Study On Mechanics Analysis Method For Stability Of Shaft Wall In Karst Region

The Keshen gas field in Tarim Basin is high and steep formation, lithology complex, strong stress, poor physical properties, obvious anisotropy, micro fracture reservoir. In addition, the deep exploratory wells with high temperature, high formation pressure, the presence of multiple pressure system in the process of drilling, borehole collapse and leakage is serious (Paleogene strata is easy to shrinkage and collapse, the Cretaceous strata are easy to leak). For some new drilling wells, due to unclear the borehole instability mechanism and inappropriate treatment, the speed of drilling is downing and the cost is increasing, most serious thing is unable to reach the target layer.This paper makes full use of geology, drilling, well logging, the measured geostress data and so on, in the view of rock mechanics, focusing on the mechanism of borehole instability analysis of Paleogene and Cretaceous formation. The research work and results are as follows:(1) Dipole acoustic logging data has more information of the rock mechanics, firstly extracting the S-P slowness and azimuth information by using STC method (the azimuth of fast shear wave is the azimuth of stress direction of SH1). Based on the in-situ experiment, fine logging interpretation of rock mechanics parameters, build the rock mechanics parameter profile which provides the basic data for calculating stress and three formation pressure and wellbore stability analysis.(2) According to the characteristics of abnormal high pressure of objective layer, using a modified Eaton method to calculate the high steep structure abnormal high pressure, small error. Through the study, with the increase of the maximum horizontal principal stress direction and formation tendency, the maximum horizontal principal stress decreases, the minimum principal stress increases. With the increase of formation dip, the maximum, minimum principal stress first increases and then decreases. Considering the strata sloping has significant effect on stress, build the calculation model of high steep structure formation stress, which gives the reliable data of calculating collapse pressure, fracture pressure and construct the wellbore stability discriminant chart.(3) When the borehole is drilled, the in-situ stress balance is destroyed, when the wall stress exceeds the rock shear strength, shear failure, manifested as wellbore collapse or shrinkage hole, when the stress exceeds the rock tensile strength, tensile failure, drilling fluid leakage. Considering the significant effect of rock strength, fracture, horizontal stress ratio on wellbore stability, lead into the shear strength, coefficient of permeability increase, SH1/SH2, build discriminant chart of borehole wall stability for Paleogene and Cretaceous stratum.(4) According to the characteristics of Paleogene and Cretaceous stratum, buried deep, complex lithology, characteristics of plastic and brittle failure, commonly used linear M-C criterion is no longer applicable, establish the collapse pressure calculation model of Paleogene and Cretaceous strata with parabolic and hyperbolic Mohr criterion. at the same time, according to the characteristics of the three formation pressure profile, the Paleogene and Cretaceous strata of the safe mud density window is established (5) Based on the logging interpretation platform of Forward.net, optimize the model develop the analysis of wellbore stability program-WBSA(Wellbore Stability Analysis), for wellbore stability analysis on the well been drilled and drilling.Through the systematic research, has formed a set of suitable evaluation methods and techniques of wellbore stability for Keshen region of Tarim Basin. From the application situation of5wells, good effect, it has an important effect on preventing wellbore instability and guiding drilling security, efficiently in high and steep structures developed for new drilling wells.