Summary The bulk “apparent-adsorption” behavior (Γapp vs. Cf) of two polymeric scale inhibitors (SIs), polyphosphino carboxylic acid (PPCA) and phosphorus-functionalized copolymer (PFC), onto carbonate mineral substrates has been studied for initial solution pH values of 2, 4, and 6. The two carbonate minerals used, calcite and dolomite, are much more chemically reactive than sandstone minerals (such as quartz, feldspars, and clays), which have already been studied extensively. In nearly all cases, precipitates formed at higher SI concentrations were caused by the formation of sparingly soluble SI/calcium (Ca) complexes. A systematic study has been performed on the SI/Ca precipitates formed by applying both environmental scanning electron microscopy energy-dispersive X-ray (ESEM-EDX) analysis and particle-size analysis (PSA), and this identifies the morphology and the approximate composition of the precipitates. For PPCA, at all initial solution pH values, regions of pure adsorption (Γ) (PPCA < 100 ppm) and coupled adsorption/precipitation (Γ/Π) are clearly observed for both calcite and dolomite. PFC at pH values of 4 and 6 also showed very similar behavior, with a region of pure adsorption (Γ) for PFC < 500 ppm and a region of coupled adsorption/precipitation (Γ/Π) above this level. However, the PFC/calcite case at pH = 2 showed only pure adsorption, whereas the PFC/dolomite case at pH = 2 again showed coupled adsorption/precipitation at higher PFC concentrations. For the SIs on both carbonate substrates, precipitation is the more dominant mechanism for SI retention than adsorption above a minimum concentration of approximately 100 to 500 ppm SI. The actual amount of precipitate formed varies from case to case, depending on the specific SI, the substrate (calcite/dolomite), and the initial pH (pH = 2, 4, and 6). Although the qualitative behaviors of both PPCA and PFC were similar on both carbonate substrates, the apparent adsorption of PPCA was higher on calcite than on dolomite, and the apparent adsorption of PFC was higher on dolomite than on calcite. We discuss here how these observations are related to the reactivity of the different carbonate minerals, the resulting final pH (which affects the dissociation of the SI), the Ca-SI binding, and the solubility of the resulting complex.

The mechanism of retention of scale inhibitors (SI) within the reservoir formation is central to a squeeze treatment having a long lifetime. Scale inhibitors are retained within porous media by the two main mechanisms of adsorption (Γ) and precipitation (∏). There is not complete agreement in the literature about when we should use one mechanistic description or another, and indeed both can occur together as coupled adsorption/precipitation (Γ/∏). Previously a general model of coupled (equilibrium) adsorption/precipitation has been derived and the agreement between the model and experiment was very good (Kahrwad et al., 2008). This model was subsequently extended to derive a consistent dynamic coupled Γ/∏ flow model for simulating non-equilibrium (kinetic) coupled processes of any type (Sorbie, 2010). This latter model has not yet been fully validated, but the work in this paper and its companion paper (Paper 2: Ibrahim et al, 2010) provides the type of data required in order to do this. In this paper (Paper 1), new static experimental adsorption/precipitation measurements are presented for two phosphonate inhibitors, DETPMP (a penta-phosphonate) and OMTHP (a hexa-phosphonate) using sand, kaolinite and siderite as the mineral phases. These experiments were carried out at a range of adsorbent mass (m)/ fluid volume (V)/ ratios and it is the "apparent adsorption", Γapp vs. the final scale inhibitor concentration, cf, which is measured and plotted. By observing how the Γapp vs. Cf, curves vary for different values of the (m/V) ratio, this indicates whether we are in the purely adsorbing (Γ) or in the coupled adsorption/precipitation (Γ/∏) regime (Kahrwad et al., 2008). For these static apparent adsorption tests, m = 10g, 20g and 30g samples of each mineral were used (with a fixed volume of SI solution, V = 80ml) to analyse the apparent adsorption behaviour. In addition, related pure precipitation/compatibility tests were carried out in the absence of any minerals in the bulk solutions. The experimental results for both phosphonate scale inhibitors show good agreement with the theory in different regions of pure adsorption and coupled adsorption/precipitation. These results show clearly how such laboratory measurements should be carried out to determine both the levels of SI retention and the precise retention mechanism. This paper characterizes the systems used in subsequent dynamic adsorption/ precipitation sand pack floods which are reported in a related paper (Ibrahim et al., 2012) and which will be used in future to validate fully dynamic coupled Γ/∏ flow models (Sorbie, 2010).

