An integrated rock-mechanics tests and numerical modelling of chalk rocks: An improved integrated workflow for borehole safety

M. K. Medetbekova, M. R. Hajiabadi*, A. Brovelli, H. F. Christensen, H. M. Nick

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review

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Abstract

Fluid withdrawal and pore pressure reduction change the effective stresses around a borehole and cause borehole instability associated with progressive localization of the damaged zone as well as potential fines production. Experimentally, chalk exhibits a complex geomechanics behaviour (pore collapse, shear failure, time/rate dependency) and modelling the behaviour of the borehole under in-situ and operational conditions requires the constitutive model to be capable of capturing the observations. This study presents a workflow that integrates rock mechanics testing on cylindrical specimens as well as specimen with a single lateral hole (SLH) and a finite element code, developed for chalk. The code incorporates post-peak softening as well as the rate dependency of the pore collapse stress in order to accurately predict the wellbore stability under in-situ stress conditions. The tested SLH specimen was CT imaged before and after testing for identifying the damaged zone and its extension. Backward numerical simulations of the SLH test data improved the accuracy of the estimated rock mechanics properties (post-peak failure and dilatancy) compared to the properties estimated by back analyses of standard triaxial tests with a single element simulator. The workflow is applied to predict the stability of a small lateral borehole (2 cm) created with Radial Jet Drilling technique with two different geometries: one with circular geometry created by a rotating nozzle; another with a circular hole with wing shaped cracks likely to develop when a static nozzle is used. Results of the wellbore stability analyses applying the chalk properties from the back analyses highlighted the importance of using experimentally verified post-peak failure and dilatancy parameters, together with a modelling tool capable of simulating shear strain localization incorporating the Cosserat approach.

Original languageEnglish
Article number109365
JournalJournal of Petroleum Science and Engineering
Volume208
Number of pages13
ISSN0920-4105
DOIs
Publication statusPublished - 2022

Bibliographical note

Funding Information:
We kindly thank Bertold Plischke from Isamgeo GmbH for helping, while this research was being carried out. Authors also would like to thank Geo for its contribution with regards to conventional rock mechanics and SLH testing. This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654662 as well as the Danish Hydrocarbon Research and Technology Centre under the Advanced Water Flooding programme. The 3D Imaging Center at The Technical University of Denmark is gratefully acknowledged for providing access to X-ray tomography equipment.

Funding Information:
We kindly thank Bertold Plischke from Isamgeo GmbH for helping, while this research was being carried out. Authors also would like to thank Geo for its contribution with regards to conventional rock mechanics and SLH testing. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654662 as well as the Danish Hydrocarbon Research and Technology Centre under the Advanced Water Flooding programme. The 3D Imaging Center at The Technical University of Denmark is gratefully acknowledged for providing access to X-ray tomography equipment.

Publisher Copyright:
© 2021 The Authors

Keywords

  • Austin chalk
  • Radial jet drilling
  • Strain localization
  • Wellbore stability

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