| Energy is the lifeblood of economic development.With the increasing population,the economy wants to maintain high-speed growth,and the consumption of energy will increase year by year.Since the 21 st century,China's demand for oil and gas energy has been increasing,but the annual output of crude oil in China has basically maintained at about 200 million tons,which makes it difficult to guarantee domestic demand.As a result,the external dependence of crude oil has increased year by year,which not only restricts China's economic development,but also has affected China's energy strategic security.Therefore,the research and development of unconventional energy is of great significance to ensure national energy security and sustained and high-speed economic development.Oil shale,as an unconventional energy source,has attracted wide attention all over the world.The reserves of oil shale resources are huge.The proven reserves of oil shale resources in China alone are 7199.37 billion tons,of which the reserves of oil shale in Jilin Province are 2527.64 billion tons;the reserves of oil shale resources in China are 476.44 billion tons;and the reserves of oil shale oil in Jilin Province are 51.78 billion tons.Therefore,speeding up the exploration and development of oil shale in China and realizing the industrialized production of oil shale oil can make up for the shortage of conventional energy production in China,readjust the energy consumption structure and guarantee the national energy security.Compared with oil shale surface distillation technology,oil shale in-situ mining technology is more concerned by scholars all over the world because of its small area and less environmental impact.However,the low thermal conductivity and permeability of natural oil shale limit the efficiency of in-situ heating of oil shale,reduce the efficiency of in-situ production technology of oil shale,and increase the cost.In order to solve this problem,hydraulic fracturing is needed to create fractures in oil shale formation,so as to improve the permeability and heat transfer efficiency of oil shale formation.In this paper,hydraulic fracturing technology in oil shale in situ transformation technology is taken as the research background,and the law of hydraulic fracture initiation and propagation in vertical wells in oil shale formation is studied.This paper mainly analyses the law of fracturing initiation and propagation of oil shale hydraulic fracturing from three aspects: theoretical analysis,numerical simulation and hydraulic fracturing laboratory simulation test.According to the research results,a fracturing test scheme is designed.Vertical well fracturing construction is carried out in Fuyu oil shale in-situ transformation pilot test base of Jilin University,which verifies that the dualwell fracturing scheme can meet in-situ production of oil shale.Process requirements.In this paper,elasticity theory and mechanics superposition principle are used to analyze the law of initiation and propagation of hydraulic fracturing cracks in vertical wells of oil shale.Three modes of initiation of hydraulic fracturing in oil shale formation are put forward when natural fractures exist: initiation from rock body,tension and shear along natural fractures,and various criteria of three modes of initiation are analyzed and summarized.The criterion of how hydraulic cracks expand when they encounter natural cracks in the process of propagation.In the aspect of numerical simulation,the ABAQUS finite element software cohesive element is used to simulate the fracture,and a two-dimensional hydraulic fracturing model is established to analyze the law of hydraulic fracture initiation and propagation when the angle between the direction of perforation and the direction of maximum horizontal principal stress exists.The effects of synchronous and time-sharing fracturing on the initiation and propagation of hydraulic fractures are analyzed.The results show that the increase of perforation direction will lead to the increase of the initiation pressure and width of hydraulic fracture,but the length of fracture will be shortened,and the fracture will gradually deflect to the direction parallel to the maximum horizontal principal stress after initiation;the choice of fracture propagation path will be affected by the horizontal geostress difference after intersection of hydraulic fracture and natural fracture;the two-well fracturing is easier to form interconnection between wells.Through the main fracture,and dual well time-sharing fracturing can reduce the fracture initiation pressure of post-fracturing wells,and the degree of influence will decrease with the increase of the distance between wells.In terms of indoor physical simulation test,a real triaxial hydraulic fracturing physical simulation test system developed by Jilin University was used to conduct vertical well hydraulic fracturing simulation test on oil shale samples in Wangqing area.Oil shale samples were cut by wire cutting machine,and the samples were put into moulds to make concrete samples.Vertical in-situ stress,maximum and minimum horizontal stress were determined by controlling variables.The effects of in-situ stress and injection displacement of fracturing fluid on the initiation and propagation of hydraulic fracturing cracks are experimentally analyzed.The research shows that with the increase of vertical in-situ stress,maximum and minimum horizontal stress and fracturing fluid displacement,the initiation pressure of hydraulic fracture will increase,and the effect of vertical in-situ stress on the initiation pressure of fracture is greater than that of normal in-situ stress.Hydraulic fracture will initiate and expand preferentially in horizontal bedding direction,but if the difference between vertical in-situ stress and maximum horizontal in-situ stress is not large,the initiation pressure of hydraulic fracture will increase.Fractures are prone to deflect under the influence of natural fractures;when large displacement fracturing is used,reservoirs are more likely to form a similar effect to volume fracturing.According to theoretical analysis,numerical simulation and indoor physical test,the related laws of vertical hydraulic fracturing crack initiation and expansion of oil shale are obtained.Double-well fracturing scheme is adopted in Fuyu oil shale in situ production site.Fracturing construction is carried out in Fuyu oil shale site.During the fracturing process,micro-seismic monitoring method is used to monitor and analyze the fracturing situation,and every time.After fracturing,the fracturing effect of oil shale is observed according to the results of microseismic monitoring imaging,which provides guidance for the next fracturing.The above theoretical analysis,numerical simulation and indoor physical test results provide theoretical guidance for hydraulic fracturing in Fuyu oil shale in-situ production,and the actual fracturing operation results verify the correctness of the theory.The research results in this paper can provide reference and guidance for similar drilling hydraulic fracturing construction. |
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