Investigation of calcium carbonate scale inhibition and scale morphology by scanning electron microscopy

384 Journal of Chemistry, Vol. 43 (3), P. 384 - 387, 2005 INVESTIGATION OF CALCIUM CARBONATE SCALE INHIBITION AND SCALE MORPHOLOGY BY SCANNING ELECTRON MICROSCOPY Received 23rd-June-2004 Nguyen Thi Phuong Phong Institute of Materials Science Hochiminh City branch, Hochiminh City SUMMARY Calcium carbonate scale inhibition in squeeze treatment by the phosphonate–type scale inhibitors, such as diethylenetriamine penta (methylene phosphonic acid) (DETPMP), ethylene diamine tetra(methyelne phosphonic acid) (EDTMP) or by the mixing of DETPMP, EDTMP and chelants (citric acid (CA), maleic acid (MA), ethylendiamintetraacetic acid (EDTA) has been studied in previous works [5, 6]. This paper focused on several aspects concerning calcium carbonate scale inhibition mechanism of DETPMP and of the mixing of DETPMP with trace of chelants by scale morphology on scanning electron microscopy (SEM). From the SEM photos, it can be observed that the presence of the inhibitors, especially with the right ones, causes deformation of the crystal morphology of both the adhered and precipitated crystals. The strong depression of adhesion of crystal (calcite) is caused by the adsorption of inhibitor on its surface. I - INTRODUCTION Oilfield scale is practically a ubiquitous problem in oilfield operations. It can form when incompatible waters mix, for example during water flood, or it can form when reservoir pressure and temperature changes are endured by self-scaling brines. Sulfat scale commonly occurs with the former mechanism and carbonat scale with the latter [1, 2]. CaCO3 scale deposition depends upon the temperature of the fluid: calcium carbonate scale exists predominantly in calcite form at temperature of < 50oC, and exists predominantly in aragonite form at temperature of > 65oC. Carbonate calcium has in practice three poly- morphs forms: calcite (hexagonal), aragonite (ortho-rhombic), and vaterite (hexagonal) depending on the increasing solubility and decreasing thermo-dynamic stability. In our works [5, 6] phosphonates are the most cost-effective inhibitors of sulfate scale at reservoir conditions, but they are limited for inhibition of CaCO3 scale. The improving CaCO3 scale inhibition has obtained by mixing of phosphonate DETPMP and a trace of chelating agent such as CA, MA, EDTA [7]. In this paper, the CaCO3 scale morphology has been observed by scanning electron microscopy (SEM) in order to find significant differences in the morphologies between the presence and the absence of inhibitors and to understand better the CaCO3 scale inhibition mechanisms. II - EXPERIMENTAL Laboratory studies were performed to evaluate of inhibition efficiency of DETPMP in the absence and the presence of trace of chelant CA. Chemicals - DETPMP (concentration 30%; pH = 6.5) is 385 synthesized by Lab of Magnetochemistry, of Institute of Materials Science Hochiminh City Branch. - Citric acid 98%, Russia, is used as solution of 1%. - EDTA pure, Merck, is used as solution of 1%. - Murexide, Merck (purpurat ammonium), is used as solution of 1%. - Synthesis brines. Procedures Inhibition efficiency was determined by testing method NACE Standard TM 03-074-95 [4]. Inhibition efficiency is calculated according to the following equation: % ICa= [ ] [ ] [ ] [ ] 100 CaCa 22 22 ×    ++  ++ innoni innonin CaCa % ICa: percent calcium inhibition [Ca2+]in: soluble calcium concentration of the inhibited sample. [Ca2+]non-in: soluble calcium concentration of the uninhibited sample [Ca2+]i: initial soluble calcium concentration Soluble calcium concentration is titrated with standard EDTA solution and murexide (ammonium purpurate) indicator. After NACE tests, all solutions in test bottles were filtered through filter paper 0.2 µm and were dried at 50oC. Investigation of crystal morphology was performed on the SEM, JOEL JSM-5300, Japan. III - RESULTS AND DISCUSSION 1. Crystal morphology and polymorphous of CaCO3 In the uninhibited system, most of the precipitates in the solution are orthorhombic calcite particles with average particle size of 10 µm were formed on the bottom of test bottles (Fig. 1). During aging process (aging temperature of 70oC; aging time of 48 hours), the population of calcite crystals adhered on the solid surface. On the other hand, the particle sizes of vaterite and aragonite crystals in the solution are about 100 µm (Fig. 2). These facts indicate that the calcite on the surface nucleates and grows by direct crystallization of the lattice ion in solution. The aragonite, vaterite crystals precipitate