Pore pressure shear slip

ESTIMATION OF CRITICAL PORE PRESSURE FOR SHEAR SLIP OF FRACTURES AT THE SOULTZ HOT DRY ROCK GEOTHERMAL RESERVOIR USING MICROSEISMIC MULTIPLETS... 

CALCULATION OF CRITICAL PORE-PRESSURE FOR SHEAR SLIP OF FRACTURE... 

We estimate distribution of critical pore-pressure of fractures for shear slip during hydraulic stimulation. 

According to Coulomb s law of friction, the relationship among shear stress, normal stress, coefficient of friction and pore-pressure at the moment of shear slip (critical condition) due to increasing of pore-pressure are described as follow. 

The estimated critical pore-pressure for shear slip suggests that the fluid pressure in fractures is higher value near the fractured zones 1 and 2 around 2,900 m, and declines as the fractures are away from the zones. 

Figure 5. Calculated critical pore-pressure for shear slip of fractures versus depth of estimated fractures. The zone of permeable fracture, which are detected by well loggings are indicated by shaded rectangles.

It is considered that some fractures opened with increasing pore-pressure maybe after shear slip because the maximum wellhead pressure is about lOMPa. However, Mode I fractures can not radiate seismic events having enough energy, and we can not detect these seismic events. Therefore, we consider only shearing fractures for estimation of critical pore-pressure using induced microseismic events. 

Earthquake Swarms, Fig. 5 Mohr-Coulomb diagram illustrating the reactivation of earthquake slip due to increase of the pore pressure Ap. The failure envelope, which is indicated by the inclined line, shows the failure shear stress Xf = — p), where p is the Mctirai coeffi-... 

Three properties of fluids under shear are discussed in detail flow boundary condition, friction, and shear viscosity. It has been shown that the no-slip boundary condition assumed in fluid mechanical formulations of Newtonian flow past solids can fail at the molecular level. The velocity profiles deviate most from the continuum linear form at small pore separations, low temperatures, high pressures, and high shear rates. Friction is controlled by two factors - interfacial strength and in-plane ordering. 

Instability in a soil mass occurs when slip surfaces develop and movements are initiated within it. Undesirable properties in a soil, such as low shearing strength, development of fissures and high pore water pressure, tend to encourage instability and are likely to lead to deterioration in slopes. In the case of open excavation, removal of material can give rise to the dissipation of residual stress that can aid instability. 

Solving this flow model for the velocity the pressure is calculated from the ideal gas law. The temperature therein is obtained from the heat balance and the mixture density is estimated from the sum of the species densities. It is noted that the viscous velocity is normally computed from the pressure gradient by use of a phenomenologically derived constitutive correlation, known as Darcy s law, which is based on laminar shear flow theory [139]. Laminar shear flow theory assumes no slip condition at the solid wall, inducing viscous shear in the fluid. Knudsen diffusion and slip flow at the solid matrix separate the gas flow behavior from Darcy-type flow. Whenever the mean free path of the gas molecules approaches the dimensions of pore diameter, the individual gas molecules are in motion at the interface and contribute an additional flux. This phenomena is called slip flow. In slip flow, the layer of gas next to the surface is in motion with respect to the solid surface. Strictly, the Darcy s law is valid only when the flow regime is laminar and dominated by viscous forces. The theoretical foundation of the dusty gas model considers that the model is applied to a transition regime between Knudsen and continuum bulk diffusion. To estimate the combined flux, the model is based on the assumption that the combined flux can be expressed as a linear sum of the Knudsen flux and the convective flux due to laminar flow. 

Fracturing is controlled by shear stress on the fault, pore-fluid pressure, cohesion and fault friction. In addition, the orientation of the fault with respect to the principal stress directions governs susceptibility of the fault to be ruptured. An essential role in the stress analysis play principal faults - the faults which are optimally oriented for shear faulting and thus being the most unstable under the given stress conditions. The type of faulting, the orientation of the activated faults and the direction of the slip along the faults can be inverted for stress and its spatial and temporal variations within the Earth s crust. The stress inversions work best if the fault orientations are known. If the fault plane cannot be uniquely identified from the focal mechanisms, the inversions are less accurate. The inversions are capable to retrieve four stress parameters the principal stress directions and the shape ratio. The accuracy of the results depends on the applied inversion method, on the number of inverted focal mechanisms, their errors and variety. The most critical parameter is the shape ratio, which... 

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