Inversion Of Formation Stresses And The Stability Of Borehole

With the increasing of depth of petroleum exploration and the appearance of various advanced drilling technique, borehole stability has attracted a lot of attention. To achieve a fast and safe drilling and to weaken the pollution to reservoir, the fundamental base is to make accurate analysis for the stability of borehole. There are four factors, the formation stresses, the nature of formation, the geometry of well and the drilling mud density, which have major influence on the borehole stability. Getting the exact data of in-situ stress would lay the foundation for borehole stability, so the study of the inversion of formation stresses and the stability of borehole is of vital practical significance.In this paper,based upon theory of elasticity,the stresses field of sidewall were analyzed. The serviceability of some classical theoretical models and two revision models for borehole stability were discussed. Both the models for formation stresses calculation with tectonic stress and without tectonic stress were compared. By using the model for formation stresses calculation with tectonic stress, the new method for the inversion of formation stresses were studied. Considering the fact that the data of hydraulic fracturing method is not easy to obtain, we proposed two methods for the inversion of formation stresses, one of them is based on collapse data and another is combined collapse data with hydraulic fracturing data. By using the formation stresses obtained from inversion, we given the method for the calculation of collapse pressure and fracture pressure and finally the mud density windows for safe drilling.Based on the formation stresses distribution characteristics and it's affected factors, by considering varies failure theories and inversion method of formation stresses, a software for the calculation of formation stresses and mud density windows is developed. This software has the advantage that it can deal with all the log data and available results to determine parameters required for formation stress calculation. It can also give the stress profile along the depth.Since data of collapse can be easily obtained, two new inversion methods of formation stresses we proposed and the relevant software may be valuable in the design of drilling mud density. It may also provide reference for the guidance of safe drilling and decrease the occurrence of accident.