Study On Rough Fracture Description And Fluid Flow In Rough Fracture

As the hydraulic fracture technology is widely used in low,ultra-low permeability reservoirs,and the research about unconventional resources,such as shale,tight sandstone reservoirs is becoming the research focus in oil and gas field development,fracture structure plays a more and more important role in reservoir development.So,it is greatly significant to understand the fluid migration in fracture structure for the development of low,ultralow permeability reservoirs and the unconventional resources such as shale,tight sandstone reservoirs.Therefore,theoretical derivation,physical experiment,and the numerical simulation method Lattice Boltzmann method are employed to investigate the fluid migration in tensile fracture,mismatch fracture based on Gaussion distribution,and then based on the simulation parameters,the approximate calculation formula is derivated based on the simulation parameters.The shale core samples from the oil field is split into artificial fractures by Brazilian test and the fracture morphology is acquired by 3D profilmeter,then the rough fracture surface characteristic parameter is calculated based on scanning principle.The validity of the characteristic parameter is ascertained by clustering method.The tensile fracture structure permeability computational formula is built,and the characteristic parameter influence to the permeability is calculated by the grey relational analysis method;To make sure the fluid migration rule in tensile fracture structure,the fluid seepage field is visualized,and the pressure and velocity distribution along various profiles along different direction is studied.Besides,the seepage field is discretized into small basic structure to analysis the flow characteristics;Aimed at the real structure has some mismatch length,the rough fracture height distribution is counted,and roughness,fractal dimension and mismatch length are employed to build the rough fracture model and various mismatch fracture structure is created.The mismatch comprehensive coefficient is introduced to study the mismatch length influence to fracture permeability.The orthogonal design and grey relational analysis method are employed to study various parameter influence to the mismatch fracture structure.For slipping fracture structure is important in volume fracture network,the mechanical model about the formation of slipping fracture is summarized.Then,the contact area is neglected,the V structure,sinusoid,groove,2D profile and 3D rough fracture structure are employed to study the slipping effect to fracture permeability separately,during the study the object length is constant.Based on the concept of fluid seepage,tortuosity calculation model is built.Then,3D rough fracture is simplified into 2D structure,the basic unit of fracture and equivalent plate model are introduced to verify the model accuracy.The result shows that: The sample tortuosity fluctuates near 1.10,the angularity is among 0.99°~8.86°,and the roughness is among 0.062~0.162mm;Tortuosity,roughness and angularity shouldn't be substituted by each other in calculating the permeability.The permeability of artificial fracture is 19%~29% less than that of parallel plate model,so the roughness must be considered;As is verified,the derived permeability calculation model satisfies the computational accuracy.Overall,the pressure distribution is fluctuated linear;Pressure distribution along the transverse serif is linear,but it is nonlinear along the longitudinal serif;The speed direction is disordered near partial area of the surface,and the recirculation zone,which is related to the roughness and its range of variation;The velocity is slightly different along the same streamline;When the flow is steady,it doesn't mean that the velocity is the same.The seepage velocity is various in different discretized unit,and its value is strongly related to the position;Because of 3D roughness of the fracture surface,the seepage velocity is 3D,mainly in the transport and vertical direction.The rough fracture permeability increases fluctuatedly and get constant as mismatch length increases.The mismatch fracture permeability is less than the match one's permeability.Aperture and roughness are the two main factors impacting the permeability,followed by fractal dimension,anisotropic coefficient,and mismatch length.The roughness distribution over fracture surface also greatly affects the fracture permeability.As slipping distance increases,the permeability of V structure,sinusoid and groove decrease greatly,then get constant gradually.The permeability of 2D profile structure shows the trend decreasing first,then increasing,but the overall trend is decreased.As the mismatch length increases,the permeability of 2D profile structure is little decreased;The permeability of 3D rough fracture has the decreasing trend.The tortuosity along seepage direction in the fracture is disordered because of slipping.The tortuosity formula,based on velocity distribution,satisfies computational accuracy,and it can be extended into 2D fracture tortuosity rapid calculation in numerical simulation.