Scale dissolvers for CaSO4, are composed of chelants, including aminocarboxylates or phosphonates, and can contain lower concentrations of other chemical additives (enhancers) that improve dissolution. We investigate the effectiveness of enhancers, whose function is to increase the dissolution rate, by examining the effect of substituted acetates. The objective is to elicit the chemical properties that make an enhancer effective, identify costs, safety considerations, and validate their application and limitations for field use.

Two types of mineral anhydrite and nine enhancer agents were used in static dissolution tests. Thin slabs were soaked in dissolver fluids, including a converter agent, at a pH between eight and ten. The enhancer concentration spanned 0.5, 5, and 10% w/v and probed both the catalytic and inhibitory effects. The dissolver was selected and optimized based on a customized screening methodology. Tests with mass/volume ratios of 0.1 and 0.02 were employed to fit mass-loss data to a kinetic model and rank the general rates of dissolution, which indicate there is a favorable chemical enhancer type of additive.

Mass-loss rates using two types of anhydrite, obtained at 200°F from one to six hours, indicate strong enhancer activity. Distinctive effects are found depending on the chemical substituent on the enhancer agent. Dissolution of anhydrite shows significantly different rates at 1 hour and mass-limiting effects with no differentiation for m/v = 0.02 at four hours and nearly a similar initial rate at one hour for m/v = 0.1. A significant differentiation is observable after one hour. Inhibitory effects are discernable at high concentrations (10% enhancer). The type of substrate also shows differences in dissolution activity among blue and grey anhydrite. We found a strong correlation with the dissolver enhancer chemical type, indicating a chemical class of agents that could have applicability in descaling operations. The enhancers have been ranked by health and safety characteristics and cost to assess the feasibility of use in oilfield environments.

The use of enhancer agents in scale dissolver fluids can provide dissolution within one to four hours, helping shorten the shut-in period for this treatment. Tests show that including highly polarizable agents can lead to faster dissolution of anhydrite.

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