This paper presents a model to simulate the monotonic and cyclic behaviours of weakly cemented sands.An elastoplastic constitutive model within the framework of bounding surface plasticity theory is adopted to predict...This paper presents a model to simulate the monotonic and cyclic behaviours of weakly cemented sands.An elastoplastic constitutive model within the framework of bounding surface plasticity theory is adopted to predict the mechanical behaviour of soft sandstone under monotonic and cyclic loadings. In this model, the loading surface always passes through the current stress state regardless of the type of loading. Destruction of the cementation bonds by plastic deformation in the model is considered as the primary mechanism responsible for the mechanical degradation of loosely cemented sands/weak rock.To model cyclic response, the unloading plastic and elastic moduli are formulated based on the loading/reloading plastic and elastic moduli. The proposed model was implemented in FLAC2D and evaluated against laboratory triaxial tests under monotonic and cyclic loadings, and the model results agreed well with the experimental observations. For cyclic tests, hysteresis loops are captured with reasonable accuracy.展开更多
In petroleum engineering, the transport phenomenon of proppants in a fracture caused by hydraulic fracturing is captured by hyperbolic partial differential equations(PDEs). The solution of this kind of PDEs may encoun...In petroleum engineering, the transport phenomenon of proppants in a fracture caused by hydraulic fracturing is captured by hyperbolic partial differential equations(PDEs). The solution of this kind of PDEs may encounter smooth transitions, or there can be large gradients of the field variables. The numerical challenge posed in a shock situation is that high-order finite difference schemes lead to significant oscillations in the vicinity of shocks despite that such schemes result in higher accuracy in smooth regions. On the other hand, first-order methods provide monotonic solution convergences near the shocks,while giving poorer accuracy in the smooth regions.Accurate numerical simulation of such systems is a challenging task using conventional numerical methods. In this paper, we investigate several shock-capturing schemes.The competency of each scheme was tested against onedimensional benchmark problems as well as published numerical experiments. The numerical results have shown good performance of high-resolution finite volume methods in capturing shocks by resolving discontinuities while maintaining accuracy in the smooth regions. Thesemethods along with Godunov splitting are applied to model proppant transport in fractures. It is concluded that the proposed scheme produces non-oscillatory and accurate results in obtaining a solution for proppant transport problems.展开更多
This paper presents a numerical investigation of hydraulic fracturing in oil sands during cold water injection by considering the aspects of both geomechanics and reservoir fluid flow.According to previous studies,the...This paper presents a numerical investigation of hydraulic fracturing in oil sands during cold water injection by considering the aspects of both geomechanics and reservoir fluid flow.According to previous studies,the low shear strengths of unconsolidated or weakly consolidated sandstone reservoirs significantly influence the hydraulic fracturing process.Therefore,classical hydraulic fracture models cannot simulate the fracturing process in weak sandstone reservoirs.In the current numerical models,the direction of a tensile fracture is predetermined based on in situ stress conditions.Additionally,the potential transformation of a shear fracture into a tensile fracture and the potential reorientation of a tensile fracture owing to shear banding at the fracture tip have not yet been addressed in the literature.In this study,a smeared fracture technique is employed to simulate tensile and shear fractures in oil sands.The model used combines many important fracture features,which include the matrix flow,poroelasticity and plasticity modeling,saturation-dependent permeability,gradual degradation of the oil sands as a result of dilative shear deformation,and the tensile fracturing and shear failure that occur with the simultaneous enhancement of permeability.Furthermore,sensitivity analyses are also performed with respect to the reservoir and geomechanical parameters,including the apparent tensile strength and cohesion of the oil sands,magnitude of the minimum and maximum principal stress,absolute permeability and elastic modulus of the oil sands and ramp-up time.All these analyses are performed to clarify the influences of these parameters on the fracturing response of the oil sands.展开更多
As a new technique in ground improvement, geosynthetic-encased columns (GECs) have promising applications in soft soil foundation. By assuming yielding occurs in the columns while the surrounding soil and the geosynth...As a new technique in ground improvement, geosynthetic-encased columns (GECs) have promising applications in soft soil foundation. By assuming yielding occurs in the columns while the surrounding soil and the geosynthetic remain elastic, an elastoplastic analytical procedure for foundations improved by GECs is proposed. The radial stresses that the geosynthetic provides and the elastoplastic deformations of the foundation resting on a rigid base are derived. A comparison with finite element analysis shows that the proposed method is effective and can provide a reasonable prediction of a GEC's deformation. Subsequent parametric analysis indicates that higher geosynthetic stiffness leads to better performance of the composite foundation. The optimum length of encasement is related to the load acting on the foundation and the permissible vertical and radial displacements of the column. Moreover, as the dilation angle of the column increases, the settlement decreases, especially under high loading. The influence of the encasement is more significant in soils with smaller elastic modulus.展开更多
This study numerically investigates the effect of material micro-and macro-parameters on the failure mechanisms and geometry of a wellbore breakout.The analysis of the borehole breakout is essential in addressing well...This study numerically investigates the effect of material micro-and macro-parameters on the failure mechanisms and geometry of a wellbore breakout.The analysis of the borehole breakout is essential in addressing wellbore stability,well completion,and sand production problems.The three-dimensional discrete element method(DEM)was used in the simulations.The numerical tool was used in numerical model simulations of drilling through sandstone in cubic samples at the laboratory scale subjected to pre-existing far-field stresses.Besides,a series of triaxial testing simulations were performed to relate the micromechanical parameters to the macromechanical material properties.The results showed that the geometry of the breakout is affected,among the material micro-parameters,by the particle contact modulus,bond normal and shear strengths,particle crushing strength,and particle size distribution.Further,it was found that the macro-parameters including Young’s modulus,friction and dilation angles,and uniaxial compression strength(UCS)also affect the type of breakout.展开更多
基金Funding for this project has been provided by BP Canada and the Nature Science and Engineering Research Council of Canada(NSERC)
文摘This paper presents a model to simulate the monotonic and cyclic behaviours of weakly cemented sands.An elastoplastic constitutive model within the framework of bounding surface plasticity theory is adopted to predict the mechanical behaviour of soft sandstone under monotonic and cyclic loadings. In this model, the loading surface always passes through the current stress state regardless of the type of loading. Destruction of the cementation bonds by plastic deformation in the model is considered as the primary mechanism responsible for the mechanical degradation of loosely cemented sands/weak rock.To model cyclic response, the unloading plastic and elastic moduli are formulated based on the loading/reloading plastic and elastic moduli. The proposed model was implemented in FLAC2D and evaluated against laboratory triaxial tests under monotonic and cyclic loadings, and the model results agreed well with the experimental observations. For cyclic tests, hysteresis loops are captured with reasonable accuracy.
