Fluid expansion-driven hydraulic fracture
Fluid-driven fractures occur ubiquitously in subsurface rocks in natural and engineering processes, such as earthquakes and hydraulic fracturing. “Hydraulic fracturing” refers to an engineering operation where pressurized fluids are injected into low-permeable rocks to create fractures that increase the flow capacity of hydrocarbons. Understanding the fracturing dynamics coupled with fluid flow is important.
It is very challenging to experimentally observe and describe the hydraulic fracturing process in stiff materials because: 1) the fast speed of fracturing makes high-resolution visualization difficult, 2) fluid flow in the fracture and the fracture propagation are highly coupled, while the complex fluid properties further complicate the process, and 3) the change in external conditions, such as confining pressure, and geometry scale may qualitatively overwrite the current understanding. Due to these challenges, very few works have been conducted to address the fast, fluid-coupled fracturing process experimentally.
We establish an experimental platform where the fracturing process can be directly visualized at high temporal resolution. We visualize the hydraulic fracturing process using a high-speed camera at a temporal resolution of micron-seconds. In the experiments, fracking fluid is injected by a high-pressure syringe pump into a small, vertical hole at the center of a 3D-printed, transparent PMMA cylinder sample. A penny-shaped fracture is induced from the notch at the bottom of the small hole and moves toward the cylinder edge. The fracture plane is roughly perpendicular to the vertical hole. A continuous video is recorded by a high-speed camera from the bottom of the cylinder. The acquired images are analyzed using image analysis software where the fluid flow, crack tip nucleation events, and fracture propagation dynamics during the cracking are identified.
The fracture propagates in a stick-break (stop/go) pattern, causing a step-like fracture front in the kymograph plot. A kymograph plots the radial location of the crack front versus fracturing time. In a break event, nucleation initiates at one or more locations near the crack tip and propagates in the tangential direction until covers the entire circle. The fluid front lags the crack front and the lag length increases with fracturing fluid viscosity when Newtonian fluid is used. This is because higher viscosity causes larger shear dissipation and lower fluid flow velocity in the fracture. However, when a high molecular weight polymer is used, the lag length is small regardless of fluid viscosity. For different polymers, the lag length increases as the polymer molecular weight decreases.
The fracture break pattern is quite unintuitive when polymers are used. The lag between the fluid front and the crack front stays small and is similar to that of water regardless of the polymer concentration and viscosity when high molecular weight polymers are used. We speculate that when the polymer flows in the fracture, a depletion zone composed of water is formed near the fracture surfaces. Most of the shear dissipation occurs within the depletion zone, which leads to a small shear loss similar to that of water. Thinner depletion zones are formed with smaller molecular weight polymers, leading to larger shear dissipations and larger lag lengths. In the middle of the fracture, the bulk polymer solutions retain their viscosities and capabilities to transport proppants.
