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Modeling the simultaneous propagation of multiple hydraulic fractures is important for stimulation of horizontal oil and gas wells. A new numerical model for the initiation and subsequent propagation of multiple hydraulic fractures allows the fractures to propagate under subcritical conditions. Including subcritical growth enables demonstration of the importance of time-dependent hydraulic fracture initiation and its impact on the subsequent growth of multiple hydraulic fractures, indicating that this phenomenon can strongly impact the behavior of simultaneously-growing hydraulic fractures, including reducing the suppression of some fractures during growth and reducing the pressure required to sustain fracture propagation.

Earthquakes, mine failures, and initiation of hydraulic fractures have all been associated with time-delayed breakage of rocks subjected to loads insufficient to incur instantaneous failure. To better model and identify failure precursors for rocks under these so-called static fatigue conditions, Acoustic Emission (AE) was monitored in beams of Coldspring Charcoal Granite subjected to constant three point loading. As a result of varying the magnitude of the loading, the times to failure range from O ( 10 1 ) to O ( 10 5 ) seconds. The experiments show a number of consistencies in the AE data. In all cases the event rate exponentially declines for a period that is about 0.4–0.6 of the total time to failure. This period is followed by a period in which the event rate exponentially increases. The total number of events generated during these two periods is also consistent among the experiments. Motivated by these observations, we propose a modified kinetic fracture theory that captures both the period of event rate decline and the period of event rate increase. It does this by firstly accounting for early time depletion of available bonds for breakage, similar to previous models. The model also accounts for generation of critically stressed bonds in the vicinity of previous bond breakages due to stress redistribution, thereby also predicting the event rate increase and further providing consistency with the observed tendency of the events to become increasingly concentrated around the eventual plane of failure as time progresses.

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.