Sand Shale Formation Sidewall Mechanical Stability And Software Prepared

Wellbore stability is one of the most important considerations in any drilling operations. At best, an unstable wellbore would mean that drilling performance is impeded through lost time. At worst, it would mean a hole collapse and total loss of a well. All this means extra costs and money-time losses. And wellbore instability also influences the perforation and sand production,ect in the reservoir development. So it is important to study wellbore instability.Wellbore instability includes mainly borehole collapse in brittle shale and borehole shrinkage in soft shale and borehole rupture in standstone. The main reasons that effect shale instability can be divided into mechanical and chemical aspects. However, only the mechanical effects are considered in this paper. Time dependent effects resulting from pore fluid migration are not considered either.In mechanical terms, when the well is drilled, the drilling fluid pressure substitutes the pressure that the formation provides, resulting from the stress re-distributing. If the redistributing stress exceeds the intension of the formation, wellbore instability will occure. The most important factors influencing wellbore instability are in-situ stresses, mud weight, formation pressure and rock properties.The logging information is gathered in or after drilling. The logging information at ervery time-interval indicates indirectly the change of pressure and rock strength around the wellbore. Meanwhile, the log data have high precision and include a lot of information about formation, and the cost of logging is lower relatively. So we can obtain the rock mechanical parameters, pore pressure and in-situ stresses througth log data. Then the mud weight window for the drilling can be predicted.In this paper, borehole instability mechanism is thoroughly studied, the methods by which the in-situ stresses, pore pressure and rock mechanical properties were obtained througth log data were described. The detailed work can be concluded as follows:(1) Mechanical reasones of borehole collapse and tensil failure were researched. With the linar elastic theory, the formula calculating the maximum drilling fluid density that causes formation tensile failure and the minimum drilling fluid density that would cause formation to produce initial shear fracture in vertical and directional borehole were derived. Three rock failure criterions have been evaluated and had drawn a conclusion that the predicted "safe" mud weight window changes with the failure cricterion selected.(2) Some model for caculating the in-situ stresses had been discussed. The changing trend of the "safe" mud weight window with the inclination and azimuth was alsoresearched, which would give some guidance to the drilling engineering. (3) Based on the theory of undercompaction, with the equivalent depth technique, theformation pressure was caculated through acoustic log data.(4) The way by which the useful information of the rock properties from log data were obtained had been studied and the effcts of the formation parameters on the "safe" mud weight window were also researched.(5) The software used to calculate "safe" mud weight window had been developed. Only a few parameters are needed , the software can calculate rock properties, formation pressure, in-situ stresses , collapse pressure and fracture pressure along the well bore.On the basis of theoretical research, the software was used to predict the "safe" mud weight window of two wells in DD gas field and in QQ oil field . Comparison between actual data related to wellbore instability and theoretical results was carried out. The results of study of this paper has important theory meaning and practical value.