Numerical Simulation Of Aseismic Energy Triggered By Injection

There exists an abundant of resources of Hot Dry Rock in deep strata which attracts more attention in recent years because of increasingly striking environmental pollution.However,the Hot Dry Rock resources are difficult to mined on account of its low porosity and low permeability.The Enhanced Geothermal System(EGH)has been proposed to stimulate the reservoir and exploit economically,which requires the key technology of hydraulic fracturing involving to hydro-mechanical coupled process with a multiphase medium.But,plenty of seism is triggered during injection into hot dry rock.Thus,the paper aims at investigating the mechanism of inducing seismicity,especially when the injection in GPK2 of Soultz-Sous-Foret is ceased,still triggering a few bigger seismicity(M=2.6).Firstly,as a result of the complicated hydro-mechanical coupled process,an efficient iterative coupling and energy releasing model are developed to reveal the genuine mechanism as much as possible.The hydro-mechanic coupling descripts the behavior of a nature fracture,including separation,contact and shear dilation,while an empirical equation(M=GAd)is used to calculate the seismic moment in the energy releasing model.Meanwhile,when predicts the total seismic moment,an assumption,“the strength of fracture element is decrease gradually descripted by Kelvin curve” is made.And the model established is confirmed by three examples.Secondly,in terms of the developed model,the logging pressure and cumulative energy in GPK2 are inversed properly,which demonstrates the validity of the developed model.Finally,another target in the paper we do is analyzing the parameter sensitivity to inducing seismicity,consisting of construction parameters(the fluid viscosity,injection rate,injection methods)and geomechanics parameters(normal stiffness in fracture,angels between the maximum principle stress and fracture,permeability).And the results are as below:(1)There is a phenomenon that fracture elements would fail again.The failure is not only happened in the front of elements and the fracture elements which have failed still would fail.Besides,The fluid pressure in the front of elements after shut in is much bigger than before,which could infer that the fluid pressure in the front of elements is the reason for still inducing seismicity after shut in;The stress transfer is another principal way to trigger seismicity,which could infer that the transfer of stress drop is another reason for inducing seismicity.(2)Cumulative releasing energy increases with increasing injection rate,fluid viscosity and normal stiffness and decreases with matrix permeability.For the different injection pattern,batch 1 and batch 2 play the positive role on decreasing releasing energy.It’s worth noting that rate of releasing energy has a character of “inertance”,which means that the rate of releasing energy still keeps the rate of last injection,and every time the injection rate is changed,the “inertance” would last more time and the subsequent releasing energy rate will increase than before.For different angles between the maximum principle stress and fracture,the tendency of releasing energy presents the feature of bigger Small on both sides and big in the middle.It is obvious that releasing energy is the biggest in a moment with 30? between the maximum principle stress and fracture and fracture elements are failed rapidly between 20? and 40?.By analyzing in details,we conclude that injection pattern、injection rate and angles between fracture and the maximum principle stress have much bigger influence on inducing seismicity.