基金the research funding for this study provided by NSERC through CRDPJ 387606-09
文摘In petroleum engineering, the transport phenomenon of proppants in a fracture caused by hydraulic fracturing is captured by hyperbolic partial differential equations(PDEs). The solution of this kind of PDEs may encounter smooth transitions, or there can be large gradients of the field variables. The numerical challenge posed in a shock situation is that high-order finite difference schemes lead to significant oscillations in the vicinity of shocks despite that such schemes result in higher accuracy in smooth regions. On the other hand, first-order methods provide monotonic solution convergences near the shocks,while giving poorer accuracy in the smooth regions.Accurate numerical simulation of such systems is a challenging task using conventional numerical methods. In this paper, we investigate several shock-capturing schemes.The competency of each scheme was tested against onedimensional benchmark problems as well as published numerical experiments. The numerical results have shown good performance of high-resolution finite volume methods in capturing shocks by resolving discontinuities while maintaining accuracy in the smooth regions. Thesemethods along with Godunov splitting are applied to model proppant transport in fractures. It is concluded that the proposed scheme produces non-oscillatory and accurate results in obtaining a solution for proppant transport problems.
基金The Natural Sciences and Engineering Research Council of Canada(Grant No.CRDPJ 387606-09).
文摘This paper presents a numerical investigation of hydraulic fracturing in oil sands during cold water injection by considering the aspects of both geomechanics and reservoir fluid flow.According to previous studies,the low shear strengths of unconsolidated or weakly consolidated sandstone reservoirs significantly influence the hydraulic fracturing process.Therefore,classical hydraulic fracture models cannot simulate the fracturing process in weak sandstone reservoirs.In the current numerical models,the direction of a tensile fracture is predetermined based on in situ stress conditions.Additionally,the potential transformation of a shear fracture into a tensile fracture and the potential reorientation of a tensile fracture owing to shear banding at the fracture tip have not yet been addressed in the literature.In this study,a smeared fracture technique is employed to simulate tensile and shear fractures in oil sands.The model used combines many important fracture features,which include the matrix flow,poroelasticity and plasticity modeling,saturation-dependent permeability,gradual degradation of the oil sands as a result of dilative shear deformation,and the tensile fracturing and shear failure that occur with the simultaneous enhancement of permeability.Furthermore,sensitivity analyses are also performed with respect to the reservoir and geomechanical parameters,including the apparent tensile strength and cohesion of the oil sands,magnitude of the minimum and maximum principal stress,absolute permeability and elastic modulus of the oil sands and ramp-up time.All these analyses are performed to clarify the influences of these parameters on the fracturing response of the oil sands.
基金Project (No. 2011FZA4021) supported by the Fundamental Research Funds for the Central Universities, China
文摘As a new technique in ground improvement, geosynthetic-encased columns (GECs) have promising applications in soft soil foundation. By assuming yielding occurs in the columns while the surrounding soil and the geosynthetic remain elastic, an elastoplastic analytical procedure for foundations improved by GECs is proposed. The radial stresses that the geosynthetic provides and the elastoplastic deformations of the foundation resting on a rigid base are derived. A comparison with finite element analysis shows that the proposed method is effective and can provide a reasonable prediction of a GEC's deformation. Subsequent parametric analysis indicates that higher geosynthetic stiffness leads to better performance of the composite foundation. The optimum length of encasement is related to the load acting on the foundation and the permissible vertical and radial displacements of the column. Moreover, as the dilation angle of the column increases, the settlement decreases, especially under high loading. The influence of the encasement is more significant in soils with smaller elastic modulus.
基金The authors would like to acknowledge the research funding for this study provided by NSERC through their Discovery Grants Program.We also thank BP for their technical advice,providing and permitting the publication of the laboratory data.
文摘This study numerically investigates the effect of material micro-and macro-parameters on the failure mechanisms and geometry of a wellbore breakout.The analysis of the borehole breakout is essential in addressing wellbore stability,well completion,and sand production problems.The three-dimensional discrete element method(DEM)was used in the simulations.The numerical tool was used in numerical model simulations of drilling through sandstone in cubic samples at the laboratory scale subjected to pre-existing far-field stresses.Besides,a series of triaxial testing simulations were performed to relate the micromechanical parameters to the macromechanical material properties.The results showed that the geometry of the breakout is affected,among the material micro-parameters,by the particle contact modulus,bond normal and shear strengths,particle crushing strength,and particle size distribution.Further,it was found that the macro-parameters including Young’s modulus,friction and dilation angles,and uniaxial compression strength(UCS)also affect the type of breakout.
