The Influence of Formation Calcium and Magnesium on the Effectiveness of Generically Different Barium Sulphate Oilfield Scale Inhibitors

Abstract

Summary Using chemical scale inhibitors is one of the most common methods of preventing downhole and topside mineral scale formation in oil fields. Several aspects of the brine composition may affect the performance of the various scale inhibitors. In this paper, we focus on the roles of calcium and magnesium ion concentrations. The calcium concentration in a particular reservoir and in the inhibitor slug often determines the extent to which the inhibitor species is retained in the near-wellbore area (i.e., on its adsorption or precipitation behavior). What is less well understood is the effect of divalent cations on the inhibition process itself. Common ion effects are well known; however, for pentaphosphonate inhibitor species (e.g., DETPMP), significant improvements in inhibition efficiency have been reported by increasing the calcium concentration in the solution. In this paper, we expand significantly on such observations. The effect of calcium and magnesium cation concentrations is examined for a wide range of generically different inhibitor species, including pentaphosphonate, hexaphosphonate, phosphinopolycarboxylate, polyvinyl sulphonate, and sulphonated polyacrylate copolymers. The results clearly indicate how different inhibitor species are affected quite differently by changes in [Ca2+] and [Mg2+] and how this difference relates to the cation affinity of the inhibitors active functional groups. The results were obtained by comparing the barium sulphate inhibition efficiency of various species in mixtures of a low/medium scaling (Brent type) formation brine and seawater (SW) and also in a more severe scaling (Forties type) formation brine/SW mixture. Barium sulphate inhibition efficiencies were examined by static inhibition efficiency tests, with residence times ranging from 30 minutes to 24 hours. Phosphonates are shown to be poor inhibitors at very low [Ca2+], indicating that their effectiveness is controlled by the formation of Ca2+/phosphonate inhibitor complexes, as discussed in previous works.1,2 On the other hand, polymeric polycarboxylate inhibitors are shown to be effective even at very low [Ca2+], indicating that the formation of multiple bonds between the polymer and the crystal surface allows for stronger adsorption and, thereby, inhibition. However, it appears that strong ionic bonds involving calcium cation bridging are required for the phosphonate-based species. Conversely, when the magnesium ion concentration is increased, the performance of the phosphonate is significantly reduced, whereas the other polymeric species are relatively unaffected. This can be accounted for in terms of the cation affinity of different inhibitor functional groups in a similar manner as comparative adsorption and inhibitor/brine compatibility effects. For the polycarboxylate inhibitor species examined in this work, a clear maximum in inhibition efficiency is observed with increasing calcium concentration. This is explained, from related experiments, in terms of complexation (incompatibility) and differences in the adsorption modes at the scale surface. Introduction and Background The most important property any oilfield scale inhibitor must possess is the ability to prevent/inhibit crystal growth at threshold (i.e., substoichiometric) concentration levels. Many works have addressed the mechanism of threshold scale inhibition.3–20 In describing the threshold effect, it is generally regarded that the inhibitor molecules adsorb at the active growth sites, which may be crystal defects, thus preventing further crystal growth by interfering with the growth process. Coupled with this, the morphology, the tendency to agglomerate, and the potential of the electric double layer (the zeta potential) of the growing nucleons are also altered by adsorption of inhibitor molecules at the growth sites.9 The overall result of adding a scale inhibitor is a reduction in the crystallization tendency and the subsequent formation of scale by two pathways.Nucleation inhibition. Disruption of the thermodynamic stability of the growing nucleons (for homogeneous crystallization). The inhibition mechanism then involves endothermic adsorption of inhibitor species, causing dissolution of the barium sulphate embryos.Crystal growth retardation. Interference/blocking the crystals' growth processes (for heterogeneous crystal growth). The inhibition mechanism then involves irreversible adsorption of the inhibitor species at active growth sites of barium sulphate crystals, resulting in their blockage. In oilfield applications, however, it is recognized that precipitation is more likely to occur on a surface that is already present. Such surfaces may be existing scale deposits, metal surfaces that offer available sites for adsorption of lattice ions (production equipment, pipelines, etc.), or the rock formation. Heterogeneous nucleation on such surfaces would be more likely than homogeneous because the free-energy barrier (for the reduction in supersaturation) may be reduced.5 The most widely used groups of downhole scale inhibitors to control barium sulphate (BaSO4) scale formation are phosphonates and small polyelectrolytes (molecular weight <104) with a polycarboxylate base. These inhibitors tend to show good performance for a range of pH and temperature conditions. However, their effectiveness deteriorates quite markedly as the pH is lowered. 3,4,20-24 This is directly related to the pKa values of the particular species and the relationship to the extent of dissociation (or ionic character) present at a particular pH. This behavior gives a strong insight into the manner in which these species interact with the scale crystals. Thus, BaSO4 scale prevention with these common inhibitor species is more difficult in low-pH environments, especially when the supersaturation is high. Alternatively, sulphonated polymers, such as polyvinyl sulphonate (PVS) (the functional groups of which are strongly acidic sulphonic acid units) provide good barium sulphate inhibition, even at low pH values (<4), because these species are still completely dissociated at these lower pH values (i.e., they have low pKa values). We conclude, then, that for barium sulphate inhibition in relatively high supersaturation brine mixtures generally found in North Sea environments, the dissociated inhibitor functional groupings are required to inhibit scale formation. The mode of attachment to the growing scale crystal is strong electrostatic binding.

