Boosting Reaction Rate of Acids for Better Acid Fracturing Stimulation of Dolomite-Rich Formations

Abstract

Summary Carbonate reservoirs can be stimulated by injecting acids to boost the rate of hydrocarbon production from low-permeability zones via the creation of conductive pathways. The reaction rate between the acidizing fluid and rock matrix is a key parameter in determining the success of stimulation treatments. Dolomite-rich formations are known to exhibit slower reaction kinetics as compared to calcite. As a result, some acid fracturing treatments fall short of creating an extended fracture or the desirable etching pattern on the fracture faces, thus limiting hydrocarbon flow. Accordingly, the development of an acid package to boost the dolomite dissolution rate will be advantageous to the efficiency of the stimulation treatment in dolomite-rich reservoirs. Accelerating the reaction rate of dolomite with strong mineral acid (i.e., hydrochloric acid, HCl) can be achieved through an additive-driven chemical approach based on the addition of judiciously selected sulfonate-based surfactants. To pinpoint the optimal surfactant(s) type and concentration, static dissolution testing was performed under ambient conditions using outcrop dolomite core samples (Silurian and Guelph). Each core sample was reacted with 28 wt% HCl in the presence and absence of sulfonate-based surfactant additives for a predetermined time. Selected surfactants are used in comprehensive reaction kinetics studies at reservoir conditions using a rotating disk apparatus (RDA). Based on the results of the ambient screening tests, three formulations were found to accelerate the reaction rate by up to 30% as compared to using 28 wt% HCl without the additive(s). The kinetics data collected at a pressure of 3,000 psi and temperatures of 175 up to 300°F showed that the reaction rate of Guelph dolomite can be accelerated by as much as 17–55% with one of the formulations. Coreflood experiments showed an increase in the acid PV to breakthrough (PVBT) when the surfactant package was added to the acid formulation pointing to a rise in the reaction rate of dolomite and the developed acid formulations. The acid formulations showed an improvement in the dolomite and acid reaction rate, which creates the opportunity to apply these formulations in the field to improve the outcome of acid fracturing treatments in dolomite-rich reservoirs.