Borehole stability in brittle rock - A tool for evaluation of borehole breakouts and in-situ stress

Examensarbete för masterexamen

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Type: Examensarbete för masterexamen
Title: Borehole stability in brittle rock - A tool for evaluation of borehole breakouts and in-situ stress
Authors: Felldin, Nils-Erik
Abstract: This thesis deals with the simulation of progressive failures in boreholes, a phenomenon known as borehole breakouts. Borehole breakouts or spalling are common in holes and tunnels subject to large differential in-situ stresses. The aim was to produce a simulation tool in MATLAB using a conformal mapping method based on the work of Gerolymatou (2019). An interactive graphical tool was coded to design a geometry and to evaluate its conformal mapping from a unit circle. A script was coded for determining stress distributions and checking for rock failure with the Mohr-Couloumb and Hoek-Brown failure criterion. Using this script it was also possible to iterate failure and generate new boundaries for simulating borehole breakouts. By using the designed tool example mappings of several geometries are shown in an attempt to quantify the performance of the mapping method. The secondary stress state of a circular hole was compared to the Kirsch (1898) solution and the stresses around a tunnel geometry was compared to a MATLAB Finite Element Model (FEM). Finally, results of a few borehole breakout simulations are presented. The mapping was shown to perform best for geometries defined by smooth continuous curves. The method of determining secondary stresses around symmetric regions is functional, however the FEM comparison of an asymmetric tunnel geometry demonstrated that the code needs further adaptation to correctly evaluate stresses around arbitrary geometries. Simulations of progressive borehole breakouts are possible if dealing with boundary oscillations stemming from the mapping. Oscillations are handled by using filters smoothing the boundary and optionally by also smoothing secondary stresses. A few ideas are presented on how the smoothing implementation can be improved to increase performance of breakout simulation. As an example of how breakout simulation can be used to find in-situ stresses from measured real world breakouts, breakout depth and width from simulations are shown as functions of the in-situ stress. Such functions could potentially be used to match measured real world breakouts to the maximum in-situ stress, if the minor in-situ stress is determined by a method such as hydraulic fracturing and if the rock can be accurately modelled by a failure criterion.
Keywords: Borehole breakouts;Conformal mapping
Issue Date: 2019
Publisher: Chalmers tekniska högskola / Institutionen för arkitektur och samhällsbyggnadsteknik (ACE)
Series/Report no.: ACEX30-19-113
Collection:Examensarbeten för masterexamen // Master Theses

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