Barite Nucleation and Inhibition at 0 to 200°C With and Without Thermodynamic Hydrate Inhibitors

“…Sorbie and Laing (2004) concluded that the mechanism for such improvement involves two steps: (1) when calcium concentration is relative high, some calcium ions can enter the barite lattice to distort the lattice, and (2) with excess calcium ions, some of the calcium ions can react with DETPMP to form Ca/DETPMP complex that can interact with the growing barite crystals and distort the barite lattice further. The similar effects of Ca 2+ on the inhibition efficiency of BHPMP as shown in Boak et al (2003) and Sorbie and Laing (2004) were also found in this study. Illustrated in Fig.10a .…”

“…Their finding explained the effect of Ca 2+ on BHPMP inhibition efficiency. The effect of Ca 2+ on inhibitor inhibition efficiency was also studied by Boak et al (2003) and Sorbie and Laing (2004). In their studies, they investigated the effects of Ca 2+ and Mg 2+ on inhibition efficiency of diethylene triamine penta (methylene phosphonic acid) (DETPMP), PPCA, and polyvinyl sulfonate (PVS) at 95°C.…”

The Effects of Barium Sulfate Saturation Ratio, Calcium, and Magnesium on the Inhibition Efficiency—Part I: Phosphonate Scale Inhibitors

“…It is well known that the MIC for barite prevention in a given brine mix increases with SR. It has also been found that the presence of calcium (Ca 2þ ) generally helps phosphonate SIs to inhibit barite, whereas the presence of magnesium (Mg 2þ ) tends to reduce their IE (Graham et al 1997(Graham et al , 2003Boak et al 1999). In this work, we have found that the MIC level of selected phosphonate SIs is controlled mainly by the SR, and these are denoted as Type 1; however, the MIC of other phosphonate SIs is controlled mainly by brine [Ca 2þ ] and [Mg 2þ ] content, and these are denoted as Type 2.…”

“…We believe that only the Ca_SI complex is active, because both Ca 2þ and the Ca_SI complex are incorporated into the barite lattice, whereas neither Mg 2þ nor Mg_SI are incorporated (Sorbie and Laing 2004;Boak et al 1999;Graham et al 1997Graham et al , 2003; hence, the SI is effectively "poisoned" by the Mg 2þ . The effectiveness of phosphonate SIs, ], and this specific effect is the main focus of this paper.…”

Scale inhibition study by turbidity measurement

“…<50 ppm. Above a certain threshold concentration, scale inhibitors can reduce the tendency for crystallization or completely prevent scale formation and growth by disrupting the thermodynamic stability of growing nuclei, causing dissolution of nucleated scale and/or interfering with the crystal growth process, resulting in blockage of the growing sites [4].…”