Mechanism Of Damage And Fracture Propagation In Shale Reservoirs Under Dynamic Hydraulic Pulsation

with the adjustment of energy structure and the green transformation of energy,shale gas,as an unconventional energy source,has become the representative of natural gas energy in China.The technically recoverable shale gas reserves are enormous,reaching 402.617 billion cubic meters by 2021.In recent years,research on theories,methods,key technologies,and equipment for shale gas development has become a hotspot,with the successful development of the Fuling gas fields.In this thesis,the mechanism of damage and fracture propagation in shale reservoirs under dynamic hydraulic pulsation is studied by theoretical analysis,apparatus development,and physical model experiments.The geological setting of the study is close to the lithology and geological characteristics of shale reservoirs in Niutitang formation,northwestern Hunan.The main contents and conclusion are as follows:(1)Firstly,four key factors affecting the fracability of shale are analyzed by testing the physical and mechanical parameters of shale samples in the middle and lower sections of the Niutitang Formation.Then,a six-index evaluation system for the engineering fractability of shale is established,and quantitative comparative analysis of the engineering fractability of target shale reservoirs is carried out.The results show that the engineering fractability coefficients of tight siliceous shale,bedding siliceous shale,carbonaceous shale in Lower Cambrian Niutitang formation,and Barnett shale in North America are 0.674,0.518,0.279,0.484–0.504,respectively.The fractability of the bedding siliceous shale is comparable to that of the Barnett shale,and the fractability of the tight siliceous shale is higher than that of the Barnett shale.In conclusion,siliceous shales have high fracturing potential.(2)A dynamic pressure pulsation output apparatus has been innovatively developed based on the generation mechanism of the waterhammer pulse.The apparatus can realize dynamic pressure pulsation output with adjustable amplitude(0-35 MPa),frequency(0-30 Hz)and duty cycle(0-100%),and the maximum pressure rise rate reaches 3500MPa/s.These properties enable the apparatus to achieve quasi-dynamic fluid pressure loading to simulate dynamic hydraulic fracturing of shale reservoirs.Then,the hydraulic fracture initiation and propagation criterion for the tight shale reservoir and the propagation criterion for the bedding shale reservoir under the action of pulsating fluid are deduced based on the theoretical analysis.(3)Firstly,a sample preparation apparatus and method for bedding shale-like materials considering the mesoscopic bedding structure of shale are developed.Then,to match the anisotropic characteristics of the natural bedding shale,the influences of the proportion of bedding material and bedding angle on the anisotropic characteristics of similar materials are analyzed.Finally,the "C+6" formulation is selected as an ideal material for simulating bedding shale,which can highly match with bedding siliceous shale in terms of structural properties,anisotropy coefficient,mechanical properties,and failure mode.(4)An innovative coupled static and dynamic hydraulic fracturing test system under the triaxial loading is developed to conduct coupled static and dynamic hydraulic fracturing physical model experiments considering the influence of dynamic pressure parameters.The evolution of characteristic parameters such as pressure,acoustic emission,and fracture propagation mode under the coupled static and dynamic hydraulic fracturing is observed.The action mechanism of dynamic pressure parameters on micro-crack propagation causing damage evolution of shale matrix around well on hydraulic macro-fracture morphology are discussed.The results show that during the dynamic injection process,the dynamic pressure parameters(pulsation amplitude,upper limit pressure)mainly affect the damage degree of the shale matrix through the size and number(frequency)of the microcracks stimulated during the pressure pulsation process.The damage degree of the shale matrix will further affect the subsequent static injection fracturing process.With the increase in pulsation amplitude and upper limit pressure,the cumulative damage to the shale matrix around the well caused by the coupled static and dynamic hydraulic fracturing shows a trend of first increasing and then decreasing,and there is an optimal dynamic pressure parameter.When the pulsation amplitude and upper limit pressure are 50% and 75% of the static initiation pressure,respectively,the optimal damage state is reached.Moreover,With the increase in pulsation amplitude and upper limit pressure,the development of micro-cracks in the shale matrix around the well presents a trend of "preferential propagation of axial section cracks–alternate propagation of axial and cross section cracks–preferential propagation of cross section cracks",which affects the subsequent static propagation patterns of hydraulic fractures.(5)Comparative true triaxial fracturing experiments of bedding shale with horizontal wells under the dynamic and static hydraulic injection are conducted.The response characteristics of pressure,acoustic emission,and propagation mode of hydraulic fractures under the influence of horizontal stress state and bedding dip angle are analyzed.The results show that hydraulic fractures are mainly dominated by the weak bedding plane in the static injection condition and exhibit progressive propagation along the bedding plane.During fracture initiation,the high energy release is lower than that of tight shale reservoirs but still significant.In contrast,when the dynamic injection is conducted,the low-amplitude dynamic pressure pulsation causes reservoir cracks to exhibit dense small-scale propagation,which can significantly enrich the secondary fractures network near the main hydraulic path.High-amplitude dynamic pressure pulsation could stimulate a large-scale fractures network along the weak bedding plane and the direction perpendicular to minimum principal stress in a short time.(6)Based on the fracturing laws of shale reservoirs,a variableamplitude coupled static and dynamic hydraulic fracturing method for tight shale reservoirs and a variable-amplitude dynamic fracturing method for bedding shale reservoirs are proposed.Moreover,the feasibility design of the dynamic fracturing equipment in the borehole is carried out.There are 116 pieces of graphs,29 pieces of sheets,and 195 pieces of reference documentations in this thesis.