The Study Of Effect Of Cyclic Variable Loadinging Fracturing On Reactivation Of Natural Fracture

Different from the ideal homogeneous model,in the actual production process of petroleum engineering,oil and gas reservoirs generally contain a large number of natural defects,including natural fractures.The distribution of natural fractures will have different effects on the physical and mechanical properties of rocks.Combining the indoor large-scale true triaxial physical simulation experiment and finite element numerical simulation method,to study the activation characteristics of natural fractures under different variable loading fracturing conditions is of great significance to the fracturing transformation of reservoir.The research work of this thesis is as follows.(1)The indoor true triaxial hydraulic fracturing physical model experiment is carried out.Through the artificial specimen with natural fracture and FBG Optical Fiber Strain Sensor prefabricated inside,the hydraulic fracturing fluid injection curve and optical fiber strain curve of the specimen are measured,and the slipping characteristics of the natural fracture area in the process of variable loading fracturing are determined.At the same time,according to the tensile strength of the artificial specimen obtained from the experiment,it lays a foundation for the further study of the numerical simulation under variable loading fracturing.(2)Based on the finite element method,a numerical model containing natural fractures and hydraulic fractures is established to compare the stress distribution at the intersection of hydraulic fractures and natural fractures under different conditions of variable loading fracturing.At the same time,combined with the physical model experiment,it is concluded that the damage caused by high pressure variable loading fracturing makes the pore pressure at the tip of hydraulic fractures symmetrically distributed when they intersect with natural fractures,Under low pressure loading and linear loading,the pore pressure distribution is uneven due to the infiltration of fracturing fluid,which promotes the activation of natural fractures;At the same time,the fracture width will increase with the increase of the number of natural fractures and the decrease of ground stress difference.(3)By combining the complex characteristics of on-site geological reservoirs,a multi-layer mud sand interbedding model is established.In high pressure variable loading fracturing,hydraulic fractures can expand through interface fractures,while hydraulic fractures in linear loading and low pressure variable loading models expand along interface fractures.Increasing the peak value of circulating fluid injection pressure can strengthen the permeability of hydraulic fractures in high pressure variable loading fracturing,but with the increase of pressure,There is a risk of activating the interface fractures that have passed through.