1. Field of the Invention
The invention relates to a method of dissolving sulfate scales from surfaces such as the equipment and wellbore surfaces of oil and gas wells.
2. Background Information and Description of Related Art
Sulfate scales normally form as a result of the mixing of incompatible waters. This may occur during crude oil and gas production when sea water containing sulfate and bicarbonate ions is mixed with reservoir water containing alkaline earth metal cations (barium, calcium and strontium). Depending on surrounding conditions such as pH, temperature, pressure and the presence of a seed crystal, if the concentrations of alkaline earth cations and sulfate ions exceed the solubility of the corresponding alkaline earth sulfates, the sulfate will precipitate and form scales. Under these conditions, the scales may also form with a single source of water containing supersaturated levels of alkaline earth metal and sulfate ions.
Adherent and occluded scales cause serious problems in many applications involving water flow. For example, during gas and oil production, scale plugging of surface and subsurface equipment, tubings, and perforations often lead to severe productivity decline and difficult operating conditions. Of the sulfate scales, barium sulfate is particularly troublesome because of its extreme insolubility. Furthermore, barium sulfate is often associated with strontium sulfate which may carry traces of radium with its potentially dangerous property of radioactivity.
Various methods have been proposed for the removal of sulfate scales including mechanical and chemical treatments. The latter treatments include contacting the scale with any of various agents in which the scale will dissolve, such as solutions of an aminopolycarboxylic acid (APCA), e.g. diethylenetriaminepentaacetic acid (DTPA) or ethylendiaminetetraacetic acid (EDTA), which acts as a sequestrant. However, while the APCA's are somewhat effective in dissolving sulfate scales, including barium sulfate, further increases in both the rate of dissolution and the amount of scale dissolved are considered highly desirable from the standpoint of increased productivity. Thus any method yielding such improvement, particularly with regard to barium and strontium sulfate scales, could be of considerable importance.
Various improvements in the dissolution of sulfate scales have been proposed, some of which are disclosed in the following prior art references which are cited and described herein in accordance with the terms of 37 CFR 1.56, 1.96 and 1.98.
U.S. Pat. No. 3,660,287, issued May 2, 1972 to Quattrini, discloses the removal of scale including calcium and barium sulfates from oil well equipment by treating it with a composition comprising a partially neutralized aminopolyacetic acid, e.g., DTPA or EDTA, and a carbonate such as ammonium bicarbonate.
U.S. Pat. No. 4,708,805, issued Nov. 24, 1987 to D'Muhala, discloses the removal of barium and strontium sulfate scales by treatment with a sequestering composition comprising citric acid, a polycarbazic acid, and an alkylenepolyaminopolycarboxylic acid, e.g., DTPA or EDTA.
U.S. Pat. No. 4,621,694, issued Nov. 11, 1986 to Wilson et al., discloses a method for removing scale from wellbore surfaces and equipment utilizing a composition comprising an external aqueous phase in which is emulsified a hydrocarbon membrane phase enveloping an internal aqueous phase, with the hydrocarbon and internal aqueous phases containing different complexing agents.
R. H. Ashcraft, "Scale Inhibition Under Harsh Conditions by 2-Phosphonobutane-1,2,4-tricarboxylic Acid (PBTC)", NACE, (National Association of Corrosion Engineers), Paper No. 123, Boston, MA, Mar. 29, 1985, discloses the use of PBTC as a threshold scale inhibitor in preventing or minimizing the formation of calcium carbonate scales. A threshold scale inhibitor is a compound capable of preventing scale formation at concentrations far below that required to sequester the scale compound which is necessary for dissolution.