Rockburst characteristics of several hard brittle rocks:A true triaxial experimental study

Rockburst is a typical rock failure which frequently threatens both human life and construction equipment during highly stressed underground excavation.Rock lithology is a control factor of rockburst.In this paper,rockburst tests were conducted on rectangular prismatic specimens of six types of intact hard brittle rocks,i.e.granodiorite,granite,marble,basalt,sandstone and limestone,under one-free-face true triaxial loading conditions.With the use of high-speed cameras,an acoustic emission(AE)system and a scanning electron microscope(SEM),rockburst of different rocks was investigated.The results show that the strainbursts of granodiorite,granite and marble were accompanied by tensile splitting near the free face,and consequently were relatively strong with a large amount of fragment ejection and kinetic energy release.For basalt,sandstone and limestone,failure was primarily dominated by shear rupture.The strainbursts of basalt and sandstone were relatively small with minor fragment ejection and kinetic energy release;while no burst failure occurred on limestone due to its relatively low peak strength.Rock strength,fracturing and fragmentation characteristics,and failure modes of different rocks can significantly affect rockburst proneness and magnitude.The AE evolution coupled with SEM analysis reveals that the differences in the inhe rent microstructures and fracture evolution under loading are the primary factors accounting for different rockbursts in various rock types.

Impact of elliptical boreholes on in situ stress estimation from leak-off test data

We developed an inversion technique to determine in situ stresses for elliptical boreholes of arbitrary trajectory. In this approach, borehole geometry, drilling-induced fracture information, and other available leak-off test data were used to construct a mathematical model, which was in turn applied to finding the inverse of an overdetermined system of equations.The method has been demonstrated by a case study in the Appalachian Basin, USA. The calculated horizontal stresses are in reasonable agreement with the reported regional stress study of the area, although there are no field measurement data of the studied well for direct calibration. The results also indicate that 2% of axis difference in the elliptical borehole geometry can cause a 5% difference in minimum horizontal stress calculation and a 10% difference in maximum horizontal stress calculation.

Fracture Reactivation Modeling in a Depleted Reservoir

Injection-induced fracture reactivation during hydraulic fracturing processes in shale gas development as well as coal bed methane(CBM)and other unconventional oil and gas recovery is widely investigated because of potential permeability enhancement impacts.Less attention is paid to induced fracture reactivation during oil and gas production and its impacts on reservoir permeability,despite its relatively common occurrence.During production,a reservoir tends to shrink as effective stresses increase,and the deviatoric effective stresses also increase.These changes in the principal effective stresses may cause Coulomb fracture slip in existing natural fractures,depending on their strength,orientation,and initial stress conditions.In this work,an extended finite element model with contact constraints is used to investigate different fracture slip scenarios induced by general reservoir pressure depletion.The numerical experiments assess the effect of Young’s modulus,the crack orientation,and the frictional coefficient of the crack surface on the distribution of stress and displacement after some reservoir depletion.Results show that the crack orientation significantly affects the state of stress and displacement,particularly in the vicinity of the crack.Slip can only occur in permitted directions,as determined by the magnitudes of the principal stresses and the frictional coefficient.Lastly,a larger frictional coefficient(i.e.,a rougher natural fracture surface)makes the crack less prone to shear slip.

In-situ stress inversion in Liard Basin, Canada, from caliper logs

This paper proposes an integrated method of analytical calculation,artificial intelligence,and probabilistic analysis to cost-effectively determine geomechanical properties and in-situ stresses from borehole deformation via caliper logs.It's also demonstrated in this paper that the actual borehole size can not be simply taken as the bit size by default,and adjusted borehole size has to be used to find the reasonable borehole deformation.In the proposed method,an artificial neural network(ANN)is applied to map the relationship among in-situ stress,adjusted borehole size,geomechanical properties,and borehole displacements.The genetic algorithm(GA)searches for the set of unknown stresses and geomechanical properties that match the objective borehole deformation function.Probabilistic analysis is conducted after ANN-GA modeling to estimate the most possible ranges of the parameters.The hybrid method has been demonstrated by a field case study to estimate the adjusted borehole size,Young's modulus,and the two horizontal in-situ stresses using borehole deformation information reported from four-arm caliper logs of a vertical borehole in Liard Basin in Canada.

A fully coupled finite element framework for thermal fracturing simulation in subsurface cold CO2 injection

Thermal fracturing could occur during cold CO2 injection into subsurface warm rock formations.It can be seen in a variety of fields such as carbon geo-sequestration,unconventional gas development,enhanced oil recovery,geothermal energy extraction,and energy geological storage systems.In CO2 geosequestion,limited degree of thermal fracturing due to the cooling effects of cold CO2 injection will enhance well injectivity,especially for those storage formations of low permeability.Thermal fracturing can therefore potentially enhance the injection efficiency and make positive impact on commercialization of CO2 geological storage.However,excessively developed fractures could break down the caprock and cause potential CO2 leakage into overlying rock formations.Risk analysis has to be done based on thermal fracturing simulation in order to maintain caprock integrity.Simulation of thermal fracturing during cold CO2 injection involves the coupled processes of heat transfer,mass transport,rock deforming as well as fracture propagation.To model such a complex coupled system,a fully coupled finite element framework for thermal fracturing simulation is presented.This framework is based on the theory of non-isothermal multiphase flow in fracturing porous media.It takes advantage of recent advances in stabilized finite element and extended finite element methods.The stabilized finite element method overcomes the numerical instability encountered when the traditional finite element method is used to solve the convection dominated heat transfer equation,while the extended finite element method overcomes the limitation with traditional finite element method that a model has to be remeshed when a fracture is initiated or propagating and fracturing paths have to be aligned with element boundaries.

Poroelastoplastic reservoir modeling by tangent stiffness matrix method

As oil and gas extraction activities move into deeper rock formations,many experimental studies and field investigations indicate rock exhibits a plastic behavior rather than a pure linear elastic behavior,so poroelastoplasticity must be taken into account in the reservoir simulation.Because reservoir rock is a porous material consisting of a compressible solid matrix and number of compressible fluids occupying the pore space,fully coupled modeling is required for reservoir simulation considering solid-fluid interaction,complex stress conditions and nonlinear behaviors.But the computational process could be cumbersome when constant tangent stiffness method is used to address the poroelastoplastic behavior.In this paper,a fully coupled poroelastoplasticity reservoir model based on Drucker-Prager yield criterion is implemented the tangent stiffness method,and the computational efficiency is compared with the constant stiffness method.The accuracy of these two methods is demonstrated in one-dimensional consolidation.In a case study,these two methods are used to analyze the stresses and pore pressure of a reservoir and computing results and running efficiency are compared.Also,the linear elastic and nonlinear solutions are compared in one-dimensional consolidation and reservoir modeling.It shows that the difference between results by constant stiffness method and tangent stiffness method is very small,while the tangent stiffness method shows significantly fewer iteration numbers and shorter running time than the constant stiffness method.