Chunxiao Gas Fields Directional Shaft Wall Stability Study

With the development of petroleum industry, the technology of directional drilling was increasingly applied in oil drill engineering, therefore, it is important for us to research the stability of directional wells. Being different from straight wells, the stability of directional wells is different according to the changeable deviation and relative azimuth angle*. However, lots of computing formulae about strata collapse pressure and fracture pressure which concentrate on the analysis of the stability of straight wells do not take deviation, relative azimuth and other factors into account. Meanwhile, the large amount of the directional drilling tasks of the Chunxiao gas field group urgently require the good research about directional wellbore stability so that it can ensure the task of directional well drilling be completed successfully.In order to solve the directional wellbore stability problems, this thesis establishes the directional wellbore stability model, using the mechanical methods to analyze the stress distribution of sidewell. Being different from the regular collapse and fracture pressure calculating formulae, this model adopts the Mohr-Coulomb strength criteria to judge the stability of wellbore according to the stress analysis and rock strength parameters. This model not only takes rock strength parameters, in-situ stress status into consideration but also takes account of the borehole attitude, which makes the calculation more accurate. On the basis of this model, this thesis analyse the relationship between wellbore stability in different stress status and all sorts of parameters including deviation angle, relative azimuth angle, formation stress, pore pressure and rock mechanical parameters. And, according to the actual situation of Chunxiao gas field group, this thesis studied the rules and characteristics of directional well stability and give some positive proposals which is benefit to the optimized drilling design in the work area. At the same time, on the basis of the established model and findings of basic parameters in the work area, this thesis designed the program of safe mud calculation for the work area.During the formation pore pressures calculation research, this thesis deduced a modifier formula to predict the pore pressure by equivalent depth method according to the basic compaction theory and effective stress theory. The amended equivalent depth method takes influence of the Biot elasticity coefficient into account. Meanwhile, it deduces the formula of strata normal compaction trend line is ln(t - tma) = aH + b rather thanln( t) = aH + b. Through the computational analysis, the amended calculation model has the similiar result to the Eaton method while calculating the pore pressure. When applied to the work area, the calculating result is superior to the original equivalent depth method.On the calculation of the formation stress, this thesis combine fracture pressure test with log interpretation to calculate the formation stress including the value and thedirection precisely, so it provides the consistent formation stress with the safe mud calculation model. Has been always a difficult problem to calculate formation stress because laboratory experiment is expensive and it can only get the test data and the in-situ stress value of core samples. And, the method which calculate the formation stress through the elliptical wellbore shape may bring considerable error because the factors which affecting the wellbore shape including not only the in-situ stress but also the others. The method presented by this thesis calculate the formation stress with diplog data and scaled by the fracture pressure test possess the merit of simplicity, facility and consectary dataout. In the end, this thesis uses the logging data to figure out the lithomechanics parameters, formation stress, pore pressure and it establishes the lateral distribution rules of various parameters. According to the work area's designed directional well trajectory data, this thesis calculated the safe mud density of the planned...