Current In-situ Stress Of Gaoshangpu Reservoir,Nanpu Sag,Bohai Bay Basin,China

In this dissertation,the 3D rock mechanics field of Gaoshangpu reservoir in Nanpu sag is constructed by combining rock mechanics test,logging data calculation and seismic attributes,and its rock mechanics properties are defined.The orientation and magnitude of current in-situ stress of the single well are determined by borehole breakouts,wave velocity anisotropy,acoustic emission,differential strain method and hydraulic fracturing method.Taking the stress state of single well as constraints,current in-situ stress field in Gaoshangpu reservoir is predicted by using Petrel and Ansys combined modeling technology.The dynamic and static factors affecting in-situ stress are analyzed and discussed.The research results of in-situ stress are applied to predicting hydraulic fracture and structural fracture.Corresponding suggestions are put forward for hydraulic fracturing of Gaoshangpu reservoir,thus realizing the research and application of current in-situ stress of low permeability reservoir in complex fault-block.Through Brazilian test,uniaxial and triaxial compression test,triaxial compression test under different fluid conditions and different temperatures,the effects of confining pressure,buried depth,lithology,fluid and temperature on the mechanical properties of Gaoshangpu reservoir are clarified.The results show that the rock mechanics properties are quite different and have strong heterogeneity.The mineral composition,structure and cementation of rocks greatly determine the mechanical properties of rocks.The mechanical effect of fluid and temperature and the physical and chemical action result in the weakening of rock mechanical properties.The different mixing ratio of multiphase fluid causes the regular change of rock mechanical properties.At the temperature of about 100?,the main factor that causes the change of rock mechanical properties is the evacuation of different forms of water.On the basis of rock mechanics test,combined with logging calculation and seismic interval velocity attribute,the 3D rock mechanics field of Es₃²⁺³ in Gaoshangpu reservoir is constructed.Its elastic modulus is mainly between 10?50 GPa,Poisson's ratio is mainly between 0.2?0.27,rock density is mainly between 2.05?2.6g/cm³.The difference within each fault block and different fault blocks is obvious.These parameters are used as rock mechanics parameters for prediction of heterogeneous stress field in Gaoshangpu reservoir and assigned to each element of the finite element model.Constrained by in-situ stress measured by method such as borehole breakouts,wave velocity anisotropy,acoustic emission,differential strain and hydraulic fracturing,a precisely heterogeneous stress field was predicted with the help of Petrel and Ansys combined modeling technology.The results show that the orientation of maximum horizontal principal stress is generally NE?NEE trending and the range of variation is NE58?86°.The orientation of maximum horizontal principal stress in the same fault block is uniform and the variation is uniform.The difference is more obvious between different fault blocks.The magnitude of in-situ stress is generally lower in the west and higher in the east.The in-situ stress state changes twice at 480m and 1500m.In the target layer,the in-situ stress state conforms to S_V>S_Hmax>S_hmin and S_hminis compressive stress,which belongs to Ia type of in-situ stress state.There are obvious differences in the inter-strata stress,and five types of in-situ stress profiles are divided as"high–low–high","low–low–high","high–low–low","low and low","inter-strata"and"low–low–low".Based on the measured data of in-situ stress,logging calculation and numerical simulation results of in-situ stress field,and combined with theoretical calculation,the effects of static factors such as burial depth,lithology,fault,structural morphology,and dynamic factors such as fluid,temperature,hydraulic fracture on in-situ stress are analyzed quantitatively.The components of in-situ stress and stress coefficients have a good linear relationship with buried depth,the dispersion of in-situ stress decreases with the buried depth increased,while the orientation of in-situ stress is less affected by buried depth;The influence of lithology on in-situ stress is embodied in rock mechanics parameters.The relationship between horizontal principal stress and lithology is remarkable,but the relationship between vertical principal stress and lithology is not obvious.The abrupt change of stress value and the deflection of stress direction often occur at the lithologic interface.The difference of lithology is an important reason for the inhomogeneous distribution of in-situ stress magnitude and the change of stress orientation in fault blocks.Fault is the most important factor affecting the distribution of in-situ stress in Gaoshangpu reservoir,and its scale,shape,combination pattern,filling,dip angle and strike have significant influence on the distribution of in-situ stress orientation and magnitude.Fluid affects the in-situ stress field by changing the mechanical properties and pore pressure.In the process of water flooding,the pore pressure is increased and the mechanical properties of rock are weakened,which leads to the change of the stress state around wellbore.The influence of pore pressure on the stress around wellhead is far greater than changing of the mechanical properties of rock.Reservoir temperature mainly comes from geotemperature and water flooding,geotemperature affects the regional distribution of in-situ stress field,but the degree of influence is not high.Water flooding in wellbore not only causes the change of magnitude of in-situ stress,but also causes the deflection of stress orientation,but the influence range is limited.Hydraulic fracture and in-situ stress influence each other.The formation and propagation of hydraulic fracture are mainly controlled by the state of in-situ stress.It can also cause disturbance of local stress field around wellbore.The change of stress field determines the propagation mode and shape of repeated hydraulic fracture.Based on the definition of natural fracture distribution,and combined with numerical simulation and theoretical calculation,the influence of stress state and approximation angle around wellbore on fracture propagation mode is clarified.The results show that when the approaching angle is low(generally less than 45°),the hydraulic fracture is easy to be captured;when the approaching angle is medium(45?60°),the hydraulic fracture captured by the natural fracture is not only likely to shift from its tip to expand,but also has the possibility of directly penetrating the natural fracture;when the approaching angle is high(nearly 90°),the hydraulic fracture is easy to penetrate.It is a favorable condition to form complex network fractures by passing through natural fracture and turning at the double tips of natural fractures.Natural fractures in wellbore should not be more or less,too many natural fractures are prone to fracturing fluid leakage,too little is not conducive to the communication of fracture network.The greater the horizontal stress difference(or ratio)around the wellbore,the easier the hydraulic fracture is to directly penetrate the natural fracture,the more the extension direction tends to the maximum horizontal principal stress direction,and the farther the extension,the more straight the fracture surface,but it is not conducive to the formation of complex fracture network.The results of in-situ stress in Gaoshangpu reservoir are applied to hydraulic fracturing development and corresponding suggestions are put forward.It is considered that the R.Douglas formula considering pore pressure and fracturing fluid permeability is an effective method for calculating breakdown pressure of Gaoshangpu reservoir.According to the minimum principal stress of different horizon profiles in Gaoshanpu area,corresponding proppant types are recommended.Quartz sand,52 MPa ceramsite and 69 MPa ceramsite should be used as proppants at different depths.Because the existence of hydraulic fracture has changed the way of seepage in reservoir,the"five-point method"should be adopted to deploy well pattern considering the influence of natural fracture and maximum horizontal principal stress direction on water injection development,and the main injection-production line should be 45°staggered from the extension direction of hydraulic fracture.