A Phase-Field Based Approach for Modeling the Cementation and Shear Slip of Fracture Networks

Modeling the geomechanical deformations of fracture networks has become an integral part of designing enhanced geothermal systems and recovery mechanisms for unconventional reservoirs. Stress changes in the reservoir can cause large variations in the apertures of fractures resulting in drastic changes in their transmissivities. At the same time, sustained high injection pressures can induce shear slipping along existing fractures and faults and trigger seismic activity. In this work, a novel approach is introduced for the simulation of cementation and shear slip of fractures on very general semi-structured grids. Natural fracture networks are represented in large scale reservoirs using the phase-field approach. The fluid flow through fractures is simulated on spatially non-conforming grids using the enhanced velocity mixed finite element method. The geomechanics equations are discretized using the continuous Galerkin finite element method. The single-phase flow and mechanics equations are decoupled using the fixed stress iterative scheme. The model can predict shear slipping and opening/closure of fractures due to induced stresses and poromechanical effects. Two synthetic examples are presented to model the effects of injection/production processes on the cementation and shear slip of fractures. The impact of the fractures' orientation and their connectivity on the hydromechanical response of the reservoir is also considered. The examples illustrate the strong impact of the dynamic behavior of fractures and the accompanying poroelastic deformations on the safety and productivity of subsurface projects.

Failure Characteristics and Mechanism of Multiface Slopes under Earthquake Load Based on PFC Method

Understanding the failure mechanism and failure modes of multiface slopes in the Wenchuan earthquake can provide a scientific guideline for the slope seismic design. In this paper, the two-dimensional particle flow code (PFC2D) and shaking table tests are used to study the failure mechanism of multiface slopes. The results show that the failure modes of slopes with different moisture content are different under seismic loads. The failure modes of slopes with the moisture content of 5%, 8%, and 12% are shattering-shallow slip, tension-shear slip, and shattering-collapse slip, respectively. The failure mechanism of slopes with different water content is different. In the initial stage of vibration, the slope with 5% moisture content produces tensile cracks on the upper surface of the slope; local shear slip occurs at the foot of the slope and develops rapidly; however, a tensile failure finally occurs. In the slope with 8% moisture content, local shear cracks first develop and then are connected into the slip plane, leading to the formation of the unstable slope. A fracture network first forms in the slope with 12% moisture content under the shear action; uneven dislocation then occurs in the slope during vibration; the whole instability failure finally occurs. In the case of low moisture content, the tensile crack plays a leading role in the failure of the slope. But the influence of shear failure becomes greater with the increase of the moisture content.

Investigation of Unstable Failure Potential of a Shear Slip Using DEM at an Underground Mine

Geological structures and discontinuities subjected to the perturbations posed by mining operations in underground mining can be re-activated and cause fault-slip rockbursts. This study investigates geomechanical stability in terms of shear slip behavior along discontinuities using 3DEC with focusing on sudden changes of shear stress and shear displacement. A direct shear test is performed using a continuously yielding joint model to examine the evolution of shear stress and shear displacement on this joint. Further, this continuously yielding joint model is applied in major discontinuities of an underground mine to examine whether an unstable shear slip behavior exists, which is represented by a significant shear stress decrease and a shear displacement increase. By referring to geological mapping of this mine, four cases are developed and each case is set up with one type of major discontinuities with identically simulated mining operations. Results imply that the amplitude of shear stress decrease and shear displacement increase along discontinuities substantially increases with the depth due to higher virgin stresses and mining-induced stresses at greater depths. The discontinuity parallel to the interface between footwall and orebody is the least safe case and subjects to the largest potential of triggering seismic events. Keywords: Shear slip behavior, Unstable failure, Mining-induced seismicity, Continuously yielding joint model, DEM.