at high temperature with large sizes. They are in the upper layer, and easily flow with fluids. Calcite precipitates at low temperature with smaller crystals. However, at high temperature and long aging time, these calcite crystals have strong adhesion and stick on the bottom of the bottle and it is difficult to treat away. It can be seen that in the absence of the inhibitor, calcite is the main crystal form. Fig. 1: SEM picture of the calcite crystals adhere on the surface in the uninhibited system Fig. 2: SEM picture of the aragonit/vaterite crystals in the uninhibited system The SEM pictures of particles adhered on the surface and precipitated in the solution in the presence of the inhibitor are shown in Figs from 3 to 6. The crystal morphology of adhered 386 calcite during aging time changes to form polycrystalline calcite by the presence of inhibitor. This morphology change may be caused by a partial covering of the crystal surface by the inhibitor. In the presence of the inhibitor, amount of the adhered calcite is markedly decreased and the particle size is relatively large compared with that in the absence of the inhibitor (Figs. 3&4). In the case of the most suitable inhibitor, DETPMP: CA (2 : 1) (the inhibition efficiency is upper 80%), the calcite crystals have loose structure with the holes and they are less adhesive (Fig. 4). The aragonite/vaterite crystals become the main polymorphs in the presence of the inhibitor. The aragonite/vaterite crystals have two beneficial effects in comparison with calcite crystals: (1) the aragonite/vaterite crystals will not adhere together to form a scale in the same way as calcite crystals would do; (2) the presence of aragonite/vaterite crystals will upset the equilibrium between the fluid and any existing scale. In the presence of DETPMP, the CaCO3 scale inhibition efficiency is low (< 35%, 10 ppm, 70oC, 48 h), but their treatment is rather easily because the aragonite crystals are predominant (Fig. 5). With the DETPMP: CA, the scale inhibition efficiency is the best, the vaterite polymorphs is prominent (Fig. 6). Fig 3: SEM picture of the calcite crystals in the presence of 10 ppm DETPMP after 48 h Fig. 4: SEM picture of the calcite crystals in the presence of 10 ppm DETPMP : CA (2 : 1) after 48 h Fig. 5: SEM picture of the aragonite/vaterite crystals in the presence of 10 ppm DETPMP after 48 h Fig. 6: SEM picture of the aragonite/veterite crystals in the presence of 10 ppm DETPMP : CA (2 : 1) after 48 h 2. Inhibition mechanism It is generally believed that the inhibitor molecules must adsorb at the active growth sites on the surface, which may be crystal defects, thus preventing further crystal growth by interference with the growth process. The inhibition of scale formation is affected by both the location of the adsorbed inhibitor at the 387 crystal surface and the extent of chemical bonding with the surface. Related study [3] shows that less than 5% of the crystal surface is covered by adsorbat molecules. This suggests that the inhibitor molecules are preferentially adsorbed at the most active growth sites (kinks) on the surface, alter the surface properties of the crystals, and may affect nucleation rate, crystal growth. DETPMP, CA adsorbed on CaCO3 surfaces by binding the carboxylic or phosphonat anions to surface of calcium ions. It was found that the inhibition effect of inhibitor is related to their surface-binding capability. Thus, we show that, the DETPMP: CA mixture contains carbonyl and phosphonic groups; therefore, its surface binding capability is stronger than that of DETPMP. Thus, DETPMP: CA possesses higher inhibition efficiency than DETPMP under the same conditions (CaCO3 inhibition efficiency of DETPMP is 30.12%; of DETPMP : CA (4 : 1) is 63.3%; of DETPMP : CA (3 : 1) is 65.14%; DETPMP : CA (2 : 1) is 80.8%; DETPMP : CA (1 : 1) is 52.2%. We also show that the synergic effect of the chelating agent CA has shown as the primary chelant and DETPMP, as the secondary one. The first agent chelates the calcium ion before it is mixed with the phosphonate scale inhibitor. This phosphonate is the second, and stronger, chelating agent. That fraction of the calcium ion is unchelated by weaker, primary chelant will be chelated by the secondary chelant, i.e., the phosphonate. This phosphonate plays a role as a strong chelating agent. This leads to a new equilibrium between primary chelant and calcium ion, and the process repeats. When the chelating capacity of the phosphonate is satisfied, additional released calcium results in precipitation of the phos- phonate. 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