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.

Summary The development of effective scale-inhibitor (SI) squeeze treatments remains a challenge for carbonate reservoirs because of their substantial chemical reactivity with the SI. This in turn might potentially lead to uncontrolled SI precipitation and induced formation damage. This work takes a systematic approach to understanding the retention mechanisms of SI in carbonate formations with respect to the detailed carbonate-formation mineralogy, type of SI, and reservoir conditions in the absence of oil. Static adsorption/compatibility experiments, described previously as apparent adsorption tests (Kahrwad 2008), were performed to evaluate the areas of different retention mechanisms [pure adsorption (Γ) and coupled adsorption/precipitation (Γ/Π)] of different SI species in brine. Experiments were conducted for five SIs at various conditions: initial pH values, mineralogical compositions (calcite, limestone, and dolomite), and temperatures. The SI species used in this study included a phosphonate [di-ethylene tetra-amine penta (DETPMP)], a phosphate ester [polyhydric alcohol phosphate ester (PAPE)], and three polymeric SIs [polyphosphino carboxylic acid (PPCA), P-functionalized copolymer (PFC), and sulfonated polyacrylic acid copolymer (VS-Co)]. All precipitates were studied using environmental scanning electron microscopy/energy dispersive X-ray (ESEM/EDX) and particle-size analysis (PSA). The overall results from these coupled Γ/Π experiments are as follows: For the polymeric SIs (PPCA, PFC, and VS-Co), the highest retention levels were observed at low pH for all carbonate substrates, because of the increase in divalent cations calcium and magnesium (Ca2+ and Mg2+, respectively) available from rock dissolution for SI–M2+ ions (divalent cations) precipitation. For DETPMP and PAPE SIs, the retention level was greatest at higher pH values, because the SI functional groups were more dissociated and, hence, available for complexation with M2+ ions. The polymeric VS-Co predominantly showed pure adsorption with only a low amount of precipitation (Γapp ≈ 1.2 mg/g) in contact with the dolomite substrate. This is because of the presence of sulfonate groups (low pKa). For polymeric inhibitors, the retention level (Γapp) was highest on calcite (highest relative calcium content), followed by limestone and dolomite. DETPMP and PAPE SIs showed the highest retention levels on dolomite (higher final solution pH and more SI dissociated), followed by limestone and calcite. For all SI species, higher retention (more precipitation, Π) was observed at elevated temperatures. At lower temperatures, an extended region of pure adsorption was observed for all SIs. The information presented in this study will be helpful in SI product selection on the basis of mineralogy and reservoir conditions. As a consequence, longer squeeze lifetimes and improved efficiency of SI deployment in carbonate reservoirs can be achieved. In addition, this study provides valuable data for validating models of the SI/carbonate/Ca/Mg system that can be incorporated into squeeze design simulations.

Summary In this paper, results are presented on the general mechanisms by which scale inhibitors (SIs) are retained within porous media. There is a generally accepted view that the main two mechanisms of SI retention are "adsorption" and "precipitation," and these are described by different but related modeling approaches in the literature. These approaches have been used quite successfully to model field squeeze treatments. To analyze in a detailed and unambiguous manner where a given retention mechanism (e.g., pure adsorption) or mechanisms (e.g., coupled adsorption and precipitation) are operating requires that we carry out careful laboratory experiments under "field relevant" conditions. In this work, we study adsorption vs. adsorption/ precipitation by performing a series of experiments where we know that the system exhibits either (a) adsorption only or (b) coupled adsorption/precipitation. Experimentally, it is straightforward to determine which regime the system is in. We present the theory describing the coupled adsorption/precipitation process. In addition, an extensive series of experimental adsorption/precipitation measurements is presented for various mineral separates including sand, chlorite, siderite, muscovite, kaolinite, and feldspar. The coupled adsorption/precipitation model is in very good agreement with the experiment.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractOne 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 Ca 2+ and Mg 2+ 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), hexaphosphonate (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 [Ca 2+ ] and also that their effectiveness is considerably reduced, or poisoned by increased [Mg 2+ ]. The performance of the phosphonate species are shown to be controlled by the performance of inhibitor/Ca 2+ complexes, whereas the formation of inhibitor/Mg 2+ complexes effectively poisons the phosphonate species ability to inhibit. The polymeric species (PPCA and PVS) are shown to be less influenced by increased [Mg 2+ ]; indeed, marginally improved performance is obtained. This paper significantly extends the findings reported previously in SPE 37273. 1

