Stimulation of Calcite-Rich Shales Using Nanoparticle-Microencapsulated Acids

Abstract

Summary Acid treatment is a common stimulation technique to increase the conductivity of calcite–rich reservoirs. Retarded acid systems such as acid–in–oil emulsions are typically used to minimize wellbore damage and long–distance propagation of acids from the wellbore. However, the stability of these emulsions under harsh reservoir conditions is still a challenge. This study investigates a novel, robust acid carrier system using silica nanoparticles (NPs) for acid treatment of calcite–rich shales. First, highly hydrophobic silica NPs were blended with concentrated acid solutions (0.5–15 wt%) to form acid–in–air powder or microencapsulated acid (MEA). The MEA was then placed in an unpropped fractured core system, and fracture closure was simulated by increasing the overburden pressure. It resulted in the mechanical crushing of the MEA and release of the acid. The conductivity of the cores before and after the MEA treatment was measured. Finally, the surface etching on the fracture face caused by acid was quantified by measuring the surface roughness using an optical profilometer. The qualitative characterization was performed using optical microscopy. The mixing of highly hydrophobic silica NPs with acids under high shear rates resulted in the formation of MEA by self–assembly of the NPs at the water/air interface. These MEA particles were found to be robust encapsulating agents for acids where the release could be triggered by means of mechanical crushing. The fracture–conductivity experiments showed that the MEA could dramatically improve (up to 40 times) the permeability of the unpropped fractures by creating concentration–dependent, nonuniform localized surface etching.