This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 17827, “Prevention of Dissolution and Reprecipitation of Calcium Sulfate While Acidizing,” by Harvey Quintero, SPE, Darren Maley, SPE, and Farah Zafar, SPE, Trican Well Service, prepared for the 2014 International Petroleum Technology Conference, Kuala Lumpur, 10–12 December. The paper has not been peer reviewed.
Calcium sulfate (CaSO4) in the form of gypsum and anhydrite is one of the more prevalent evaporite minerals typically found in the carbonate rocks of the western Canadian sedimentary basin (WCSB). Most calcium sulfate scale inhibitors used for acid treatments rely on either the retardation of CaSO4 crystal growth or the creation of soluble complex salts with the calcium ions. A broad-spectrum scale inhibitor has been specially formulated for high-salinity and acid solutions that not only prevents the precipitation of CaSO4 but also helps inhibit the initial dissolution of CaSO4.
Covering a vast extension of 1.4 million km2, the WCSB is between the southwestern border of the Canadian shield in Manitoba and the eastern flank of the Canadian Rocky Mountain system in British Columbia. Approximately half of the WCSB is composed of carbonate reservoirs.
Commonly, these carbonate formations are stimulated with acid. Depending on the formation itself and the presence of formation damage, two main acid-stimulation techniques are used: matrix acidizing and acid fracturing. In a matrix-acidizing treatment, the objective is to squeeze the acid into the formation below fracturing pressures to create wormholes to increase production and to bypass formation damage, such as that caused by drilling. In an acid-fracturing treatment, the acid is pumped at high rates above the fracturing pressure. As the treatment is pumped, the acid reacts with the fracture face, etching it. Once the treatment is complete, a highly conductive path is left by the etching of the fracture face by the acid.
The acid treatments are commonly completed with hydrochloric acid (HCl) or with acetic acid (CH3COOH), formic acid (CHOOH), or other organic acids. Conditions throughout the WCSB vary dramatically, and conditions such as bottomhole temperature, metals used in tubulars or downhole tools, minerals present in the formation, and composition of the oil play a factor in the customization of acid blends.