Impact of fluid acidity on the time-dependent initiation of hydraulic fractures in carbonate rocks

Carbonate-rich rocks, such as limestones, comprise commonly-encountered reservoir rocks in the oil and gas industry. This paper shows the impact of acidic fluid on the hydraulic fracture initiation through laboratory experimentation. The results show that, compared to water injection, acid injection results in more rapid initiation of the hydraulic fractures under socalled 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. The breakdown of the specimen is, in addition, 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, or specimen lifetime, 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 firstly that acid injection can be expected 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.