In produced waters from fields undergoing seawater (SW) flooding, inhibiting mineral scaling can be problematic because the SW/formation water (FW) ratio is constantly changing. For barium-sulfate scale, for example, the barite saturation ratio (SR), the yield of barite precipitate, and the molar ratio Ca2+/Mg2+ in the produced waters all evolve over time. This paper describes the effects of SR and molar ratio Ca2+/Mg2+ on the barium-sulfate inhibition efficiency (IE) of nine phosphonate scale inhibitors (SIs): OMTHP (hexa-phosphonate), DETPMP and HMTPMP (penta-phosphonates), HMDP and EDTMPA (tetra-phosphonates), NTP (tri-phosphonate), EABMPA and HEDP (di-phosphonates), and HPAA (mono-phosphonate and mono-carboxylate). IE experiments were carried out testing a range of SW/FW compositions (i.e., SR and molar ratio Ca2+/Mg2+ varying). The minimum inhibitor concentration (MIC) level of these phosphonate SIs might correlate with either the level of SR for the SW/FW mixing ratio in question (Type 1) or the Ca2+ and Mg2+ levels in solution (Type 2). When experiments were repeated, but the produced brine molar ratio Ca2+/Mg2+ was fixed, the MIC for both Type 1 and Type 2 species always correlated with the SR. The performance of these phosphonate SIs in consumption experiments, where supernatant [SI] and [Ba2+] are both assayed by inductively coupled plasma (ICP) spectroscopy at multiple residence times, is also briefly discussed. In this paper, the reasons behind Type 1 and Type 2 IE behavior in phosphonate SIs are discussed, in terms of SI molecular structure, pH, SI speciation, SI binding constants to Ca2+ and Mg2+ cations, and the possible mononuclear or polynuclear chelate structures with M2+ cations that can form under the test conditions. Possible SI-M2+ complex structures are proposed, and through molecular modeling, explanations are provided for why Type 1 and Type 2 behavior is exhibited by phosphonate SIs.
Summary Barium sulfate is one of the most-difficult types of scale to inhibit in oil- and gas-production systems because of its physical hardness and its chemical and thermal stability. Barium sulfate is most commonly inhibited with either phosphonate or polymeric scale inhibitors (SIs) deployed at substoichiometric concentrations. What is less well-known in the oil industry is the effect resulting from the use of combinations of two (or more) SIs synergistically for enhanced scale-inhibition performance. A positive “synergistic” effect would be one in which, for example, 5 ppm of A + 5 ppm of B performed better than 10 ppm of either A or B. In this paper, a series of static barium sulfate inhibition-efficiency (IE) test results are presented, in which a series of pairs of SIs have been tested to determine their synergistic properties at pH 5.5 and 95°C. Polymers can be blended with phosphonates or, alternatively, pairs of phosphonates or polymers may be applied. In all cases, the synergistic IE is compared with the IE of each SI tested independently at the mass dosage (i.e., the same concentration in mg/L or ppm). Each separate single SI used in the work has been tested previously for barium sulfate IE at pH 5.5 and 95°C to determine the minimum inhibitor concentration (MIC) for each species (Shaw et al 2012a, b). Previously, nine phosphonate and nine polymeric SIs were tested individually; in this work, 34 SI combinations have been tested to examine their synergistic properties. The MICs of the synergistic blends are compared with the normal MICs of the individual SIs. Surprisingly, in most cases, the IE of the blends is usually higher over the range of SI concentrations tested (i.e., the MIC of the blend is lower), compared with that of each SI tested separately. Certain “pairs” of SIs used together yield a significantly beneficial effect (e.g., DETPMP and HMTPMP, both pentaphosphonates). Some mechanistic reasons for these synergistic pairs working particularly well are suggested.
Summary In oilfield-produced waters, the barite mineral-scaling problem is a "moving target" because the seawater/formation-water (SW/ FW) mixing ratio is constantly changing. Therefore, the barite saturation ratio (SR), the yield of barite precipitate, and molar ratio Ca2+/Mg2+ in the produced waters are all evolving over time. This paper describes the effects of SR and molar ratio Ca2+/ Mg2+ on the barium sulfate inhibition efficiency (IE) of nine polymeric scale inhibitors (SIs): phosphino poly carboxylic acid (PPCA); maleic acid ter-polymer (MAT)—a green SI; sulfonated PPCA (SPPCA); phosphino methylated polyamine (PMPA)—a poly-phosphonate; a generic P-functionalized copolymer (PFC); polyvinyl sulfonate (PVS); vinyl sulfonate acrylic acid copolymer (VS-Co); and cationic ter-polymers A and B (CTP-A and CTP-B). The behavior for polymers is compared with similar results for phosphonate SIs (Shaw et al. 2010). IE experiments were carried out for a wide range of SW/FW compositions (i.e., SR and molar ratio Ca2+/Mg2+ varying). The minimum-inhibitor-concentration (MIC) levels of these polymeric SIs sometimes correlate with the level of SR, but not always, which is because of Ca2+ and Mg2+ effects. When experiments were repeated but the produced-brine molar ratio Ca2+/Mg2+ was fixed, MICs always correlate with the level of SR for all nine polymers studied. However, it was observed that for SIs PPCA, MAT, and PFC, the base-case MICs (i.e., molar ratio Ca2+/Mg2+ varying) were less than the fixed-case MICs (molar ratio Ca2+/Mg2+ fixed), whereas in testing SPPCA, PMPA, PVS, VS-Co, and both the cationic terpolymers, the converse is true (i.e., the fixed-case MIC is less than the base-case MIC). It has been demonstrated conclusively that the high [Ca2+] in the fixed-case tests causes some SI precipitation with brine Ca2+ when PPCA is being evaluated (as PPCA-Ca). Sulfonate functional groups present on polymeric SI molecules may help to prevent such incompatibility problems encountered with brine Ca2+ (e.g., SPPCA does not precipitate as SPPCA-Ca). Low levels of brine calcium (e.g., approximately 500 to 700 ppm) can be very beneficial to the IE performance of PPCA. However, when the [Ca2+] reaches a certain level (approximately 1,000 ppm or higher), this causes some precipitation of a Ca–PPCA compound. It has been illustrated that low-to-moderate Ca2+ levels are often best, and it is concluded that this is why SIs PPCA, MAT, and PFC perform better under base-case experimental conditions (lower Ca2+ mix). It has been illustrated that PMPA behaves remarkably similarly to conventional phosphonate SIs with regard to Ca2+ and Mg2+. In fact, the behavior of PMPA suggests that it is not polymeric in nature. PVS, VS-Co, and both the cationic ter-polymers are mildly affected by divalent ions Ca2+ and Mg2+, with the latter being affected more than PVS because of the presence of carboxylate functional groups.
