Wellbore stability is a critical issue when drilling through tectonically stressed and complex geological conditions. Understanding wellbore stability issues before a well is drilled enables better planning of the drilling operation and helps to avoid borehole failure. This is of particular importance in underbalanced drilling where we are limited with our choice of drilling mud densities. This thesis examines the impact of fluid pressure change on wellbore stability during underbalanced drilling by using a timedependent poroelastic model. The poroelastic behaviour is analysed using numerical and analytical models. The finite element method (FEM) is used for the numerical model. Some simple techniques are developed and implemented to increase the speed and stability of the FEM solution. The common assumptions of plane strain and plane stress are explored. It is shown that the plane strain assumption results in high error while the error for plane stress is low. It is also shown that use of plane strain predicts more instability than use of plane stress and the stability difference is significant. From this it is concluded that the plane stress assumption should be used instead of the commonly used plane strain assumption. A sensitivity analysis is conducted to demonstrate the effect of several variables on wellbore stability during underbalanced drilling. These variables include mean in-situ horizontal stress, deviatoric in-situ horizontal stress, bulk compressibility and permeability. I various ways changes in these variables were shown to change the chance of shear failure, early time tensile failure through exfoliation and late time tensile failure through hydraulic fracture initiation.
| Identifer | oai:union.ndltd.org:ADTP/234920 |
| Date | January 2006 |
| Creators | Hodge, Martin Owen, Petroleum Engineering, Faculty of Engineering, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Petroleum Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Martin Owen Hodge, http://unsworks.unsw.edu.au/copyright |
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