Research On Deep In-situ Stress Field And Wellbore Stability

The accurate evaluation of deep in-situ stress is the base of the safety,high efficiency of underground engineering.In the field of petroleum engineering,the evaluation of the in-situ stress is very important for development the theory of exploration and exploitation and raise economic benefits.In this paper,taken TaHe oil field for instance,with oil and gas exploration and exploitation data,basing on the analyses of the geological structure,deep geological environment(formation pore pressure,formation temperature),structural plane and the mechanical properties of deep rock,deep in-situ stress field was studied systematically and the wellbore satiability of oil well was analyzed,the main results are as follows:(1)Utilizing core identification and image logging analyses,the growth characteristic of fracture was been evaluated.The result indicated that:for the most fracture,the orientation is 65°～75°and 275°～295°,and the dip is high;The fracture development is mainly controlled by structure axis,structure high spot and faults.In addition,the lithology and depth of formation have significant effect on the fracture development.(2)On the base of analyses P-wave velocity modeling,formation bulk density is been considered.The Logging Integrated Evaluation Model for formation pressure had been improvement.Combined to measure data,the pore pressure in TaHe oil field was been evaluated by this model.The result shows that the formation pressure coefficient is about 0.80～1.35,belong to normal pressure system and become large with the depth increasing.(3)In the TaHe oil field shows that the geothermal gradient is about 0.0191～0.0273℃/m,Slightly higher than the average geothermal gradient of the Tarim basin,and lower than China's other large and sedimentary basin,is lower geothermal system.(4)Through the analyses heat exchange mechanism along the formation,drill sting liquid,annular liquid during the drill circulation,basing on the energy balance and the heat transfer theory,the wellbore temperature field model has been set up.The borehole temperature distribution has been analyzed during drill circulation by using this model.The analysis result indicated that there is a depth where the formation temperature of the borehole and the formation temperature is the same.Under this depth,the borehole temperature is lower than the original formation temperature,and it is opposite upside this depth.At the well bottom,the welbore temperature reduces the scope to be biggest.The impact of circulation time,geothermal gradient,the stratum heat conductivity as well as the well depth over the wellbore temperature distribution and the maximal perturbation temperature has been studied.(5)Using the rock mechanics test and the petroleum engineering logging analysis, the deep rock deformation and failure characteristics has been studied under the condition of the different pressure and temperature.Further,basing on logging analysis,the spatial distribution of rock mechanics parameter has been studied.(6)In the region stress analysis's foundation,the deep stress direction has been carried out studies systematically,through the analysis of the real-time detection of artificial fracture,drill-induced fracture and wellbore breakouts.The studied results shows that in the research region,modern stress field is controlled mainly by regional tectonic stress,and the maximum principal stress directions is NE40°～NE65°.(7)The Horizontal biggest principal stress inversion computation model has been derived under the different well completion condition.Using this model,fracturing data for stimulation has been analyzed in the 15 wells,the result show that the magnitude of the maximum principal stress is 108.0～129.0Mpa and the magnitude of the minimum principal stress is 77.0～107.0Mpa where the depth is 5440～6100m. The order of three principal stress magnitudes isσ_v≥σ_H≥σ_h·(8)The basic theory has been discussed that utilizing the finite element method with artificial neural network to inverse deep stress field.And the research thinking and technique process has been established.Using this theory and way,numerical simulation inversion analysis of the 2D and 3D stress field in the research region was been done in different scale.(9)The mechanical instability of deep wellbore was analyzed.(a)The common mechanical mode of wellbore instability was been summed up systematically,and the relation between mechanical mode of wellbore instability and the density of drilling fluid,the wellbore azimuth has been analyzed.(b)The quantity evaluation index of wellbore mechanical instability has been established.It was analyzed that In-situ stress state,formation temperature perturbation,formation pressure,week plane and well trajectory affect on wellbore instability using the index.(c)Existing evaluation method and calculation model of drilling fluid density that maintains wellbore stability has been improvement,the result is more reasonable and reliable.(10)Primary factor that induce wellbore instability has been analyzed.And the drilling fluid density,which maintains wellbore mechanics stability,was been calculated.It provides the important reference for drilling engineering design.