Research On Deep Shale Hydraulic Fracturing Experiments And Complexity Evaluation Model

Natural gas,with its advantages of safety and reliability,cleanliness and environmental protection,is currently the focus of new energy development in the world.The 20th Party Congress emphasized the need to control fossil energy consumption and actively promote carbon peaking and carbon neutrality.Shale gas is a kind of unconventional natural gas in mud shale,and its development and utilization is of great significance for China’s energy security and long-term stable economic development.Since the exploration and development of shale gas,China has made remarkable achievements in the field of shallow shale gas,and the development focus is tilted to deep shale gas.However,deep shale gas reservoirs are characterized by high temperature,high pressure and high fracture pressure,and the understanding of the mechanism and law of reservoir fracture expansion is still unclear,leading to high development cost and unsatisfactory fracturing effect.Therefore,it is important to master the fracture expansion law of deep shale hydraulic fracturing and explore the mechanism of fracture network formation to promote deep shale gas development.This paper takes Longmaxi Formation shale as the research object,carries out reservoir geological characteristics testing and hydraulic fracturing simulation experiments;based on acoustic emission monitoring,core profiling and fracture three-dimensional reconstruction technology,inversion obtains the change characteristics of fracture expansion during fracturing;comprehensive analysis of fracture expansion mechanism and influencing factors,establishes a suitable evaluation method for the complexity of hydraulic fracture network in deep shale,and proposes a secondary fracturing.The main contents and conclusions are as follows:(1)Geological characteristics testing of shale reservoirs: The mineral components of outcrop shale and downhole cores of the Longmaxi Formation were tested by X-ray diffraction analysis;the physical and mechanical properties of outcrop shale and downhole cores were tested by triaxial compression test and Brazilian splitting test;the ground stress distribution of deep shale reservoirs was tested by ground stress test test.(2)Large-scale hydraulic fracturing test: The experimental parameters of the real triaxial hydraulic fracturing experiment were designed according to the similar theory;the acoustic signals generated by the fracture expansion were monitored in real time by the acoustic emission monitoring system;the three-dimensional fracture patterns were reproduced by core sectioning and fracture three-dimensional reconstruction;and the hydraulic fracture characteristics were analyzed by integrating the fracture expansion patterns,pump pressuredisplacement-time curves and acoustic emission localization results.The results show that the experiments have formed various forms of fracture patterns,and the "T" shaped fracture is a relatively large number of fracture patterns in complex fractures;the acoustic emission localization results match with the actual fractures,and the localization accuracy is high,which is an effective method for real-time monitoring of fracture expansion and evolution.(3)Evaluation method of hydraulic fracture network complexity: Based on fractal theory and topology theory,the fractal dimension and topology of each group of fractures in hydraulic fracturing experiments were calculated respectively,and the two methods were improved and supplemented;the intersection length of intersecting fracture surfaces was proposed as one of the features for evaluating the complexity of seam networks,and a method for evaluating the complexity of deep shale hydraulic fracture seam networks based on a three-dimensional spatial system was established.(4)Hydraulic fracture expansion mechanism and influencing factors: Based on the fracturing experiment results,acoustic monitoring results and the complexity of the fracture network,the influencing factors of hydraulic fracture expansion pattern and fracture morphology were analyzed.The results show that the presence of developed natural fractures is a prerequisite for the formation of a complex fracture network;the hydraulic fractures produced by fracturing deep shale reservoirs in the Longmaxi Formation tend to turn and expand along horizontal natural fractures;the use of high-viscosity fracturing fluid and increased discharge volume can effectively maintain the net pressure in the fractures and increase the longitudinal fracture height;both stepped discharge volume and multi-phase injection are effective means to increase the complexity of fractures.(5)Deep shale secondary fracturing process: The first injection of high-displacement fracturing fluid increases the vertical fracture height and changes the local stress field and reduces the horizontal ground stress difference;the second injection adopts medium to high displacement to open the horizontal laminae and activate more natural fractures,thus communicating more reservoirs and increasing production.The process of hydraulic fracturing to transform the reservoir is a process of rock fracture,and many factors jointly affect the fracturing results.The research in this paper grasps the fracture expansion law of deep shale hydraulic fracturing,clarifies the influence of different factors on shale fracture,and proposes a proven method to evaluate the complexity of fracture seam network.This study provides support for the formation of complex fracture networks in deep shale hydraulic fracturing,and provides a basis for the parameters and process methods of ground equipment.