Precipitants transport during post-flush and its impacts

Abstract

Wellbore stimulation is often needed for different situations in the field ranging from deepwater frac-packs to enhanced geothermal systems. Acidizing is commonly designed for cleaning perforation, recovering perforation functions, and improving injectivity during completion operations such as frac-pack, aiming to reduce the fracturing pressure and enhance productivity. However, some field observations have shown unexpected breakdown pressure increases after the post-flush period of acid treatment in sandstone reservoirs. To understand this problem, a 3D model has been established in this paper to simulate the acidizing process through a perforation hole, and then the subsequent post-flush resulting in precipitant transport. Finally, the breakdown process where fluid is injected into the rock is modeled to compare breakdown pressures in different cases. Our numerical results reveal that although minerals can be effectively dissolved by chemical reactions during the main flush period, the generated precipitants transported during the post-flush period may affect the rock breakdown process. The precipitant transport might develop high-porosity channels, potentially leading to the breakthrough along the precipitant deposits and establishing a string hydraulic communication between the perforation hole and the surrounding rock. The existence of these high-porosity channels enables fluids to flow swiftly into the surroundings, impeding enough pressure accumulation to crack the rock. The formation of high-porosity channels in the post-flush period and its potential impact on the breakdown pressure is influenced by several factors such as post-flush fluid injection rate, acid concentration and viscosity, and the initial rock permeability.