Continental Facies Shale Formation Rock Mechanical Characteristics And Stress Field Evaluation Technique

Xinchang Xu₅ layer of Xujiahe Formation in the western Sichuan Depression was a main shale layer with thin-sandstone interbedded,which had diversity rock types and fractures development.And the distribution characteristics of the in-situ stress field was complex in Xu₅ layer.Fracturing development is the main development mode of shale gas reservoir,and the design of fracture construction scheme can't be proceeded without the distribution of the in-situ stress field.In this paper,based on rock mechanics,geostatistics,petroleum geology,geophysical log,geomechanics and reservoir engineering,the study are carried out respectively on the identification of lithology and fractures,rock mechanical characteristics,equivalent rock mechanics parameter evaluation of the rock with fracture,characteristics of in-situ stress and well logging interpretation and numerical simulation of the in-situ stress field with fracture-bearing non-homogeneous rock medium.The main points in this paper are as follow:?1?By conventional logging response characteristics of formation lithology and fractures in the Xu₅ layer,quadratic discriminant analysis method are used for lithology identification.On the basis of lithologic identification,the discriminant equations of fractures with different lithology are built by the use of conventional logging parameters as AC,DEN,RD and ACL,which have obvious fractures response characteristics.This work laid the foundation for geologic model.?2?The research of rock mechanics parameter characteristic in Xu5 layer is carried out.The basic mechanical properties of sandstone and mudstone were made clear by mechanics experiment.The results of rock acoustic testing showed that the factors affecting the velocity of P-wave and S-wave mainly included lithology and gas-bearing properties.The LM-BP neural network is trained to calculate the S-wave velocity value under different conditions as lithology and gas bearing properties.Based on the results of mechanics–acoustics synchronization test,the transformation relation between the dynamic and static parameters are established.And logging interpretation profile of rock mechanics parameters had been acquired at last.According to the strain energy theory,the equivalent elastic parameter computing method of fracture-bearing rocks were established under two-dimension condition.This work effectively solved the problem about the fracture-bearing rock mechanics evaluation.?3?The magnitude and direction of the stress in the single well in the research area are obtained respectively by the way of acoustic emission test,differential strain test,fracturing test data,anisotropy test and paleomagnetism core orientation test data.The in-situ stress interpretation profile of 24 wells,which were calibrated by using the existing single well stress test data,were calculated by ADS model.The stress profile would be used to verify the simulation of in-situ stress field in Xu₅ layer.?4?Finite element numerical modeling of the in-situ stress field characteristics in Xu₅ layer was carried out,which was based on the study of lithology,fractures,rock mechanics parameters and stress characteristics.According to tectonic characteristics,sedimentary characteristics and the distribution of lithology and fractures in Xu₅ layer,the heterogeneous geological model with fractures is established.Based on the test results of rock mechanics,the Duncan-Chang elastic-plastic constitutive model was determined in in-situ stress field simulation.The mechanical model is established by use of finite element method.At last,the accurate distribution model of stress field in the research area were obtained through well stress test results,which were used to calibrate the numerical modeling results.?5?The in-situ stress distributions in Xu₅ layer were influenced by lithology,fracture zone and faults on the basis of the results of stress simulation.The in-situ stress states in Xu₅ layer were mainly?_x>?_z>?_y,and the fracturing effect in?_x>?_z>?_y state was better than?_z>?_x>?_y,which were based on the analysis about the relationship between in-situ stress and rupture mode of hydraulic fracture,prediction of the hydraulic fracture direction and longitudinal extension of hydraulic fracture.Finally,the change of hydraulic fracture direction had been predicted under the condition of distribution of horizontal maximum/minimum principal compressive stress.In this paper,a set of comprehensive in-situ stress simulation and evaluation research methods,which focused on a main shale layer with thin-sandstone interbedded,had been launched from lithology,fractures and fault.The results of this paper provide a strong technical support for the fracturing technics design of Xu₅layer of Xujiahe formation shale gas stratum.The research method and technical means of the paper have reference significance for the same type of gas reservoir research.