An Experimental and Numerical Modelling Study on Stability of Wellbores With Pre-Existing Breakouts

Abstract

Wellbore instability is a major problem in the oil and gas industry and has been an area of intensive research for a number of decades. Previous research has, however, focused on initial stability for circular wellbores by comparing wellbore stress with formation rock strength, and once the former is greater than the latter, wellbore failure is assumed. There have been very limited studies on stability for wellbores that have already experienced some forms of rock failure on the wellbore wall such as breakouts. Since it is inevitable that wellbore and/or reservoir conditions will change during drilling and production, it is important to understand how wellbores with existing breakouts would response to the changed wellbore and reservoir conditions, and whether the breakouts would remain stable or enlarged further resulting in wellbore collapse. This paper presents a preliminary experimental and numerical study on stability of boreholes with pre-existing breakouts. A series of borehole stability experiments were carried out using a polyaxial cell on weak sandstone samples with a pre-drilled central borehole. The test samples with a dimension of 85mm by 85mm by 100 mm were subjected to 3 independent stresses. The tests were performed by increasing the outer boundary stresses at a given stress ratio until sufficient breakouts were produced. The test samples were then unloaded and the failed materials within the breakout removed manually. The samples with the new borehole geometry were then re-tested with further failure monitored with a borehole camera. It was observed that the failure started from the breakout tip at about 50% of the previously applied maximum stress, but the breakout did not widen until the external load was equal to or greater than the previously applied maximum stress. The borehole stability tests were simulated numerically using FEM and a constitutive model that takes into account strain softening, large deformation and strain. The numerical modelling was conducted to follow the entire experimental process with breakouts formed by removing elements within which the effective plastic strain had reached a critical value. It has been observed that the numerical simulation was able to qualitatively re-produce the experimental observations. The experimental and numerical modelling study revealed new insights on effects of constitutive behaviour and loading history and path on the evolution of stability of wellbores with pre-existing breakouts.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015