Effect of different stress path regimes on borehole instability in poorly cemented granular formations

• A series of TWHC tests was conducted on synthetic poorly cemented sand specimens.
• Different stress paths were designed to investigate the effect of paths on borehole failure.
• The confining stress had more significant effect on the instability than the cement content.
• The effect of the supporting stress on ε1ε1 was more pronounced for smaller boreholes.
• A new failure criterion is suggested based on the conducted tests for these weak formations.

Boreholes are drilled for different purposes such as discovering potential new deposits of underground minerals, extraction of petroleum, underground strata investigation, etc. Although no specific significant problems have been reported on drilling through hard rocks and strong formations, considerable problems have been observed in areas consisting of a sandy formation where particles are not strongly cemented by natural cement agents such as clay, iron oxide or calcite. In this study, a series of the thick-walled hollow cylinder (TWHC) laboratory tests was conducted on synthetic poorly cemented sand specimens in which the applied stresses were at levels of generating breakout on the borehole wall. Five different stress paths were designed and applied to the specimens to investigate the effect of stress paths on the borehole failure. Two borehole diameter sizes (10 mm and 20 mm) and three different cement contents (6%, 7% and 8%) were considered to evaluate the effect of the borehole size and grain bonding strength on the borehole failure in poorly cemented sandy formations. The results showed that in these weak formations the confining pressure has a more significant effect on the instability of the borehole than the cement agent content. It was found that for any stress path the effect of the supporting stress on ε1ε1 was more significant for 10 mm borehole sizes. In addition, based on the applied stress paths a new failure quadrilateral was determined for poorly cemented sands. Results showed that with increasing the cement content, similar quadrilaterals with almost parallel sides will be established. These outcomes can contribute to improving the design of the supporting systems and can be utilised to predict the borehole instability prior to drilling.