Chemical dissolution of oilfield strontium sulfate (SrSO4) scale by chelating agents

This paper investigates the chemical dissolution of strontium sulfate (SrSO4) scale, a minimally studied and common oilfield scale, in the presence of two types of chelating agents: ethylenediametetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA). Formation of SrSO4 significantly decreases the well production and costs millions of dollars per year by seriously damaging the drilling and production equipment. Mineral scales can be chemically treated by addition of chelating agents to complex the metal. Chemical methods are cheaper and are easily applicable to exotic scales which are not easily accessible at high temperature and high pressure wells. In this work, the effects of chelate concentrations (12.50 mM–50.00 mM), temperatures (40 °C–80 °C), and pH (8 and 12) were investigated to determine optimal dissolution conditions. 24 h batch experiments conducted at pH 12 showed that increasing temperature from 40 °C to 80 °C results in an increase of overall SrSO4 dissolution, but the effect is more noticeable for EDTA than DTPA. The maximum dissolution is observed when the solution is slightly supersaturated with respect to EDTA or DTPA (Sr:EDTA or Sr:DTPA of 1:1.1 and 1:1.18). Kinetic experiments conducted at pH 12 showed that dissolution rate increases with increasing temperature and chelate concentrations for both EDTA and DTPA. Dissolution rates were lower at pH 8 than pH 12, suggesting optimal dissolution occurs at higher pH. Activation energies of 13.5–10.8 kcal/mol for 15 mM and 25 mM EDTA as well as 7.4–6.5 kcal/mol for 15 mM and 25 mM DTPA indicate SrSO4 dissolution as a surface controlled process. Thus, these results have provided the foundation for in-situ SrSO4 dissolution or dissolution of other common or exotic mineral scales.