An experimental study on the relationship between localised zones and borehole instability in poorly cemented sands

- Hashemi, Sam, Taheri, Abbas, Melkoumian, Nouné

  • Authors: Hashemi, Sam , Taheri, Abbas , Melkoumian, Nouné
  • Date: 2015
  • Type: Text , Journal article
  • Relation: Journal of Petroleum Science and Engineering Vol. 135, no. (2015), p. 101-117
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  • Description: Poorly cemented sands are mainly located in areas where layers of unconsolidated formations exist. Drilling a borehole in the ground causes stress perturbation and induces tangential stresses on the borehole wall. If the cohesion between sand particles generated by existing cementation is not high enough, the tensile stress concentration may cause grain debonding and, consequently, borehole breakout. In this study a series of solid and thick-walled hollow cylinder (TWHC) laboratory tests was performed on synthetic poorly cemented sand specimens. The applied stresses were high enough to generate breakout on the borehole wall. Simultaneous real-time monitoring and deformation measurement identified the development of localised breakout zones and compaction bands at the borehole wall during the tests. The results from the video recording of the tests showed that a narrow localised zone develops in the direction of the horizontal stress, where stress concentration causes the full breakout in specimens. Dilation occurred at lower confining pressures in TWHC specimens and contracting behaviour was observed during the onset of shear bands at higher pressures. Scanning electron microscopy (SEM) studies showed that sand particles stayed intact under the applied stresses and micro- and macrocracks develops along their boundaries. The SEM imaging was also used to investigate and characterize pre-existing microcracks on the borehole wall developed due to the specimen preparation. It showed that boring the solid specimen in order to produce a TWHC specimen could generate microcracks on the borehole wall prior to testing which affects the process of borehole failure development during the test. © 2015 Elsevier B.V.

A strain energy criterion based on grain dislodgment at borehole wall in poorly cemented sands

- Hashemi, Sam, Melkoumian, Nouné

  • Authors: Hashemi, Sam , Melkoumian, Nouné
  • Date: 2016
  • Type: Text , Journal article
  • Relation: International Journal of Rock Mechanics and Mining Sciences Vol. 87, no. (2016), p. 90-103
  • Full Text: false
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  • Description: The breakage of bonding between sand particles at a borehole wall usually precedes the borehole failure and it can be considered as a sign that the onset of the borehole collapse is imminent in granular formations. Detecting the particle detachment point and introducing an appropriate failure criterion will play a key role in borehole stability analysis. To investigate the influence of different factors on the initiation of particle debonding at the borehole wall, a series of new laboratory tests was designed and performed on synthetic poorly cemented sand specimens. The tests were devised to allow visual observation of the onset of dislodgment of particles from the borehole wall under various stress paths, for two different borehole sizes and various cement contents. In order to simulate the conducted laboratory tests on TWHC a series of numerical modelling has been conducted by discrete element method to estimate the stresses and strains at the borehole wall. The total strain energy up to the point of the observed particle debonding was calculated for each stress path and a failure criterion based on the total strain energy was introduced. The results showed that the particle detachment point at the borehole wall was reached both before and after the peak strength of the TWHC specimens depending on the stress path and cement content. Also, it was concluded that the stress path has a significant effect on the onset of the particle detachment. The introduced criterion based on absorbed strain energy will help to design more effective support systems for boreholes. © 2016 Elsevier Ltd.

The failure behaviour of poorly cemented sands at a borehole wall using laboratory tests

- Hashemi, Sam, Melkoumian, Nouné, Taheri, Abbas, Jaksa, Mark

  • Authors: Hashemi, Sam , Melkoumian, Nouné , Taheri, Abbas , Jaksa, Mark
  • Date: 2015
  • Type: Text , Journal article , Technical Note
  • Relation: International Journal of Rock Mechanics and Mining Sciences Vol. 77, no. (2015/07/01/ 2015), p. 348-357
  • Full Text: false
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  • Description: Borehole stability analysis is an important challenge for researchers in the field of geotechnical, mining and petroleum engineering. Several borehole instability problems during or after the completion of drilling, have been reported by a number of exploration companies in Australia. Many of these problems are reported in drilling projects in poorly cemented sand formations at depths of up to 200 m beneath the ground. The sand production problem, as it is known, has also been observed in weakly bonded sandstones where the debonding of sand grains can be triggered by fluid pressure and induced stresses leading to the failure of the sandstone at the borehole wall. The strength of a granular material formation is generated mainly by a natural cementing agent that bonds sand grains together.
  • Description: Borehole stability analysis is an important challenge for researchers in the field of geotechnical, mining and petroleum engineering. Several borehole instability problems during or after the completion of drilling, have been reported by a number of exploration companies in Australia. Many of these problems are reported in drilling projects in poorly cemented sand formations at depths of up to 200 m beneath the ground. The sand production problem, as it is known, has also been observed in weakly bonded sandstones where the debonding of sand grains can be triggered by fluid pressure and induced stresses leading to the failure of the sandstone at the borehole wall [1], [2]. The strength of a granular material formation is generated mainly by a natural cementing agent that bonds sand grains together [3].
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