Abstract
Hydraulic fracturing is vital in the stimulation of oil and gas reservoirs, whereas the dynamic process during hydraulic fracturing is still unclear due to the difficulty in capturing the behavior of both fluid and fracture in the transient process. For the first time, the direct observations and theoretical analyses of the relationship between the crack tip and the fluid front in a dynamic hydraulic fracture are presented. A laboratory-scale hydraulic fracturing device is built. The momentum-balance equation of the fracturing fluid is established and numerically solved. The theoretical predictions conform well to the directly observed relationship between the crack tip and the fluid front. The kinetic energy of the fluid occupies over half of the total input energy. Using dimensionless analyses, the existence of equilibrium state of the driving fluid in this dynamic system is theoretically established and experimentally verified. The dimensionless separation criterion of the crack tip and the fluid front in the dynamic situation is established and conforms well to the experimental data. The dynamic analyses show that the separation of crack tip and fluid front is dominated by the crack profile and the equilibrium fluid velocity. This study provides a better understanding of the dynamic hydraulic fracture.
Highlights
Hydraulic fracturing is an inherently unstable process
The FC starts to neutralize most of the driving force when the fluid runs close to the crack tip and the results are sensitive to the accuracy of the approximation of the pressure distribution
The boundary condition only implies that the pressure at the fluid front is 0 before reaching the crack tip, it reveals no details of the pressure distribution of the fluid
Summary
Even the controlled and overall stable hydraulic fracture propagation is unstable at the small scale [1]. Most previous studies on hydraulic fracture are based on the quasi-static treatment of the fracturing fluid and the inertial force of the fluid is neglected [2,3,4]. The dynamic situation is fundamentally different from the quasi-static simulation. It has been shown that in the quasi-static fluid-driven crack, the fluid front has a relationship of x ∼ t1/2 with time t in a quasi-static situation where the inertial force is neglected and viscosity dominates the fluid behavior [4]. A recent numerical study [5] on a dynamic hydraulic fracture taking the inertia effect into account has shown that the hydraulic fracture propagates in a stepwise manner with fluid
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