Structure, Stoichiometry, and Modelling of Mixed Calcium Magnesium Phosphonate Scale Inhibitor Complexes for Application in Precipitation Squeeze Processes

Abstract

In this paper, the properties of precipitated "mixed" calcium and magnesium phosphonate scale inhibitor (SI) complexes formed by 9 common phosphonate species are investigated. These complexes are of the form SI_ Ca Nl _Mg N2 where the stoichiometry (Ca 2ϩ /SI and Mg 2ϩ /SI molar ratios, i.e. N 1 and N 2 ) in various precipitates are established experimentally, and the effect of solution pH on the stoichiometry is determined. Static precipitation tests were performed varying the amounts of Ca 2ϩ and Mg 2ϩ present in the system (at a constant ionic strength), at test temperatures ranging from 20°C to 95°C, at a fixed [SI] ϭ 2, 000ppm. The stoichiometries of the solid precipitates were determined by assaying for Ca 2ϩ , Mg 2ϩ , and P in the supernatant liquid, under each test condition, by ICP spectroscopy. It is shown experimentally that, for all 9 phosphonates tested, these stoichiometries (i.e. N 1 and N 2 in SI_ Ca N1 _Mg N2 ) depend on (i) the nature of the SI (i.e. M 2ϩ binding sites per molecule); (ii) solution pH, which affects the speciation of the SI; (iii) the relative magnitude of the SI binding constants to Ca 2ϩ and Mg 2ϩ at the test pH (K b1 and K b2 , respectively); and (iv) the solution molar ratio of Mg 2ϩ /Ca 2ϩ . It is found that, as pH increases, the combined molar ratio of Ca 2ϩ and Mg 2ϩ to SI, i.e. N t ϭ N 1 ϩ N 2 in the complex, increases up to a theoretical maximum, N t,max , depending on the chemical structure of the phosphonate (corroborating earlier work, SPE 155114, SPE 164051). In addition, the precipitation behaviour of the various compounds is modelled theoretically by developing and solving a set of simplified equilibrium equations. Very good agreement is seen between the modelling and experimental results. Such models can be used directly in the simulation of field phosphonate precipitation squeeze treatments in order to design and optimize squeeze lifetimes.

Synergistic Properties of Phosphonate and Polymeric Scale Inhibitor Blends for Barium Sulphate Scale Inhibition

“…Interestingly, when PPCA was tested with EDTMPA ( Figure 16), only the 2 hour IE was enhanced marginally, whereas the 22 hour synergistic IE was lower than the individual EDTMPA IE. These observations are most likely indicative of the predominant mechanism by which PPCA operates, which is probably mainly by nucleation inhibition, although PPCA is still capable of some crystal growth inhibition properties, as stated in the literature (Boak, 2013;Sorbie and Laing, 2004;Sorbie et al, 2000;Shaw and Sorbie, 2013c;Yuan et al, 1998). Generally, most polymers function mainly by nucleation inhibition, and this results in 2 hour IE often being significantly better than their long-term IE (e.g.…”

“…Presumably this is because the mechanistic benefits of both inhibitors are exploited in the synergistic tests. For example, earlier basic IE tests have shown that DETPMP is more efficient than HMTPMP at 22 hours (Shaw et al, 2012a) because DETPMP is probably a better crystal growth blocker (Sorbie et al, 2000;Sorbie and Laing, 2004;Yuan et al, 1998). In some cases, HMTPMP has been shown to be more efficient than DETPMP at 2 hours, particularly in higher [Ca 2ϩ ] brine mixes (Shaw, 2012;Shaw et al, 2012a) and hence HMTPMP may be a better nucleation inhibitor than DETPMP, which may be related to the HMTPMP chemical structure.…”

“…Also, in the EDTMPA/PVS results (Figure 20), it is probably the PVS which is enhancing the 2 hour IE in the blend. PVS is known to be the polymeric SI which operates almost entirely by nucleation inhibition (Boak, 2013;Shaw, 2012;Shaw et al, 2012b;Shaw and Sorbie, 2013c;Sorbie et al, 2000;Sorbie and Laing, 2004;Yuan et al, 1998). Figures 23 to 28 show phosphonate products DETPMP, HMTPMP, EDTMPA, HMDP, NTP, and HEDP, respectively, tested synergistically with VS-Co. VS-Co is similar to PVS, except VS-Co also contains carboxylic acid functional groups.…”

“…In such tests, a number of variables can be changed, for example, brine composition (notably Ba 2ϩ , Ca 2ϩ and Mg 2ϩ ), the barite Saturation Ratio (SR), salinity, pH, temperature, etc. (Boak et al, 1999;Boak, 2013;Cushner et al, 1988;Graham et al, 1997Graham et al, , 2002Graham et al, , 2003Laing et al, 2003;Read and Ringen, 1982;Shaw, 2012;Shaw et al, 2012aShaw et al, , 2012b2012c;Shaw and Sorbie, 2012, 2013c, 2014bSorbie et al, 2000;Sorbie and Laing, 2004;Yuan et al, 1997aYuan et al, , 1997b1998;Yuan, 2001Yuan, , 2002. In some cases, [SI] may also be assayed over time in addition to IE -these experiments are referred to as SI consumption tests (Shaw, 2012;Shaw and Sorbie, 2013c).…”

“…In some cases, [SI] may also be assayed over time in addition to IE -these experiments are referred to as SI consumption tests (Shaw, 2012;Shaw and Sorbie, 2013c). In this work, all tests were conducted at pH 5.5, taking this as a "typical" reservoir pH; pH is an important variable since it affects the speciation of SIs in aqueous solution (Browning and Fogler, 1995;Oddo and Tomson, 1990;Shaw and Sorbie, 2012, 2013a, 2013d, 2014a, 2014cStone et al, 2002). In all static IE tests described in this paper, Nelson Forties formation water (FW which has a [Ba 2ϩ ] ϭ 269ppm) was used, which (with North Sea seawater, NSSW) gives a moderately severe scaling brine mixture.…”