Characteristics Of Present-day In-situ Stress And Fracture Effectiveness In Chang 7 Shale Oil Resources Of Huaqing Region

With the development of oil and gas exploration,unconventional oil and gas has become an important part of global energy,which is changing the world energy pattern.Ordos Basin is rich in unconventional oil and gas.There are extensive shale oil resources in Chang 7 oil-bearing layer of Yanchang Formation in the basin,which has great exploration and development potential.The present-day in-situ stress state plays an important role in the development of shale oil.As an important reservoir space and seepage channel of shale oil,the effectiveness of natural fracture system is of great significance to the efficient development of shale oil under the influence of present-day in-situ stress state.In order to solve the above problems,based on the systematic analysis of imaging data in the Chang 7 oil-bearing layer of Ordos Basin,this paper reveals the orientation and influencing of in-situ stress.The rock mechanics parameters are calculated by conventional logs data.The spatial distribution of in-situ stress is obtained by choosing the suitable in-situ stress calculation model,and on this basis,the three-dimensional in-situ stress model of the target layer is constructed.Then the factors that affect the development of the in-situ stress are analyzed.Finally,the development characteristics of natural fractures in Chang 7 oil-bearing layer are identified based on imaging data,and the effectiveness of natural fractures in shale oil reservoir under the influence of present-day in-situ stress is evaluated.The results show that:1)The in-situ stress orientation is ENE-WSW or near E-W in the Yanchang Formation Chang 7 oil-bearing layer of Huaqing region.Due to the influence of lithology,bedding and natural fractures and other factors,there are some differences in different areas.The orientation of in-situ stress is affected by lithology,bedding,natural fractures and other factors,resulting in some differences in different areas.2)The distribution range of dynamic elastic modulus of shale oil reservoir in the study area is 16.56～86.11 GPa,and the distribution range of dynamic Poisson’s ratio is0.11～0.34.The distribution range of static elastic modulus in the study area is19.87～34.42 Gpa,and the distribution range of static Poisson ratio is 0.07～0.47.The distribution of rock mechanical parameters in Chang 7₁ and Chang 7₂ sub-oil-bearing layer is relatively concentrated,while that in Chang 7₃ sub-oil-bearing layer is relatively scattered.3)The horizontal maximum principal stressσ_H of Chang 7 oil-bearing layer in the study area is 26.52～64.29 MPa.The horizontal minimum principal stressσ_h is17.19～60.36 MPa,and the vertical principal stressσ_v is 37.33～52.09 MPa.The present-day in-situ stress in the Chang 7 oil-bearing layer presents a normal faulting stress regime ofσ_v≥σ_H≥σ_h.The present-day distribution of in-situ stress is obviously heterogeneous.Laterally,the horizontal stress varies greatly in different areas,but it is not obvious in vertical stress.Compared with the central region,the present-day in-situ stress in the marginal region of Huaqing region is higher.Vertically,the in-situ stress of Chang 7₃ sub-oil-bearing layer is higher than that of Chang 7₁ and Chang 7₂ sub-oil-bearing layer.The distribution characteristics of the present-day in-situ stress are influenced by the burial depth,lithology,mechanical property,natural fractures and other factors.4)The fractures in the study area are mainly high-angle fractures with some inclined fractures.The fractures strike is near E-W direction.The number of fractures in Chang 7₁ and Chang 7₂ sub-oil-bearing layer are more than that in Chang7₃ sub-oil-bearing layer.5)In the study area,the fractures whose strike is close to theσ_H direction and are more likely to become effective fractures.The effective critical pressure increment of this change of fracture is about 17.4～27.7 k Pa/m.In this paper,there are 51 figures,8 tables and 127 references.