China has abundant shale gas reserves and the proven shale gas reserves rank the first in the world, but its reservoirs are characterized by low porosity and low permeability, and more effective hydraulic fracturing is needed to improve shale gas Permeability in rock, so the mining is much more difficult than other countries.At present, the basic problem of hydraulic fracturing in our country is how to form a large-scale cross-linked crack network in shale with the combination of desorption and displacement, Gas quickly mined out.
Recently, the team of Zhao Ya-Pu, State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, conducted a research work on the law of crack propagation around the solid-liquid interfacial shear stress of rock-fracturing fluid in hydraulic fracturing. The shear stress produced by the flow at the solid-liquid interface has an important influence on the crack propagation law. This finding explains the fact that wellbore pressure is much higher than the predicted value of the traditional model and can be used to guide the selection of fracturing fluid and fracturing methods The optimization, has important application value.
It is generally accepted in the industry that the fracturing fluid pressure remains substantially constant over the entire crack and appears only near the crack tip. In theoretical analysis, the researchers also found that the pressure at the crack tip is singular and the singularity and crack propagation dominate Mechanism (dominated by fracture of rock and workability of fracturing fluid). Under the dominant mechanism of viscous workmanship of fracturing fluid, the -1/2 order singularity of classical linear elastic fracture mechanics disappears and the crack tip fluid pressure Of the singularity of -1 / 3 order, the solid stress singularity is also -1 / 3 order, which is caused by the stress-phase matching of the solid-liquid phase.By the balance of forces we can see that the singularity of shear stress is stronger than the pressure , So a detailed study of the effects of shear stress is needed.
Therefore, the stress-strain singularity at the crack tip is used to qualitatively analyze the shear stress at the solid-liquid interface.By using the asymptotic properties of stress and displacement at the crack tip and combining with the newly established boundary integral of displacement and stress field The results show that shear stress tends to lead to crack closure. Commonly used stress intensity factor and energy release rate calculation methods need to introduce shear stress correction. There is a potential instability of crack propagation. And through the stress field near the crack tip and Strain energy density and the tendency of shear stress to be cracked perpendicularly to the crack surface.These findings have theoretically proved that the crack propagation can be instable by controlling the fracturing parameters and provide a new idea for the formation of a cracked network by hydraulic fracturing Based on this, the researchers established a power-law fluid-hydraulic fracturing model with coupled stress-strain at the solid-liquid interface for common disc fracturing and corrected the crack propagation criterion based on the stress intensity factor. The model Can be extended to commonly used two-dimensional and quasi-three-dimensional hydraulic fracturing model to provide fast and accurate hydraulic fracturing set And simulation schemes.Further studies have found that shear stress leads to longer and narrower cracks and higher wellbore pressure with viscous flow depletion of fracturing fluid negligible compared to the depletion of rock fracture and that excessively high Shear stress causes simultaneous instability of the crack at the wellbore and the crack tip. There are two dominant mechanisms of pressure loss and shear stress on the solid-liquid interface, depletion of rock fracture, and viscous flow of fracturing fluid Mechanism changes during the fracturing process and the direction of transition is related to the fluidity index of the power-law fracturing fluid.These discoveries explain the phenomenon that wellbore pressure is much higher than the predicted value of the traditional model, Cracked flow control provides the direction.
The above research deeply explores the important influence of shear stress at solid-liquid interface on stress singularity and dominant mechanism in hydraulic fracturing, and provides a new idea for solving the basic problems in shale gas exploitation.
Relevant research results have been published in the ASME Journal of Applied Mechanics, which was supported by the National Natural Science Foundation of China Petroleum & Chemical Joint Fund, the Chinese Academy of Sciences Innovation Cross Team Project, the Frontier Science Research Key Project and the Strategic Pilot Science and Technology Project.
Fig.1 Normalized stress and strain energy density distribution near crack tip under different dominant mechanisms
Figure 2. (a) Schematic diagram of disk-shaped hydraulic fracturing; (b) Transition of dominant mechanism with different flow indices. N <0.5 时, 由断裂耗功向黏性耗功主导机制转变; n>0.5, the shear stress dominated the early crack propagation
