Surface Chemistry of Phosphonate Scale Inhibitor Retention Mechanisms in Carbonate Reservoirs

Abstract

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.

Mitigation of Mineral Deposition in Carbonate and Sandstone Rocks Using Green Scale Inhibitors

“…Surprisingly, it seems that inulin had performed even poorer than the solution without the SI, although the amounts of calcium ion concentrations (Table ) and inhibition efficiency (Table ) imply the other way round. This could be potentially rationalized based on the retention mechanism of SIs , and by the CT scan analysis as it will be discussed in Section .…”

Experimental Analysis of Scale Inhibitors Retention in Carbonate Formations for Application in Squeeze Treatments

“…Corresponding compatibility experiments (with no mineral present) were carried out to establish whether the inhibitor is fully compatible with the test brine (in this case, with NSSW). Thus, any reduction in SI concentration for this test is due to the incompatibility between SI and NSSW, that is, it results in the formation of SI/Ca 2+ precipitation. ,, …”

“…Carbonate rocks provide a much more chemically reactive substrate than sandstone reservoirs and so the scale inhibitor retention mechanism can be more complex than simple adsorption. ,,, In carbonate formations, scale inhibitors such as DETPMP (which are often deployed in an acidic form) may react with the formation and form precipitated complexes, that is, as sparingly soluble calcium salts of the SI as reported by Tomson et al Precipitating of the SI in this way can result in longer scale squeeze lifetimes and, hence, can give an improved efficiency of SI deployment. Indeed, even fully neutralized inhibitors react with carbonates to form precipitates since the SI species are generally strong chelators for divalent ions, such as Ca 2+ .…”

“…These techniques revealed direct information on the morphology of any precipitates formed, that is, whether they are formed as free crystalline (or amorphous) precipitates or whether they form as “coated structures” on the surface of the calcite grains. The EDX measurements also yield an approximate chemical composition of the precipitates; for example, establishing the presence of phosphorus (P from the SI) and Ca, and in what approximate (P/Ca) atomic ratios. , …”

A sustainable approach to synthesize phosphonated chitosan using ball milling and its application for oilfield scale management

“…1 It has been stated that the phospho-nate group improves the adsorption activities onto the reservoir rock, affording a prolonged scale prevention lifetime. 3,10 Moreover, it was found that the injected dosage of organophosphorus-based SIs can be tracked in the formation brines via several procedures (e.g., mass spectrometry, high-pressure liquid chromatography (HPLC), and inductively coupled plasma mass spectrometry, ICP-MS). 11 However, most organophosphorus-based SIs have several disadvantages, such as poor compatibility with the production system, forming complex precipitation (e.g., SI-Ca 2+ ).…”