Investigating heterogeneous distribution of fluid pressure in hydraulic fractures during pulsating hydraulic fracturing

Abstract

Pulsating hydraulic fracturing (PHF) has been proven to be a potential fracturing method. Compared with conventional hydraulic fracturing, the PHF can generate a more complex fracture network with lower breakdown pressure and less induced seismicity. A distribution of fluid pressure in hydraulic fractures determines the initiation and propagation of new fractures, which has not been fully investigated in the PHF. In this paper, a distribution of fluid pressure in pulsating hydraulic fractures is innovatively investigated through a transient flow model (TFM). This model is solved by the method of characteristics (MOC). This solution method is then verified against experimental data. Through this model, the mechanism of the pressure distribution is analyzed and the optimization method for PHF is proposed. The effects of input frequency, friction, fracture roughness and fracture length on the fluid pressure distribution during the PHF are investigated, respectively. Results show that the fluid pressure at each point along hydraulic fractures fluctuates in the same frequency, and the pressure fluctuation amplitudes are heterogeneously distributed in the hydraulic fractures during the PHF. This phenomenon results from the fact that a standing wave is formed in the hydraulic fractures during the PHF. Moreover, according to the number of wave nodes in the standing wave, the heterogeneous distribution of fluid pressure amplitudes can be divided into three categories for input frequency from 1 Hz to 20 Hz. The frequency ranges of these three pressure distribution categories are centered by three orders of resonance frequency, and the number of wave nodes for these three pressure distribution categories equals the order of resonance frequency. Although the standing wave in the hydraulic fractures is partially changed by a friction effect, the fluid pressure amplitude distribution is still heterogenous. In addition, the friction has a dissipation effect on a resonance amplitude. The increasing fracture roughness reduces the heterogeneity of a fluid pressure distribution in hydraulic fractures during the PHF. The first maximum fluid pressure amplitude near a fracture entrance may not exist for high fracture roughness. Finally, the effect of the heterogeneous distribution of fluid pressure on the initiation and propagation of new fractures during the PHF is addressed, and then a novel and effective design method of input frequency and amplitude for the PHF is proposed. Multiple initiations of new fractures can be obtained by optimizing an input frequency and amplitude.