Novel Scale Removers Are Developed for Dissolving Alkaline Earth Deposits

Abstract

Abstract Scales are minerals that form on a surface due to the saturation of the local environment with an inorganic salt. The major categories of scales are: carbonates (Ca, Mg, Fe(II)), sulfates (Ca, Ba, Sr, Ra), oxides and hydroxides (Fe(II), Fe(III), Mg), sulfides (Fe(II) and Zn) and silicates (Ca, Mg, Al and Na). The testing and categorizing of various classes of scale removal chemicals is described in this paper. The primary deposits studied were calcite, gypsum and barium sulfate. A variety of solvents, including organic acids and chelating agents were tested. Total capacity tests as well as kinetic tests were conducted to develop improved scale removal formulations. One result has been the development of solvent formulations based on hydroxyaminocarboxylic acid (HACA) chelating agents. The tests demonstrated that HACAs are better overall complexers and solvents than aminocarboxylic acids or carboxylic acid type chelating agents in many scale dissolution formulations. A major benefit of HACAs are their high solubility in aqueous solutions from pH values <1 to >12. Thus, an almost infinite number of custom formulations can be constructed. Some of the formulations have dissolution capacities that are equivalent to 7.5% HCl. One of the new materials, hydroxyethyliminodiacetic acid (HEIDA) is completely biodegradable and this contributes to the "greening" of oilfield chemicals. Introduction Before a well is drilled and completed, the fluids in the formation are in equilibrium with their surroundings. However, when the well is drilled and starts to flow, the equilibrium is disturbed and solids may start to precipitate. Inorganic deposits are called "scales" and organic deposits are referred to as "waxes (saturated hydrocarbons)" or "asphaltenes (unsaturated and cyclic hydrocarbons)." Scales are minerals that form on a surface due to the saturation of the local nvironment with an inorganic salt. The major categories of scales of interest to oil and gas production are: carbonates (Ca, Mg, Fe(II)), sulfates (Ca, Ba, Sr, Ra), oxides and hydroxides (Fe(II), Fe(III), Mg), sulfides (Fe(II) and Zn) and silicates (Ca, Mg, Al and Na). Foulants and debris are materials that form elsewhere and are transported to a surface where they cause a disruption in flow and/or heat transfer. Much of the basic information on scale control comes from the cooling water literature, where it is difficult to separate corrosion inhibition from scale inhibition. Some of this technology has been applied to oilfield scale inhibition and removal since companies that develop inhibitors are involved in both industries. Weber and Knopp1 reviewed scaling and corrosion inhibitors used in industrial cooling water systems. This review, as well as the book by Ostroff2 stress that corrosion and scaling in natural waters exposed to CO2 are intimately intertwined and involve the equilibrium of:Equation (1) The Weber review notes that some anti-scaling materials may have a positive effect on corrosion, but their main task is to keep the surfaces clean, and this function also may adversely affect the performance of corrosion inhibitors. Prediction of scaling tendencies can be accomplished if the composition of the water down hole is known with a high degree of accuracy. One of the first methods was developed by Langulier3 and used thermodynamic and empirical values to estimate the "Saturation Index" of the water and thus predict the scaling tendency of the water source. Under the best of conditions, scaling and corrosion can be controlled with minimal use of chemicals if pH, [Ca2+] and p[CO2 are tightly controlled.