inhibitors studied include: penta-phosphonate (DETPMP), hexa-phosphonate (TETHMP), phosphino-poly carboxylate. (PPCA) and poly-vinyl sulphonate (PVS).
SPE 50771 The Influence of Divalent Cations on the Performance of BaSO4 Scale Inhibitor Species L.S.Boak, G.M. Graham, SPE and K.S. Sorbie, SPE, Heriot Watt University
Copyright 1999, Society of Petroleum Engineers Inc. This paper was prepared for presentation at the 1999 SPE International Symposium on Oilfield Chemistry held in Houston, Texas, 16–19 February 1999. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.
Abstract One of the most common methods of preventing downhole and topside mineral scale formation in oilfields is through the use of chemical scale inhibitors. Several aspects of the brine composition may affect the performance of the various scale inhibitors used in oilfield applications. In this paper, we focus on the influence of divalent cations such as calcium and magnesium. The concentration of such divalent cations is shown to effect the barium sulphate inhibition efficiency of generically different species of scale inhibitors by varying degrees, and in different directions. In previous work,1 the effect of calcium cations on the performance of various inhibitor species was demonstrated. In this paper, these observations are extended to include the effects of the magnesium cations in solution. The influence of divalent cations such as Ca2+ and Mg2+ on the barium sulphate inhibition efficiency of generically different inhibitor species has been examined here in a variety of brine systems including low scaling “Brent type” and medium scaling “Forties type” brines. In addition, low salinity “ideal” brine systems have also been examined. The scale inhibitors studied include: penta-phosphonate (DETPMP), hexa-phosphonate (TETHMP), phosphino-poly carboxylate (PPCA) and poly-vinyl sulphonate (PVS). Results are presented from a series of static barium sulphate inhibition efficiency tests conducted at 95°C over 2 and 22 hour residence times. The results presented here demonstrate how phosphonate inhibitors are significantly improved in the presence of [Ca2+] and also that their effectiveness is considerably reduced, or poisoned by increased [Mg2+]. The performance of the
phosphonate species are shown to be controlled by the performance of inhibitor/Ca2+ complexes, whereas the formation of inhibitor/Mg2+ complexes effectively poisons the phosphonate species ability to inhibit. The polymeric species (PPCA and PVS) are shown to be less influenced by increased [Mg2+]; indeed, marginally improved performance is obtained. This paper significantly extends the findings reported previously in SPE 37273.1 Introduction and Background The most important property which any oilfield scale inhibitor must possess is the ability to prevent or retard crystal growth of mineral scales at threshold (i.e. sub-stoichiometric) concentration levels. Many workers have studied the mechanism of threshold scale inhibition and a full reference list is included in a previous SPE paper.1 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 interference with the growth process. Coupled with this, the morphology, tendency to agglomerate and the potential of the electric double layer (the zeta potential) of the growing nucleons are also altered by the adsorption of inhibitor molecules at the growth sites.2 The overall result of adding a scale inhibitor is a reduction in the tendency for crystallisation and the subsequent formation of scale, by the following two main mechanisms: (i) Nucleation Inhibition: Disruption of the thermodynamic stability of the growing nucleons (for homogeneous crystallisation). The mechanism of inhibition involves endothermic adsorption of inhibitor species, thus causing re-dissolution of the barium sulphate embryos; and (ii) Crystal Growth Retardation: Interference/blocking of the growth processes of the growing crystals (for heterogeneous crystal growth). The mechanism of inhibition involves irreversible adsorption of inhibitor species at the active growth sites of barium sulphate crystals, resulting in the blockage of further growth. In oilfield applications however, it is recognised that precipitation of mineral scales is more likely to occur on a surface which is already present. Such surfaces may be existing scale deposits, metal surfaces offering available sites
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for adsorption of lattice ions (production equipment, pipelines etc.) or sites on the rock mineral surfaces. Heterogeneous nucleation on such surfaces would be more likely than homogeneous nucleation since the free energy barrier (for the reduction in supersaturation) may be reduced.3 The most widely used groups of downhole scale inhibitors for the control of barium sulphate (BaSO4) scale formation are phosphonates and small polyelectrolytes (molecular weights typically
