Calcium sulfate in the form of gypsum (CaSO4.2H2O) and anhydrite (CaSO4) is one of the most prevalent evaporite minerals typically found in the prolific middle Devonian carbonate rocks of the Western Canadian Sedimentary Basin (WCSB). Strong mineral acids, in particular hydrochloric acid (HCl), are employed to enhance permeability in the near wellbore area of the oil wells in these carbonate-bearing formations during matrix stimulation and hydraulic fracturing treatments. When calcium sulfate (CaSO4) comes in contact with the live acid, partial dissolution can occur. As the acid solution is progressively spent inside the carbonate rock, the concentration of unassociated calcium ions will increase. These ions will become readily available to react with the sulfate ions in the neutralized solution and cause, the unavoidable re-precipitation of CaSO4 crystals in the pore throat, therefore severely plugging the newly created flow channels.
Based on Le Chatelier's principle and the common ion effect, the addition of a soluble calcium salt to the treating acid package has been an economical oil field practice established to suppress the initial dissolution of CaSO4. However, a secondary protection mechanism is still required because sulfate-rich connate water could commingle with the spent acid solution during swabbing and/or flowback operations, reaching the ideal conditions for CaSO4 precipitation.
To date, most of the CaSO4 scale inhibitors that have been applied for acid treatments relied on either the retardation of CaSO4 crystal growth, or the creation of soluble complex salts with the calcium ions. This paper intends to detail the development and laboratory testing of a broad spectrum scale inhibitor specially formulated for high salinity and acid solutions that not only prevents the precipitation of CaSO4, but also helps to inhibit the initial dissolution of CaSO4.
Covering a vast extension of 1.4 million square kilometers (Bowers 1997), The WCSB defines its coordinates between the southwestern border of the Canadian shield in Manitoba and the eastern flank of the Canadian Rocky Mountain system in British Columbia. Within the WCSB, approximately half is composed of carbonate reservoirs of the Devonian age. These Carbonate formations combine for a project reserve of 15 billion barrels of oil and 35 trillion cubic feet of natural gas (Li 2002). These important reserves of hydrocarbons have been successfully exploited since the late 1940's (Milligan 1998).
