Permeability evolution during fluid-pressure induced shear slip in saw-cut and natural granite fractures

SUMMARY Mechanisms controlling fracture permeability enhancement during injection-induced and natural dynamic stressing remain unresolved. We explore pressure-driven permeability (k) evolution by step-increasing fluid pressure (p) on near-critically stressed laboratory fractures in shale and schist as representative of faults in sedimentary reservoirs/seals and basement rocks. Fluid is pulsed through the fracture with successively incremented pressure to first examine sub-reactivation permeability response that then progresses through fracture reactivation. Transient pore pressure pulses result in a permeability increase that persists even after the return of spiked pore pressure to the null background level. We show that fracture sealing is systematically reversible with the perturbing pressure pulses and pressure-driven permeability enhancement is eminently reproducible even absent shear slip and in the very short term (order of minutes). These characteristics of the observed fracture sealing following a pressure perturbation appear similar to those of the response by rate-and-state frictional healing upon stress/velocity perturbations. Dynamic permeability increase scales with the pore pressure magnitude and fracture sealing controls the following per-pulse permeability increase, both in the absence and presence of reactivation. However, initiation of the injection-induced reactivation results in a significant increase in the rate of permeability enhancement (dk/dp). These results demonstrate the role of frictional healing and sealing of fractures at interplay with other probable processes in pore pressure-driven permeability stimulation, such as particle mobilization.

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Permeability Evolution of an Intact Marble Core during Shearing under High Fluid Pressure

Geofluids ◽

10.1155/2021/8870890 ◽

2021 ◽

Vol 2021 ◽

pp. 1-18

Author(s):

Yuan Wang ◽

Yu Jiao ◽

Shaobin Hu

Keyword(s):

Normal Stress ◽

Laser Scanning ◽

Shear Failure ◽

Fluid Pressure ◽

Shear Displacement ◽

Permeability Evolution ◽

Fracture Permeability ◽

Effective Normal Stress ◽

High Fluid ◽

High Fluid Pressure

The progressive shear failure of a rock mass under hydromechanical coupling is a key aspect of the long-term stability of deeply buried, high fluid pressure diversion tunnels. In this study, we use experimental and numerical analysis to quantify the permeability variations that occur in an intact marble sample as it evolves from shear failure to shear slip under different confining pressures and fluid pressures. The experimental results reveal that at low effective normal stress, the fracture permeability is positively correlated with the shear displacement. The permeability is lower at higher effective normal stress and exhibits an episodic change with increasing shear displacement. After establishing a numerical model based on the point cloud data generated by the three-dimensional (3D) laser scanning of the fracture surfaces, we found that there are some contact areas that block the percolation channels under high effective stress conditions. This type of contact area plays a key role in determining the evolution of the fracture permeability in a given rock sample.

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Laboratory Study on the Effect of Fluid Pressurization Rate on Fracture Instability

Geofluids ◽

10.1155/2021/6084032 ◽

2021 ◽

Vol 2021 ◽

pp. 1-8

Author(s):

Xinyao Wang ◽

Quanchen Gao ◽

Xiao Li ◽

Dianzhu Liu

Keyword(s):

Dominant Role ◽

Fluid Pressure ◽

Slip Mode ◽

Permeability Evolution ◽

Induced Earthquakes ◽

Stick Slip ◽

Pressurization Rate ◽

Fracture Instability ◽

Fluid Pressurization ◽

Fracture Stability

Fluid injection-induced earthquakes have been a scientific and social issue of wide concern, and fluid pressurization rate may be an important inducement. Therefore, a series of stepwise and conventional injection-induced shear tests were carried out under different fluid pressurization rates and effective normal stresses. The results show that the magnitude of fluid pressure is the main factor controlling the initiation of fracture slipping. The contribution of fluid pressure heterogeneity and permeability evolution on the initiation of fracture slipping is different with the increase of fluid pressurization rate. When the fluid pressurization rate is small, permeability evolution plays a dominant role. On the contrary, the fluid pressure heterogeneity plays a dominant role. The increase of fluid pressurization rate may lead to the transition from creep slip mode to slow stick-slip mode. Under the laboratory scale, the fluid pressure heterogeneity causes the coulomb failure stress to increase by about one times than the predicted value at the initiation of fracture slipping, and the coulomb stress increment threshold of 1.65 MPa is disadvantageous to the fracture stability.

