability of reactive species. The common approach to sulphide and carbonate scale prediction often does not account for three phase CO2 and H2S partitioning at different temperatures and pressures along the system. This leads to the use of inaccurate compositions and pH values for the mineral scaling calculations. In this paper, we apply a rigorous workflow (step-by-step procedure) based on a compositional PVT model to calculate molecular CO2 and H2S distribution, three phase relative volume changes, compositional changes and scale precipitation trends from the reservoir to the wellhead separation stage using commonly available surface field data, a full PVT software and a scale prediction software of user's choice. A simplified version of the workflow was previously applied to high CO2 and H2S gas/condensate wells with production of condensed water only (Verri et al., 2016). This paper focuses on the field application of our general workflow to North Sea oil wells with high water cut and medium H2S levels (≈2200 ppmv in the separator gas phase). By following this rigorous procedure and applying the concept of Maximum Dissolved Iron (MDI) we were able to provide accurate sulphide and carbonate scale prediction profiles from reservoir to separator. The combination of reservoir, production and chemical engineering models using specific iterative processes (within the workflow) has provided a new and reliable step-by-step procedure for the prediction of combined sulphide and carbonate scales in oil and gas wells which can be implemented by anyone using any PVT and scale prediction software. However, in our ongoing work, we are building a model which incorporates all of this workflow by coupling together PVT/VLE models with our aqueous scale prediction software.
A series of static adsorption/precipitation tests has been carried out to examine the behaviour of Diethylenetriamine Penta Methylene Phosphonic (DETPMP), a commonly used commercial scale inhibitor (SI), when in contact with brine and calcium carbonate (CaCO3). These tests are carried out at pH 2 and pH 4 and at a range of DETPMP concentrations from 0ppm to 5000 ppm. In these tests, the addition of disaggregated CaCO3 to the brine/SI mixture was accompanied by a rise in the pH and a commensurate precipitation of a DETPMP_Ca complex. Apparent adsorption displayed a common 2:1 trend indicating that no adsorption took place, only precipitation. These results changed somewhat when selected divalents (Ca and Mg) were removed from the brine.
The bulk "apparent adsorption" behavior (Γapp, vs. Cf) of 2 polymeric scale inhibitors (SI), PPCA and PFC, onto carbonate mineral substrates has been studied for initial solution pH values of pH 2, 4 and 6. The 2 carbonate minerals used, calcite and dolomite, are much more chemically reactive than sandstone minerals (e.g. quartz, feldspars, clays etc.) which have already been studied extensively. In nearly all cases, precipitates formed at higher SI concentrations were due to the formation of sparingly soluble SI/Ca complexes. A systematic study has been carried out on the SI/Ca precipitates formed, by applying both ESEM/EDX 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] <100ppm) and coupled adsorption/ precipitation (Γ/Π) are clearly observed for both calcite and dolomite. PFC at pH = 4 and 6 also showed very similar behavior with a region of pure adsorption (Γ) for [PFC] < 500ppm and a region of coupled adsorption/precipitation (Γ/Π) above this level. However, the PFC/calcite case at pH 2 showed only pure adsorption, while the PFC/dolomite case at pH 2 again showed coupled adsorption/ precipitation at higher PFC concentrations. For both SIs on both carbonate substrates, precipitation is the more dominant mechanism for SI retention than adsorption above a minimum concentration of ~100 – 500 ppm SI. The actual amount of precipitate formed varies from case to case, depending on the specific SI, substrate (calcite/dolomite) and initial pH (pH 2, 4 and 6). Although the qualitative behavior of both PPCA and PFC was similar on both carbonate substrates, the apparent adsorption of PPCA was higher on calcite than on dolomite; PFC apparent adsorption was higher on dolomite than on calcite. It is discussed in the paper how these observations are related to the reactivity of the different carbonate minerals, the resulting final pH (which affects the dissociation of the SI), Ca-SI binding and the solubility of the resulting complex.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.