Impact of Fluid Acidity on the Time-Dependent Initiation of Hydraulic Fractures in Carbonate Rocks

Carbonate-rich rocks are commonly encountered in oil and gas reservoirs, and as such, mechanisms of hydraulic fracture initiation and growth in carbonates are important for effective reservoir stimulation. Thus motivated, this paper shows the impact of acidic fluid on the hydraulic fracture initiation in laboratory experiments. The results demonstrate that compared to water injection, acid injection results in more rapid initiation of the hydraulic fractures under so-called static fatigue or pressure-delay conditions wherein a certain pressure, insufficient to instantaneously generate a hydraulic fracture, is maintained until a hydraulic fracture grows. Acid injection also is shown to generate a dissolution cavity in the vicinity of the wellbore, and the breakdown of the specimen is observed to be explosive in the case of acid injection, probably due to the generation and the subsequent rapid expansion of carbon dioxide as a part of the dissolution reaction. Finally, the time to breakdown is shown to be related not only to the magnitude of the wellbore pressure, but also to the apparent permeability of the specimen. Altogether, the results indicate first that acid injection has the potential to improve the initiation of multiple hydraulic fractures within multistage hydraulic fracturing of horizontal wells by decreasing the time required for initiation at subcritical wellbore pressures. The results also show that the current theoretical framework can capture the overall negative exponential relationship between the time to breakdown and the wellbore pressure, but it is insufficient to account for the secondary dependence on rock permeability.