AbstractsEarth & Environmental Science

Carbonate-rich rocks such as limestone comprise commonly encountered reservoir rocks in the oil and gas industry. This thesis is aimed at showing the impact of acidic fluid on 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 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. The breakdown of the specimen is also explosive in the case of acid injection, probably due to generate 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. Taken together, the results indicate firstly that acid injection can be expected to improve 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.