Identifying crack initiation stress threshold in brittle rocks using axial strain stiffness characteristics

As an estimate for the in-situ spalling strength around massive underground excavations to moderately jointed brittle rocks, crack initiation stress marks the initiation of rock micro fracturing. It is crucial to accurately identify crack initiation stress level by proper method. In this study, confined compression tests of sandstone samples are used to examine the validity/applicability of proposed axial strain stiffness method. The results show that by highlighting the minuscule changes in stress-strain curve, the axial strain stiffness curve provided further insight into rock failure process and revealed five stages:(a) irregular fluctuation,(b) nearly horizontal regular fluctuation,(c) irregular fluctuation gradually decreasing to zero,(d) extreme fluctuation, and(e) near zero, which mainly correspond to five stages of stress–strain curve. The ratio of crack-initiation stress to peak strength determined using this approach is 0.44–0.51, similar to the ranges previously reported by other researchers. In this method, the key is to accurately detect the end point of the stage(b), "nearly horizontal regular fluctuation" characterized by a sudden change in axial strain stiffness curve, and the sudden change signifies crack initiation in rock sample. Finally, the research indicates that the axial strain stiffness curve can provide a mean to identify the crack-initiation stress thresholds in brittle rocks.

Hydraulic Fracturing Leads to Wolframite Deposition at Magmatic-hydrothermal transition

Objective The Nanling Range in southern China is a world-class tungsten province. It is disputed how wolframite as the main ore mineral in the tungsten deposits in this area is precipitated. Wolframite solubility is only weakly dependent on pressure （Wood and Samson, 2000）. Therefore, a pressure change cannot alter wolframite solubility. However, fluid inclusions record high-pressure fluids that trigger the initiation and propagation of vein- bearing fractures in these deposits. It is enigmatic whether a decrease in fluid pressure after hydraulic fracturing causes wolframite precipitation from hydrothermal solutions. We first demonstrate that fluid pressure drop after hydraulic fracturing could cause a significant decrease in CO2 solubility, increase pH, and facilitate wolframite precipitation at magmatic-hydrothermal transition using finite element-based numerical experiments.

Recent Development in Numerical Simulation of Enhanced Geothermal Reservoirs

This paper briefly introduces the current state in computer modelling of geothermal reservoir system and then focuses on our research efforts in high performance simulation of enhanced geothermal reservoir system. A novel supercomputer simulation tool has been developing towards simulating the highly non-linear coupled geomechanical-fiuid flow-thermal systems involving heterogeneously fractured geomaterials at different spatial and temporal scales. It is applied here to simulate and visualise the enhanced geothermal system （EGS）, such as （1） visualisation of the microseismic events to monitor and determine where/how the underground rupture proceeds during a hydraulic stimulation, to generate the mesh using the recorded data for determining the domain of the ruptured zone and to evaluate the material parameters （i.e., the permeability） for the further numerical analysis and evaluation of the enhanced geothermal reservoir; （2） converting the avail- able fractured rock image/fracture data as well as the reservoir geological geometry to suitable meshes/grids and further simulating the fluid flow in the complicated fractures involving the detailed description of fracture dimension and geometry by the lattice Boltzmann method and/or finite element method; （3） interacting fault system simulation to determine the relevant complicated rup- ture process for evaluating the geological setting and the in-situ reservoir properties; （4） coupled thermo-fluid flow analysis of a geothermal reservoir system for an optimised geothermal reservoir design and management. A few of application examples are presented to show its usefulness in simulating the enhanced geothermal reservoir system.

A review on hydraulic fracturing of unconventional reservoir

Hydraulic fracturing is widely accepted and applied to improve the gas recovery in unconventional reservoirs.Unconventional reservoirs to be addressed here are with very low permeability,complicated geological settings and in-situ stress field etc.All of these make the hydraulic fracturing process a challenging task.In order to effectively and economically recover gas from such reservoirs,the initiation and propagation of hydraulic fracturing in the heterogeneous fractured/porous media under such complicated conditions should be mastered.In this paper,some issues related to hydraulic fracturing have been reviewed,including the experimental study,field study and numerical simulation.Finally the existing problems that need to be solved on the subject of hydraulic fracturing have been proposed.