Summary Phosphonate scale inhibitors (SIs) applied in downhole-squeeze applications may be retained in the near-well formation through adsorption and/or precipitation mechanisms. In this paper, we focus on the properties of precipitated calcium phosphonate complexes formed by nine common phosphonate species. The stoichiometry [calcium ion to phosphorous (Ca2+/P) ratios] in various precipitates is established experimentally, and the effect of solution pH on the molar ratio of Ca2+/P in the precipitate is investigated. All static precipitation tests were carried out in distilled water (DW), with only Ca2+ [as calcium chloride (CaCl2)] and SI present in the system at test temperatures from 20 to 95°C. The molar ratio of Ca2+/P in the solid precipitate was determined by assaying for Ca2+ and P in the supernatant liquid under each test condition by inductively coupled plasma (ICP) spectroscopy (Ca0 and P0 are known, but are also measured experimentally). We show experimentally that the molar ratio of precipitated Ca2+/P (or Ca2+/SI; or n in the SI–Can complex) depends on the SI itself and is a function of pH for all phosphonates tested. It is found that, as pH increases, the molar ratio of Ca2+/P (n in the SI–Can) in the precipitate increases up to a theoretical maximum, depending on the chemical structure of the phosphonate. Our findings corroborate proposed SI-metal complex-ion structures, which were presented previously in Shaw et al. (2012c), as discussed in detail in this paper. In addition, the precipitation behavior of the various compounds is modeled theoretically by developing and solving a set of simplified equilibrium equations. We find that the precipitation behavior can be modeled, but only if a fraction (β) of “non-SI” of the initial phosphonate SI is taken into account. The quantity β can be as high as 0.2 (i.e., approximately 20% non-SI), although there is a degree of variability in this factor from product to product. However, good quantitative agreement is shown comparing the predictions of the equilibrium-solubility model with the experiment. Such models can be used directly in the modeling of field phosphonate precipitation-squeeze treatments.
Summary Scale-inhibitor (SI) squeeze treatments are applied extensively for controlling scale formation during oil and gas production. The current research involves phosphonate/metal precipitate studies in the context of precipitation-squeeze treatments. The main focus here is on the precipitation and solubility behavior of the SI_calcium (Ca)_magnesium (Mg) complexes of HEDP (a diphosphonate), DETPMP (a pentaphosphonate), and OMTHP (a hexaphosphonate); these mixed phosphonate/divalent precipitates are denoted as SI_Can1_Mgn2, where n1 and n2 are the stoichiometric ratios of Ca and Mg to SI, respectively. Precipitation experiments with SI_Can1_Mgn2 species were carried out over a temperature range of 20 to 95°C, while varying the Mg/Ca molar ratio over a wide range from all Ca to all Mg. These precipitates were formed in MgCl2·6H2O/CaCl2·6H2O brine solutions with appropriate molar ratios of metals, then separated from the supernatant by filtration. Subsequently, the solubility of the collected precipitate was found in a solution of the same Mg/Ca molar composition from which it was prepared. In this type of experiment, the solubility of the SI_Can1_Mgn2 precipitate without any respeciation is determined. In addition, another type of solubility experiment was carried out for a precipitate formed in a brine with one fixed Mg/Ca ratio; this was subsequently placed into a solution with different Mg/Ca compositions (from all Ca to all Mg). In these experiments, respeciation of the precipitate may occur. We have been able to establish the solubility (Cs) of the precipitates of three SIs (HEDP, OMTHP, and DETPMP) as a function of both temperature and Mg/Ca molar ratio. It has been shown that the solubility of precipitate is in equilibrium with Mg and Ca concentrations in solution, and any change of these parameters leads to solubility variation. All phosphonate/metal precipitates become less soluble with increasing temperature and much more soluble with an increasing proportion of Mg. We have found that any change in Mg/Ca ratio of brine does lead to a redistribution of Ca, Mg, and SI concentrations in a given precipitate and bulk solution, and, hence, leads to some variation in the precipitate solubility. Additionally, the inhibition efficiency (IE) of precipitated and then redissolved HEDP, OMTHP, and DETPMP SIs was tested and compared with the IE of industrial stock products. We show that, unlike polymeric SI precipitates, the inhibition activity of phosphonate SIs does not depend significantly on the precipitation process, and the IE of precipitated and redissolved SI_Ca and SI_Ca_Mg complexes is very close to that of the industrial stock solutions. These results can be used directly for modeling phosphonate precipitation-squeeze treatments, and the significance of these results for field applications is explained.
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