Abstract
Wellbore drilling operations frequently entail the combination of a wide range of variables. This is underpinned by the numerous factors that must be considered in order to ensure safety and productivity. The heterogeneity and sometimes unpredictable behaviour of underground systems increases the sensitivity of drilling activities. Quite often the operating parameters are set to certify effective and efficient working processes. However, failings in the management of drilling and operating conditions sometimes result in catastrophes such as well collapse or fluid loss. This study investigates the hypothesis that optimising drilling parameters, for instance mud pressure, is crucial if the margin of safe operating conditions is to be properly defined. This was conducted via two main stages: first a deterministic analysis—where the operating conditions are predicted by conventional modelling procedures—and then a probabilistic analysis via stochastic simulations—where a window of optimised operation conditions can be obtained. The outcome of additional stochastic analyses can be used to improve results derived from deterministic models. The incorporation of stochastic techniques in the evaluation of wellbore instability indicates that margins of the safe mud weight window are adjustable and can be extended considerably beyond the limits of deterministic predictions. The safe mud window is influenced and hence can also be amended based on the degree of uncertainty and the permissible level of confidence. The refinement of results from deterministic analyses by additional stochastic simulations is vital if a more accurate and reliable representation of safe in situ and operating conditions is to be obtained during wellbore operations.
Highlights
An overview of experiences during the drilling and production of hydrocarbon from wells indicates rampant incidences arising from wellbore instability
The conventional approach to ensure that the rock surrounding the wellbore during drilling or production remains intact is by the radial application of mud pressure using fluids with specialised properties
Knowing the correct magnitude of mud pressure to exert is crucial in order not to instigate instabilities that may lead to wellbore failure
Summary
An overview of experiences during the drilling and production of hydrocarbon from wells indicates rampant incidences arising from wellbore instability. Wellbore instability poses a major problem during drilling, and its causes can be Edited by Yan-Hua Sun categorised into mechanical and chemical effects. (2007) classify the factors contributing to wellbore instability as uncontrollable (natural) and controllable factors. Natural factors include the presence of naturally fractured or faulted formations, tectonically stressed formations, high in situ stresses, mobile formations, unconsolidated formations, naturally over-pressured rock collapse and induced over-pressure rock collapse; controllable factors include bottom-hole pressure (mud density), well inclination and azimuth, transient pore pressures, physicochemical rock–fluid interaction, drill string vibrations, erosion and temperature. Other factors which affect wellbore stability are the orientation of in situ stress fields, the mechanical properties of rock and bedding planes, and pore pressure (Chen et al 1997)
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