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 and, in this paper, we will focus on the role of calcium ion concentration. 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 well bore area, i.e. on its adsorption or precipitation behaviour. What is less well understood is the effect of calcium on the inhibition process itself. Common ion effects are well known. However, for penta-phosphonate inhibitor species (e.g. DETPMP), significant improvements in inhibition efficiency have been reported with increasing calcium concentration in solution. In this paper, we expand significantly on such observations. The effect of calcium cp tion concentration is examined for a wide range of generically different inhibitor species including penta-phosphonate, hexaphosphonate phosphino-poly carboxylate, polyvinyl sulphonate and sulphonated polyacrylate co-polymers. The results indicate clearly how different inhibitor species are affected quite differently by increased [Ca2+] and how this relates to the calcium 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 sea water and also in a more severe scaling (Forties type) formation brine/sea water mixture. Barium sulphate inhibition efficiencies were examined by static inhibition efficiency tests with residence times ranging from 30 minutes to 24 hours. Phosphonate inhibitors 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 has been discussed by previous workers. 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, for the phosphonate based species it appears that strong ionic bonds involving calcium cation bridging is required. As [Ca2+], in the mixed brine is increased the magnitude of the effect (i.e. calcium enhanced inhibition) is related to the functional groups present on the inhibitor in the order phosphonate >acrylate >sulphonate. Phosphonate inhibitor species are very dramatically affected whereas sulphonated inhibitor species are less affected. This can be accounted for in terms of the calcium affinity of the different inhibitor functional groups in a similar way to 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 modes of adsorption at the scale surface. Introduction and Background The most important property which any oilfield scale inhibitor must possess is that it has the ability to prevent/inhibit crystal growth at threshold (i.e. sub-stoichiometric) concentration levels. Many workers have studied the mechanism of threshold scale inhibition. 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. The overall result of adding a scale inhibitor is a reduction in the tendency for crystallisation and the subsequent formation of scale, by two pathways:Nucleation Inhibition: Disruption of the thermodynamic stability of the growing nucleons, (for homogeneous crystallisation). P. 611

Building a fundamental understanding of the reactions between scale inhibitor (SI) and formation minerals is essential for effectively designing SI “squeeze” treatments. Results of bulk “apparent adsorption” (Γapp) experiments are presented for a widely used phosphonate SI, DETPMP, on calcite and dolomite mineral substrates. The apparent adsorption results are supported by (i) measuring the corresponding solution [Ca2+] and pH values in solution, (ii) studying the surface chemistry of the resulting SI/Ca precipitates using environmental scanning electron microscopy–energy-dispersive X-ray (ESEM-EDX) analysis to identify the morphology/composition of the SI/Ca precipitates, and (iii) a detailed mass balance analysis, indicating the fate of the Ca2+ and the SI. Results revealed that DETPMP was dominantly retained by both calcite and dolomite via a precipitation mechanism (actually coupled adsorption/precipitation) for all initial pH values (pH0 2, 4, and 6) and T = 95 °C, although a small region of pure adsorption (Γ) was observed at [DETPMP] < 100 ppm. Moreover, higher Γapp occurred on dolomite than on calcite for all initial pH0. This result is counterintuitive, because it is well-known that calcite is much more reactive than dolomite. However, final equilibrium pH values are higher for dolomite, compared to calcite. Thus, a higher pHfinal led to a more dissociated DETPMP and this effect had a greater effect on SI/Ca precipitation than the higher [Ca2+] by rock dissolution. EDX analysis confirmed scale-inhibitor phosphorus in the deposited solids, indicating coupled adsorption/precipitation. Supporting mass balance calculations correlated very well with our experimental observations, showing higher generated calcium in calcite than dolomite and less calcium generation at higher initial pH0 (lower rock dissolution). Finally, an equilibrium mechanistic model describing the inhibitor dissociation, Ca-binding to the dissociated SI species, and precipitation of the SI_Ca n complex, coupled to the carbonate system, is proposed to qualitatively explain these experimental findings.