Shear failure analysis of a shallow depth unsupported borehole drilled through poorly cemented granular rock
- Authors: Hashemi, Sam , Taheri, Abbas , Melkoumian, Noume
- Date: 2014
- Type: Text , Journal article
- Relation: Engineering Geology Vol. 183, no. (2014), p. 39-52
- Full Text: false
- Reviewed:
- Description: Adopting an appropriate failure criterion plays a key role in the borehole stability analysis. In this paper the induced stresses on a vertical borehole wall were calculated based on the elastic theory. Then, to predict the stability of a borehole drilled through a poorly cemented sand formation, failure envelopes in different failure criterion domains were derived using the results from a series of precise laboratory tests conducted on solid and hollow cylinder specimens. The mixture used in specimen preparation was designed to simulate the properties of the samples collected from depths up to 200m at a drilling site in South Australia. The hollow cylinder test apparatus was developed by modifying a Hoek triaxial cell. These modifications allowed observing the process of debonding of sand grains from the borehole wall during the test and consequently, acquiring a better understanding on the failure mechanisms of a borehole drilled through poorly cemented sand formations. Three well-known failure criterion domains; Coulomb, Drucker–Prager and Mogi, were considered versus the laboratory test data to investigate their capability to predict the shear failure of a borehole using the data from hollow cylinder tests. The obtained results showed the significance of selecting an appropriate failure domain for predicting the shear failure behavior of poorly cemented sands near the borehole wall. The results also showed that the Coulomb criterion is not well suited for predicting the borehole failure when there is no pressure acting inside the borehole. A failure envelope based on the Mogi domain was developed which can be used for the far-field stress states. The introduced failure envelope allows predicting the stability of a borehole drilled in poorly cemented sands.
An experimental study on the relationship between localised zones and borehole instability in poorly cemented sands
- 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
- Full Text: false
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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.
The failure behaviour of poorly cemented sands at a borehole wall using laboratory tests
- 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
- Reviewed:
- 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].
A Failure criterion for unsupported boreholes in poorly cemented sands
- Authors: Hashemi, Sam
- Date: 2016
- Type: Text , Journal article
- Relation: International Journal of Civil, Environmental, Structural, Construction and Architectural Engineering. Vol. 3, no. 11 (2016), p. 1707-1712
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- Reviewed:
- Description: The breakage of bonding between sand particles and their dislodgement from the borehole wall are the main factors resulting in a borehole failure in poorly cemented granular formations. The grain debonding usually precedes the borehole failure and it can be considered as sign that the onset of the borehole collapse ia imminent. Detecting the bonding breakage point and introducing an appropriate failure criterion will play an important role in borehole stability analysis. To study the influence of different factors on the initiation of sand bonding breakage at the borehole wall, a series of laboratory tests was designed and conducted on poorly cemented sand samples.
A strain energy criterion based on grain dislodgment at borehole wall in poorly cemented sands
- 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
- Reviewed:
- 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.