Prediction Of Fracture Of Low Permeability Reservoir Of The Sha-3 Member In The 5th Block In Bonan Oilfield

On the basis of the core fracture observation,the analysis of seismic data and logging data,we obtained the original data of the fractures.The status of ancient and current in-situ stress field was simulated by the means of finite element numerical simulation.On the guide of ‘the fractures is made by the ancient in-situ stress,changed by the current in-situ stress',the distribution of the fractures in the 5th block in Bonan Oilfield was calculated based on the quantitative relationship between in-situ stress and fractures.Based on the observation of the core and thin section analysis in the 5th block in Bonan Oilfield,it is found that the vertical and high angle fractures are developed well,and only a small amount of shear cracks are developed,the scale and intensity of macroscopic cracks are small and the microcracks are wildly developed.According to the the core measurement and the result of the paleomagnetism method,the advantage of the fractures in the 5th block of Bonan was NE-SW,and the fracture of NNW-SSE was developed slightly.It is concluded that the Es1 is the main period of the formation of cracks in the 5th block of Bonan based on the previous studies and the fault rate analysis.The fractures have poor permeability,about 53% of the cracks are fully filled,about 27% of the cracks are semi-full filled or semi-filled,most of the fills are muddy,accounting for about 54%,followed by calcium,accounting for about 23%.The magnitude of the present geostress is determined by the differential strain experiment and the acoustic emission experiment.The direction of the present geostress is determined by the fracturing monitoring data and the acoustic logging data.It is found that the in-situ stress in the 5th block of the Bonan has the characteristics of east-west zonation,the NE-SW is in the east and NW-SE in the west.Based on the top surface structure of the sand layer in the third member of the Shahejie Formation in the Bonan five areas,the geological model was established by using ANSYS15.0.The rock mechanics parameters which were measured by the three-axis mechanics experiment and the logging data were loaded to the geological model after static and dynamic calibration.The numerical simulation of the geostress field is carried out under the constraint condition of the measured geostress field and the characteristics of east-west zonation.The simulation results are in good agreement with the experimental results.The direction of the ancient ground stress field is determined by the regional tectonic setting and the detection results of natural fractures,the size of the paleo-stress field is determined according to the deformation characteristics of the strata and the degree of rock fracture.The numerical simulation of the paleo-stress field can be carried out after the size and direction of it is determined.The simulation results show that the present geostress and the depth of burial are generally positive correlation,the fault has a relatively high value,the small faults in the middle of the study area have a significant impact for the ground stress distribution;ancient horizontal stress and buried depth is negatively correlated,and the fault has a low value.Due to the small stress,the lithologic parameters have a great influence on the in-situ stress,and the vertical stress increases with the increase of the buried depth.According to the relationship between ground stress and fracture,the fracture's parameters in the Bonan five areas are calculated.The results show that the microcracks in the Bonan five areas are more developed with a line density of 37~55/m.The fracture porosity is concentrated in the range of 0.0073%~0.013%,and the distribution is uniform.The distribution of fracture porosity of the faults is between 0.015% and 0.022%,and increases with increasing depth,and is higher than the fracture porosity of the nearby strata obviously.The permeability of the fracture is small,generally between 0~0.04 m D,and decreases with increasing depth.The permeability of the fault is greatly improved due to the fracture of the rock,and the distribution range is 0.09 ~ 0.23 m D.in this paper,we have a conclusion that the sedimentary facies and rock components of the reservoir are the internal factors of the fracture development,and the regional tectonic stress field is the external main controlling factor of the fracture development,and the secondary cracks can be reconstructed later.