Abstract
Wellbore collapse and drilling fluid leakage are prone to occur when drilling a conglomerate formation. The conventional method for predicting wellbore stability cannot achieve sufficient accuracy when used in conglomerate formations. In this study, the particle size distribution in the conglomerate formation is characterized by the statistics of particle size analysis. Based on microstructure analysis and laboratory tri-axial tests, physical and mechanical properties of conglomerate can be obtained. Discrete element method, by particle flow code software $$\hbox {PFC}^{2\mathrm{D}}$$ , is applied to simulate the failure process of the conglomerate formation near the borehole region and investigate the influence of gravel size distribution, drilling mud density and fluid infiltration on the severity of wellbore instability. The simulation results show that the process of wellbore instability within the conglomerate formation correlates with the particle size of the gravels and their spatial distribution: in the fine-grained conglomerate formation, the cracks dominantly extend toward the direction of maximum horizontal stress; in the coarse-grained conglomerate formation, the cracks initially generate around large gravels and preferentially extend toward the gravel-concentrated zone. In addition, the variation of drilling mud density substantially affects the number of cracks in terms of mechanical analysis. The favorable mud density for specific conglomerate formation is found to minimize crack numbers. Further, while considering fluid infiltration into formation, more cracks should be accounted and worse instability state is reached in comparison with counterpart without infiltration. In this sense, practical operations are recommended to allow for the pressure equilibrium and sealing capability of drilling fluid.
Published Version
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