Research On Natural Gas Pipeline Risk Assessment Based On Fuzzy BN And Scenario Analysis

The safe and efficient development of shale gas has an important impact on the energy supply pattern of China.With the deepening of shale gas exploration and development in China,the increasingly prominent casing deformation problem in the multi-stage fracturing process has seriously restricted the shale gas development process in China.Engineering field test results show that shear deformation is the main form of casing deformation,and multi-stage fracture-induced fault activation and then slip is the main cause of casing shear deformation.However,the relationship between multi-stage fracturing engineering parameters,fault activation critical conditions,and the spatial and temporal evolution law of fault slip is not clear,which makes it difficult to propose corresponding prevention and control measures.In this thesis,the evolution of fault activation,slip and casing shear deformation during multi-stage fracturing of shale gas wells is studied in depth,and corresponding prevention and control measures are proposed.The main research work carried out in this thesis is as follows:(1)Shale rock mechanics and friction characteristics tests under fracturing fluid immersion conditions were carried out.The effects of different immersion lengths and external loading conditions on the mechanical parameters and friction coefficients of shale rocks were studied.The evolution of the friction coefficient of the fault interface under the influence of the coupling of immersion time and external load is revealed.The results showed that the compressive strength,tensile strength,modulus of elasticity and cohesion of shale decreased with the increase of soaking time,and Poisson’s ratio increased with the increase of soaking time;The shale friction coefficient decreases with increasing immersion time and normal stress,and then both level off;Fault friction coefficient under stratigraphic stress conditions decreases only with increasing immersion time and is less affected by fault interface stress;The friction coefficient decreases rapidly at the initial stage of fault activation,and continues to decrease and gradually stabilize after the end of fracturing.(2)A numerical model of segmented multi-cluster hydraulic fracturing flow-solid coupling was established based on the theory of porous elastodynamics.The mechanism of dynamic change of near-wellbore ground stress during multi-stage fracturing was studied.The dynamic changes of pore pressure and effective ground stress in the formation under the fault conditions are clarified.The casing load spectrum characteristics under dynamic ground stress were analyzed.The results of the study indicate that multi-stage fracturing leads to higher pore pressure,lower effective ground stress and higher ground stress difference in the near-wellbore formation;The pore pressure and effective ground stress at the fault location change abruptly;the casing stress increases under dynamic ground stress,the safety factor decreases,and the risk of casing deformation increases.(3)A dynamic ground-stress fault activation model was developed.The characteristics of pore pressure and effective ground stress changes in the fault area during multi-stage fracturing are clarified.On the basis of this,a fault activation evaluation model was developed by combining the Mohr-Coulomb criterion.Considering the variability of activation coefficients at different locations of fault interfaces,the spatial and temporal evolution of fault activation lengths in the multi-stage fracturing process is clarified;Considering the time-varying characteristics of the friction coefficient at the fault interface,a transient spatial-temporal evolution model of the fault slip volume was established to clarify the spatial-temporal evolution law of the fault slip volume and casing deformation during the fracturing process.The results of the study show that the fracturing fluid starts to activate upon entering the fault,while slip occurs;Fault activation length increases at a slower rate as fracturing proceeds,and fault slip and casing deformation increases at a faster rate;The decrease in friction coefficient at the fault interface and the increase in fault activation length lead to an increase in slip,and the accumulation of slip leads to deformation of the casing.Fault activation,slip and casing deformation onset moments are positively correlated with fractured bottomhole pressure and fault modulus of elasticity,and negatively correlated with fault pinch angle and rock internal friction angle.(4)Compressible material-based casing deformation control method and sequentialsynchronous coupled fracturing mode optimization method to avoid fault activation were developed.(1)Based on the characteristics of compressible material cement sheath absorbing fault slip,the mechanism of compressible material cement sheath controlling casing shear deformation is revealed,and the feasibility of the method is verified by indoor tests.A compressible cement sheath equivalent outer diameter model was established to achieve quantitative evaluation of the effect of compressible cement sheath on casing deformation control.(2)The mechanism of fracture activation induced by conventional fracturing process in neighboring wells is revealed,and a sequentialsynchronous coupled fracture mode optimization method to avoid fault activation is proposed.That is,the coupled fracturing mode is used to optimize the sequence of multiwell multi-stage fracturing before the fault to avoid fault activation.The results show that the method can avoid casing deformation in neighboring unfractured wells,reduce the length of fractured section loss due to fault slip,and achieve the effect of reducing the impact of casing deformation on fracturing construction.Through this thesis,the dynamic change law of near-wellbore ground stress,casing load characteristics,spatial and temporal evolution of fault slip and casing deformation during multi-stage fracturing of shale gas wells are clarified,and the corresponding casing deformation control methods are proposed.The research results can provide reference and guidance for shale gas well casing deformation control.