Factors Affecting Scale Inhibitor Retention in Carbonate-Rich Formation During Squeeze Treatment
- Amy T. Kan (Rice U.) | Gongmin Fu (Rice U.) | Mason B. Tomson (Rice U.) | Musaed Al-Thubaiti (Saudi Aramco) | Alan J. Xiao (Cognigen Corp.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 2004
- Document Type
- Journal Paper
- 280 - 289
- 2004. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.3.4 Scale, 4.1.2 Separation and Treating, 4.2.3 Materials and Corrosion, 2.2.2 Perforating
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Significant progress has been made toward developing a quantitative understanding of the inhibitor/rock interaction. In this study, four common oil field inhibitors (three phosphonates and one polyacrylate) are compared using carbonate-rich formation material. In addition to calcite (CaCO3 ) in the reservoir rock, several calcium inhibitor (Inh) solid phases are also important. Two reactions are central to the inhibitor retention in carbonate-rich formation: first, reduction of calcite dissolution because of surface poisoning by the Ca-Inh coating; and second, precipitation of Ca-Inh solid with either low Ca or high Ca stoichiometry. For NTMP, an acidic Ca-NTMP salt is formed at a low-pH environment. In addition, two crystalline Ca-NTMP phases and an amorphous Ca-NTMP salt may form, depending on the aquatic environment. Quantitative relationships between types of inhibitors, inhibitor acidity and concentration, and kinetics of calcite dissolution and calcium-phosphonate precipitation are developed. Consequences of the observations on squeeze design and scale inhibition will be discussed.
The reactions of inhibitor with formation rock determine the inhibitor retention and release following an inhibitor squeeze. There are several aspects to understanding and optimizing, in a predictable manner, inhibitor squeeze designs. Earlier efforts have focused on describing what happens and when to resqueeze.1,2 More recent papers have advanced the knowledge of inhibitor reactions under various production conditions.3-13 Missing in previous inhibitor/core interaction studies have been the order of the reactions, kinetics, and solid-phase stoichiometry following the inhibitor/core reaction. A question with inhibitor/core reactions is to what extent the reaction will be quenched by adsorbed inhibitors. It is also necessary to know the differences among inhibitors, how to optimize a squeeze design, and if there is an optimum inhibitor concentration and acidity. A primary objective of this study is to develop mechanistic understanding of how inhibitors react with core material and to develop a methodology to predict and control inhibitor/core reactions. In the following, the chemistry of inhibitor/carbonate-rich rock interaction has been characterized for four commonly used oilfield-scale inhibitors.
Inhibitor Solution Speciation and Metal Salt Solubilities.
To predict the fate of inhibitors in the diverse brine compositions that occur in production fluids, it is necessary either to know or to be able to predict the simple acid-base and complexation equilibrium of inhibitors and divalent metals, such as calcium, magnesium, barium, and iron, vs. T and TDS. In Table 1 is listed the ionization and complexation stability constants for three phosphonic acids: aminotri(methylene phosphonic acid or NTMP, diethylenetriamine penta(methylene phosphonic acid or DTPMP, and bis-hexamethylenetriamine penta(methylene phosphonic acid or BHPMP, and one polyacrylic acid (phosphinopolycarboxylic acid or PPCA). These inhibitors are some of the most common oilfield-scale inhibitors. Note that the unit of polymer concentration used in calculating PPCA speciation is expressed as the acrylic acid monomer (A), which may be extended to polymers of different sizes. Similarly, the stoichiometry of Ca-PPCA salt is expressed as three Ca and two acrylic acid trimers. To our knowledge, this is the first attempt to establish a simple set of substi-tuent constants for inhibitor acids at realistic oilfield conditions. At 1.0 M ionic strength and 70°C, bC a=0.56 for NTMP, 0.62 for DTPMP, and 0.38 for BHPMP (see Table 1). This would correspond to a stability constants of 101.12 , 101.24, and 10 0.76 M-1 for the complexation of Ca27 with a dinegative phosphonate ion in the order of NTMP, DTPMP, and BHPMP, respectively. This is comparable to the stability constants of Ca/methyl and aminomethyl phosphonates.14 The average stability constant of Ca with 12 common monophosphonates is 101.55+0.14 M-1 at 0.1 M I and 25°C. It should be noted that the stability constant for Ca2+HPO4 2- ?CaHPO4 0 is approximately 101.3 M-1 at 1 M I and 25°C. The difference between the stability constants of the polyphosphonates measured by this research group and those reported in the literature may be caused by the electrostatic effect of these molecules, because of an ionic strength effect, or related to more complicated relations of structure, but clearly the complexation is mostly electrostatic in origin.
Using these speciation models, we are able to establish the solubility products of various metal phosphonate and metal-polymer solubility products (Table 2) and their ionic strength and temperature dependence using statistical programs to correlate the laboratory-measured solubilities.15-17 For all three phosphonate salts and the Ca-PPCA salt, we typically observed the formation of a high-solubility amorphous phase of metal inhibitor salt when mixing the inhibitor with calcium at high concentrations.15-20 The amorphous high-solubility material will eventually develop into a crystalline phase with much lower solubility, often by flowing brine over the metal inhibitor salt by a membrane filter to remove the readily solubilized amorphous Ca-Inh salt. The crystalinity of both Ca-NTMP and Ca-DTPMP solid phases has been confirmed by XRD analyses. The solubilities of Ca-NTMP and Ca-DTPMP are very similar, while Ca-BHPMP is significantly more soluble than that of Ca-NTMP (approximately seven times higher than that of Ca-NTMP at 70°C, 1 M ionic strength, 4000 mg/l Ca, and 5.5 pH). The solubility of Ca-PPCA is lower than Ca-BHPMP and higher than Ca-NTMP. Note that the solubility product of Fe-NTMP is many orders of magnitude lower than Ca-NTMP. Therefore, the solubility of Fe-NTMP may also play a significant role in controlling the fate of phosphonates during squeeze and production, even though iron concentration is typically much lower than the calcium concentration in brine.21
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