Mitigation of Calcium Sulfate Scale Formation When Seawater Is Used To Prepare HCl-Based Acids
- Jia He (Texas A&M University) | Ibrahim Mohamed Mohamed (Texas A&M University) | Hisham A. Nasr-El-Din (Texas A&M University)
- Document ID
- Society of Petroleum Engineers
- SPE International Symposium and Exhibition on Formation Damage Control, 15-17 February, Lafayette, Louisiana, USA
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 4.2.3 Materials and Corrosion, 3.2.2 Downhole intervention and remediation (including wireline and coiled tubing), 1.8 Formation Damage, 4.3.4 Scale, 4.1.2 Separation and Treating, 4.3.1 Hydrates, 3.2.4 Acidising, 5.8.7 Carbonate Reservoir
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The damaging effect of calcium sulfate precipitation on the permeability of carbonate cores when mixing hydrochloric acid with seawater for matrix acid treatments has been identified in our recent work (SPE 143855). The objective of this work is to mitigate calcium sulfate precipitation by using a suitable scale inhibitor in hydrochloric acid. Another objective is to determine the scale inhibitor type, concentration, and whether it is needed in the preflush or post-flush stages.
Core flood tests were conducted using Austin Chalk cores (1.5 in. × 6 in.) with a permeability of 5 md, to investigate the effectiveness of scale inhibitor. A synthetic seawater was prepared according to the composition of seawater in the Arabian Gulf. Calcium, sulfate ions, and scale inhibitor concentrations were analyzed in the core effluent samples. The minimum concentration of scale inhibitor was determined over a wide range of temperatures (77 to 210°F).
A scale inhibitor (sulfonated terpolymer) was found to be compatible with hydrochloric acid systems, and can tolerate high concentrations of calcium (30,000 mg/l). Analysis of the core effluent indicated that the new treatment successfully eliminated calcium sulfate scale deposition. The concentration of scale inhibitor ranged from 20 to 250 ppm, depending on the scaling tendencies of calcium sulfate. This work confirms that an appropriate scale inhibitor can be added to acid, to avoid calcium sulfate precipitation when seawater is used to prepare hydrochloric acid for matrix acidizing.
The potential of calcium sulfate precipitation is greatly increased when seawater is used for matrix acidizing treatments (Oddo et al. 1991; Nasr-El-Din et al. 1996; He et al. 2011). In those processes, calcium concentration is greatly increased, due to the reaction of the hydrochloric acid with calcite in the carbonate reservoir rocks. High concentrations of calcium will combine with high levels of sulfate in seawater, and significantly increase the scaling tendency of calcium sulfate. Moreover, the solubility of calcium sulfate drops greatly once acid is spent (Flint 1968; Kruchenko and Beremzhanov 1976; Li and Demopoulos 2002). Those two factors account for the precipitation of calcium sulfate once it exceeds the critical scaling tendency of calcium sulfate (Yeboah et al. 1993a, b; Raju and Nasr-El-Din 2004). Calcium sulfate will deposit in the stimulated flow channels (Delorey et al. 1996), possibly causing blockage of pore throats and severely impairing the formation permeability (Tahmasebi et al. 2007); hence, decrease well injectivity or productivity (Smith et al. 1967; Shen and Crosby 1983). Even worse, the nucleation and precipitation process of calcium sulfate during acidizing treatments may affect the acid reaction with carbonate rocks, and cause a limited acid stimulation effect (He et al. 2011).
Typically, the application of scale inhibitors has been one of the economical methods used to mitigate calcium sulfate (Yuan 2004; Fan et al. 2010; Hoang et al. 2009). The use of scale inhibitors may act either as chelating agents to form a soluble complex (Moore et al. 1972; Jamialahmadi and Mueller 1991; Al-Khaldi et al. 2011), or as threshold inhibitors, which block the development of the supercritical nuclei (He et al. 1994; Tomson et al. 2003), or as retarders of the growth of the calcium sulfate crystals (Amjad and Hooley 1986; Amjad 1988; Liu and Nancollas 1973). By simply adding an acid-soluble and effective scale inhibitor into the acid system, mineral scale formation can be prevented during the process of acid stimulation (Smith et al. 2000; Sam 2003; Nasr-El-Din et al. 2004). In addition, combining scale inhibition and acid stimulation into a single package inherently reduces well intervention costs and well downtime, and therefore achieves significant economic benefits (Smith et al. 2001).
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