Post-acid flush and its impact on breakdown pressure

Acidizing is a common technique for cleaning perforations, mitigating near-wellbore formation damage, and enhancing perforation functions during production and completion works like frac-pack. Post-perforation acid flush is designed to favorably reduce the breakdown pressure, improve well injectivity, and help with more effective fracking to follow. However, some field observations indicate that, counterintuitively, the breakdown pressure may increase during subsequent fracturing operations after the post-perforation acid flush, which can adversely affect fracturing performance. To investigate the potential causes of this phenomenon, a three-dimensional finite element model is established to systematically simulate multiple processes: sandstone acidizing through a perforation, the transport of resulting precipitates during the post-flush period, and the injection of fracking fluid during the main frac treatment. The resultant breakdown pressures are then evaluated and compared to identify possible influencing factors, which may help with the optimization of acidizing treatment design and subsequent fracking operations. This study demonstrates that precipitates generated during the main flush are transported by the post-flush fluid, leading to the formation of high-porosity channels. These channels may subsequently break through the precipitation zone and facilitates the easy flow of the fluid into the surrounding rock, preventing pressure buildup at the onset of fracking which may result in a higher breakdown pressure. Several factors were investigated to qualitatively evaluate their influence on breakdown pressure. Among these, fluid injection rate, initial average porosity, and mineral content were found to significantly affect breakdown pressure. A lower fluid injection rate was observed to reduce breakdown pressure. Under identical acidizing conditions, formations with higher initial porosity and lower contents of fast-reacting minerals exhibited increased breakdown pressures. This study presents an innovative investigation of how precipitate transport during post-acid flush affects breakdown pressure, while identifying influencing rock property parameters contributing to the observed counterintuitive field occurrences of elevated breakdown pressure. The findings would help to optimize acidizing design prior to main frac treatments.