Solubility and Inhibition Efficiency of Phosphonate Scale Inhibitor_Calcium_Magnesium Complexes for Application in a Precipitation-Squeeze Treatment

Abstract

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.

Experimental Investigation of the Interaction between a Phosphate Ester Scale Inhibitor and Carbonate Rocks for Application in Squeeze Treatments

“…In the PAPE/calcite system, the normalized calcium concentration in PAPE/calcite system at 60 °C leveled off at ∼1, which means that all calcium generated in situ was consumed, and there was no excess calcium in solution, whereas in the same system at 80 °C, normalized [Ca 2+ ] decreased even below the baseline, which means that some calcium was consumed from the NSSW. Indeed, the calcium involved in complexation for PAPE/calcite system at 60 °C was less than that at 80 °C, meaning that less reaction occurred at lower temperature, leading to less apparent adsorption. , …”

“…For pH 0 4 at 80 °C compatibility tests (no substrate), there was no noticeable change in final pH (see Figure (a)), showing that PAPE was compatible with NSSW at these conditions. The final pH increased when the tests were repeated in the presence of 5 or 10 g of either carbonate substrate because of the dissolution of calcite or dolomite with the associated liberation of Ca 2+ (and Mg 2+ for dolomite) and carbonate (CO 3 2– ). , This trend was observed until the retention regime changed from pure adsorption to coupled adsorption/precipitation, at which point the divalent cations generated in situ were consumed by the formation of an SI–M 2+ complex. , The final pH for dolomite was higher than that of the calcite system, resulting in a greater degree of PAPE dissociation and a higher tendency for apparent adsorption (as shown in Figure (a,b)).…”

“…When the scale inhibitor is injected into carbonate formations, the SI (weak polyacid) solution contacts the carbonate mineral substrate and causes rock dissolution, leading to an increase of initial pH to ∼7 and 8 for calcite and dolomite substrates, respectively. The increased solution pH causes the inhibitor to become more dissociated, and these negatively charged species (dissociated scale inhibitor) are then able to be adsorbed onto the positively charged rock surface through electrostatic interactions. , In addition, if the scale-inhibitor concentration is sufficiently high, Ca 2+ binds with the dissociated scale inhibitor (SI–Ca 2+ complexation). , Thus, the entire system is coupled, and the three parts of the equilibrium system are the dissociation of SI as a weak polyacid (H n A) to form dissociated species, such as H n – m A m – right up to A n – at very high pH values; these dissociated SI species, which are very strong chelating agents, then bind with Ca 2+ (and/or Mg 2+ ) ions to form SI/Ca complexes, which are known to be sparingly soluble; the above reactions are coupled to the carbonate system, because the lower pH solution conditions and the chelating power of the dissociated SI cause the carbonate equilibrium to change. …”

“…The level of Mg 2+ should also be taken into account when examining the retention of PAPE, and research has shown that magnesium has a poisoning effect for phosphonate scale inhibition performance . The normalized magnesium concentration is presented in Figure (a,b); note that Mg is present in the initial NSSW brine ([Mg 2+ ] = 1368 ppm), and it may be available from the dolomite substrate (CaMg­(CO 3 ) 2 ), but it is only present at trace levels in calcite.…”

Effect of Carbon Nanoparticles on the Crystallization of Calcium Carbonate in Aqueous Solution

“…Because of the strong complexing effect of their functional groups with calcium ions, the solubility of calcium ions in water can increase, which is considered to be the main reason for the scale inhibition of chemical scale inhibitors. In addition, the scale inhibitors can also adsorb on the crystal surface of the sediment, resulting in the change in the crystal form of calcium carbonate [37]. Therefore, most of the chemical scale inhibitors have both solution scale inhibition performance and surface scale inhibition performance.…”