The development of a new model to predict rock strength in-situ conditions is presented in this paper. This model was obtained using experimental triaxial compression data for limestones, sandstones and shales under confining pressure conditions up to 16,000 psi. A non-linear Mohr failure envelope is considered for rock strength under triaxial pressure conditions. Results of wellbore stability analysis using several failure models are presented and comparisons are made. Rock porosity is also included in the computational procedure through correlations of this parameter with the log-derived uniaxial compressive strength of the rock. The correlations developed are also applicable for proper rock bit selection.
Field evaluation of rock strength in-situ conditions is fundamental in today's drilling engineering. Knowledge of rock failure behavior as a function of rock type, porosity; and triaxial stresses would provide information required to properly establish wellbore stability and optimum rock bit type selection, among others. This information is critical during offshore well planning and often is a contributing factor to the successful drilling of directional, highly inclined and horizontal wells.
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