Chelating Agents as Acid-Fracturing Fluids: Experimental and Modeling Studies

Acid-fracturing operations are applied to improve the productivity of carbonate reservoirs by creating rough fracture surfaces that remain conductive for reservoir fluids after the fracture closure. Different acid types have been used over the history of acid fracturing, including weak organic and strong HCl acids. Organic acids have low dissolving power, generating low fracture conductivity, while HCl acid is corrosive and requires numerous additives. For the first time, this study investigates the performance of the chelating agents, namely, ethylenediaminetetraacetic acid (EDTA) and glutamic acid N,N-diacetic acid (GLDA), in creating sufficient fracture conductivity. Several experiments/measurements were performed, such as coreflooding, fracture conductivity, computed tomography scan imaging, fracture face permeability, scratch test strength, impulse hammer hardness, inductively coupled plasma analysis of effluents, and nuclear magnetic resonance (NMR). The lab-scale outcomes were reproduced numerically, and field-scale simulations were carried. EDTA and GLDA treatments resulted in comparable fracture conductivity outcomes to the HCl acid treatment. However, they were associated with less rock strength reduction, which is essential for long-term sustainable fracture conductivity. The connectivity of the fracture and rock matrix was improved, as concluded from the NMR measurements. The study recommends injecting chelating agents in different cycles separated with soaking periods if higher dissolution/conductivity is desirable. Field-scale simulations showed that EDTA and GLDA chelating agents would result in lower dissolution magnitudes compared to HCl acid. Nevertheless, they could etch 60–70% of the created fracture length compared to 20% etched length created by HCl acid.