In-situ Stress Characteristics Of Chang 7 Shale Oil Reservoir In Northern Shaanxi And Evaluation Of Fracture Turning Experiment After Plugging

The shale oil is enriched in Chang 7 reservoir of Yanchang Formation in northern Shaanxi of Ordos Basin,with huge potential for exploration and development.According to the characteristics of sand-shale-mudstone thin interlayers and strong heterogeneity in the Chang 7 reservoir,the regional in-situ stress field distribution and interlayer stress differences have an impact on the initiation,propagation,and secondary fracturing of volume fracturing fractures.Based on the interpretation of more than 600 conventional well logging big data,combined with fracturing experiments to optimize the rock mechanics parameter model of the Chang 7 member,a three-dimensional model of the regional stress field in northern Shaanxi was established,and the distribution law of the stress field was mastered.Combined with the indoor true triaxial hydraulic pressure Fracture experiments explore the ability of fracture plugging and diversion under different stress differences,and provide an experimental basis for the optimization design of secondary fracturing plugging.The main work of this thesis includes the following parts:(1)Based on the geological characteristics of key blocks in northern Shaanxi and rock mechanics parameter testing experiments,it can be seen that:in the geological characteristics,the Chang 7 Member is vertically stacked in multiple layers,with strong interlayer heterogeneity and significant differences in the characteristics of natural fractures,and the development of natural fractures is long.The Chang 7-2 section is the highest,followed by the Chang 7-3 section,and the Chang 7-1 section is the lowest;in terms of mechanical characteristics,the Chang 7-1 section has the largest elastic modulus,the smallest Poisson’s ratio,and the highest degree of brittleness,followed by the Chang7-2 section,and the Chang 7-3 section are less brittle.(2)Carried out the Kaiser acoustic emission in-situ stress experiment,then optimized the in-situ stress structural stress coefficient of the key wells 1,2,and 3 of the Chang 7 reservoirs,and interpreted the longitudinal in-situ stress profile of a single well,finally plotted the stress field distribution based on more than 600 wells.Among them,the three-dimensional in-situ stress distribution characteristics of the H326 well area in Xin’anbian,northern Shaanxi were deeply analyzed.The simulation showed that the three-dimensional stress in the H326 well area was larger in the surrounding area and smaller in the middle,and the relationship remained σ_v＞σ_H＞σ_h.The interlayer of the three-dimensional stress values is as follows:Chang 7-1 section>Chang 7-2section>Chang 7-3 section,and the horizontal stress difference is expressed as Chang 7-3 section>Chang 7-2 section>Chang 7-1 section,combined with deep structural analysis,the interlayer in-situ stress is significantly affected by lithology.(3)In view of the large difference in the turning and propagation of hydraulic fractures under different stress differences and fracture initiation angles,hydraulic fracturing plugging and turning experiments were designed and carried out to explore the influencing factors of new fracture initiation and turning laws after the original fractures were plugged.Experiments show that the crack initiation after plugging is affected by the induced stress of the original fracture.When the preset angle is 45°,60°,and 90°,the larger the crack initiation angle is,the more significant the turning effect is;when the horizontal stress difference is low,the fractures are prone to divert and communicate with nearby natural weak surfaces to form complex fractures.(4)The 15 key Geology and Engineering parameters affecting the effect of secondary fracturing of Chang 7 Member were optimized,and a grey correlation method was established to evaluate the optimal model of reservoir sweet spot,which was beneficial to optimizing reservoir fracturing.The results show that in the mechanical parameters,the horizontal in-situ stress difference is the main controlling factor affecting the production of the Chang 7 shale reservoir in northern Shaanxi.The field fracturing should give priority to the well area with a lower horizontal stress difference,which is conducive to the realization of secondary Fracturing creates high-efficiency fractures.