Investigation of borehole stability in poorly cemented granular formations by discrete element method

Behaviour of poorly cemented formations in case of drilling a vertical exploration borehole will be studied to achieve an in-depth understanding of borehole stability problem. Analysis of the granular formation behaviour has a significant importance in identifying stability issues, designing adequate borehole supports and choosing an efficient drilling method. This paper presents numerical investigations on the behaviour of poorly cemented formations in the vicinity of an unsupported vertical cylindrical borehole. Due to poor cementation and therefore granular behaviour of these formations, Discrete Element Method (DEM) was identified as being well suited for developing realistic models. To conduct the numerical studies a cube of 8 m3 made up of spherical particles with diameters ranging from 5 mm to 70 mm was constructed and analysed in three-dimensional Particle Flow Code (PFC 3D). It is a discontinuum code used in analysis of the granular materials where the interaction of discrete grains is considered. A cylindrical opening with the diameter of 0.3 m runs along the central vertical axis of the cube simulating the presence of a borehole. The stresses applied to the cube simulate the underground conditions around an exploration borehole at the depth of 80 m. The effects of in situ stresses around the borehole, strength of particle bonding and fluid flow pressure on the stability of the formation around the borehole have been investigated. It has been shown that the development of in situ stresses in the ground due to drilling a borehole results in the formation of a plastic zone around that borehole. When there is lack of sufficient bonding between the sand grains, the interaction between them results in their movement towards the borehole opening and thus eventuates the collapse of the borehole wall. Furthermore, the presence of high pressure water flow expedites the process of the borehole collapse.