Research And Application Of In-situ Stress Characteristics Of Deep Xujiahe Formation In West Sichuan Depression

Sichuan Basin is China's major oil and gas basins.Southwestern China is also an important production base of natural gas.There are gigantic thick terrestrial sediments about Upper Triassic in west Sichuan depression.From above to below there are many of main gas reservoirs.The gas reservoir,especially deep Xujiahe formation, has characteristics of tight stratum,deep bury,high pressure,complex trap,difficult exploration and different petrophysical properties to make the engineering geology character is complex.This paper take the petroleum geology,the structural geology, the geophysical well logging,the oil gas pool engineering and the multi-disciplinary theory and the method as the instruction,take the gas reservoir of Xu 2 member of Xinchang gas filed which is lie in middle of west Sichuan depression as research object,thoroughly analyzes the characteristics of west Sichuan area deep stratum rock mechanics,in-situ earth stress and which acts on hole stability applied research.This paper takes core laboratory results as calibrated scale,takes well logging calculating method as oriented,combines with field data,thoroughly to establish the rock mechanics parameters(elastic modulus,Poisson's ratio,compressive strength, tensile strength,cohesive force,internal frictional angle) forecasting model.To establish a vertical profiling of rock mechanics parameters and a 3D spatial distribution picture of plane's characteristics and we can establish initially a reliability and accuracy computational method of rock mechanics parameters and forecasting model.Finally it can provide a reliable basis data for the calculation of the in-situ stress and three reservoir pressures.To analyze the various types pressure test results about deep well in west Sichuan and we have carried out to determine the size and direction of in-situ stress in deep layer.On the basis of the fracturing data,well logging and the laboratory data for pressure state,we can use three-dimensional numerical simulation software which called FLAC3D to simulate three of main production formation' stress distribution, through the results analyze the distribution character.The research results show that there was extrusion stress generated by the napped structure which is lie in front of Long-men mountain,it makes the study area a strike-slip type,so the main stress state isσ_H/＞σ_v＞σ_h,and the direction of maximum horizontal principal is E-W.The stress's size distribution is controlled by fault distribution,burial depth and the size and direction of the tectonic stress.In view of the formation of abnormal high pressure in Upper Triassic strata in the western Sichuan foreland basin is the result of combined action which is composed of formation un-compaction,hydrocarbon cause step up pressure,clay mineral dehydration cause step up pressure and structure uplift.Therefore,the equivalent depth method which is a traditional method was unfit to calculate the abnormally high pressure in tight reservoir.For avoiding the complication of pressure origin,and considering that pressure index proposed by Eaton can reflect the relation between mechanism of pressure formation,conserving condition of abnormal pressure,and hydrodynamic force both sand and mudstone.On the basis of pressure tests results by drilling,this paper use Eaton method and effective stress method synthetically to establish formation porosity pressure sections of the study area.Furthermore,it is consider that geologic characteristic of the study area,then it refer to the mature computational modeling of collapse pressure and fracture pressure,finally we can establish the safe mud window.The application shows that the rock mechanics parameters,in-situ stress,three of main formation pressures and the safe mud window data satisfy the requirements for direct construction.