Evolution of Fracture Surface Parameters in Heterogeneous Carbonate Reservoirs Employing Chelating Agents as Fracturing Fluids

Abstract

Acid fracturing is a technique that can be used to increase the formation productivity in carbonates. The injection of acid causes dissolution along the fracture, which results in an improved conductivity. The success of the acid fracturing process is influenced by several factors, including the acid type, acid concentration, formation type, and contact time. Many fluids have been examined for their ability to maintain conductivity, beginning with strong acids, such as hydrochloric acid (HCl), and progressing to weak acids, such as organic acids. The main issues with HCl are the deleterious impacts of rapid and strong reactions around the wellbore, which can corrode the wellbore tubular and reduce the potency of the acid, therefore limiting its impact on the reservoir rock. Chelating agents are less reactive than HCl and do not require a corrosion inhibitor. This study investigated the impact of using chelating agents (GLDA and DTPA) as hydraulic fracturing fluids (HFFs) on fracture stimulation in heterogeneous carbonate formations, composed of mixed calcite and dolomite by conducting a series of core flooding experiments and comparing them against the traditional HCl-based HFF. The physicochemical evolution of the initially smooth fracture surface is tracked using laser profilometry (surface roughness), impulse hammer (rock hardness), and inductively coupled plasma optical emission spectrometry (effluent concentration) techniques. Fracture conductivity was measured before and after acid etching with a core flooding system at various flow rates and closure stresses. The results showed that the chelating agent-based HFF improved fracture conductivity similar to the HCl-based HFF for mixed carbonate formation. The HCl-based HFF resulted in the formation of a single deep channel in the middle of the fracture, the GLDA-based HFF resulted in the formation of a wide shallow channel on the side, and the DTPA-based HFF resulted in face dissolution. The DTPA-based HFF softened the rock and increased the surface roughness more than the GLDA-based HFF, while the GLDA-based HFF improved fracture conductivity more than the other one. Furthermore, ICP analysis revealed that DTPA dissolved more calcium and magnesium ions. For the first time, chelating agents were investigated and tested as acid fracturing fluids on heterogeneous carbonate reservoirs to generate sustainable conductivity.