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Fault valving and pore pressure evolution in simulations of earthquake sequences and aseismic slip

Nature Communications ◽

10.1038/s41467-020-18598-z ◽

2020 ◽

Vol 11 (1) ◽

Cited By ~ 1

Author(s):

Weiqiang Zhu ◽

Kali L. Allison ◽

Eric M. Dunham ◽

Yuyun Yang

Keyword(s):

Fault Zone ◽

Fluid Pressure ◽

Seismogenic Zone ◽

Permeability Evolution ◽

Aseismic Slip ◽

Effective Normal Stress ◽

Pressure Stress ◽

Cyclic Changes ◽

Earthquake Sequences ◽

Pressure Evolution

Abstract Fault-zone fluids control effective normal stress and fault strength. While most earthquake models assume a fixed pore fluid pressure distribution, geologists have documented fault valving behavior, that is, cyclic changes in pressure and unsteady fluid migration along faults. Here we quantify fault valving through 2-D antiplane shear simulations of earthquake sequences on a strike-slip fault with rate-and-state friction, upward Darcy flow along a permeable fault zone, and permeability evolution. Fluid overpressure develops during the interseismic period, when healing/sealing reduces fault permeability, and is released after earthquakes enhance permeability. Coupling between fluid flow, permeability and pressure evolution, and slip produces fluid-driven aseismic slip near the base of the seismogenic zone and earthquake swarms within the seismogenic zone, as ascending fluids pressurize and weaken the fault. This model might explain observations of late interseismic fault unlocking, slow slip and creep transients, swarm seismicity, and rapid pressure/stress transmission in induced seismicity sequences.

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Inversion in the permeability evolution of deforming Westerly granite near the brittle–ductile transition

Scientific Reports ◽

10.1038/s41598-021-03435-0 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Claudio Petrini ◽

Claudio Madonna ◽

Taras Gerya

Keyword(s):

Fluid Flow ◽

Oil And Gas ◽

Axial Strain ◽

Fluid Pressure ◽

Pressure Control ◽

Permeability Evolution ◽

Gas Recovery ◽

Permeability Enhancement ◽

Triaxial Apparatus ◽

Brittle Ductile Transition

AbstractFluid flow through crustal rocks is controlled by permeability. Underground fluid flow is crucial in many geotechnical endeavors, such as CO2 sequestration, geothermal energy, and oil and gas recovery. Pervasive fluid flow and pore fluid pressure control the strength of a rock and affect seismicity in tectonic and geotechnical settings. Despite its relevance, the evolution of permeability with changing temperature and during deformation remains elusive. In this study, the permeability of Westerly granite at an effective pressure of 100 MPa was measured under conditions near its brittle–ductile transition, between 650 °C and 850 °C, with a strain rate on the order of 2·10–6 s−1. To capture the evolution of permeability with increasing axial strain, the samples were continuously deformed in a Paterson gas-medium triaxial apparatus. The microstructures of the rock were studied after testing. The experiments reveal an inversion in the permeability evolution: an initial decrease in permeability due to compaction and then an increase in permeability shortly before and immediately after failure. The increase in permeability after failure, also present at high temperatures, is attributed to the creation of interconnected fluid pathways along the induced fractures. This systematic increase demonstrates the subordinate role that temperature dilatancy plays in permeability control compared to stress and its related deformation. These new experimental results thus demonstrate that permeability enhancement under brittle–ductile conditions unveils the potential for EGS exploitation in high-temperature rocks.

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