Discrete Element Analysis of Wellbore Stability Mechanisms in Broken Formations
Authors: Liang Hongjun , Fang Zheng , Zhang Zhen , Yan Mingfei , Li Jiaxin , Zhou Bo , Zhao Li , Lu Yunhu 
Published: August 2024

ABSTRACT:
Clarifying the mechanism of wellbore stability in broken formations is crucial due to the discontinuous deformation and stress characteristics of such rocks. Conventional continuum-based rock mechanics methods need to be improved for these formations. The discrete element method (DEM) is more suitable as it treats the rock as a combination of rigid bodies, effectively modelling discontinuities. This paper examines wellbore stability in fractured formations using DEM. It first analyzes the drilling characteristics of broken formations and then studies fracture data from the Tarim exploration. Based on discrete particle theory and contact constitutive relation, a 3D model of wellbore stability of broken formation is established, and the influence of various engineering geological factors on wellbore stability is evaluated. The study reveals that wellbore instability is primarily caused by drilling fluid filtrate entering the formation through weak surfaces (bedding and fractures), rapidly weakening the rock strength and expanding fractures into a network, leading to rock disintegration. Initial collapse occurs around the wellbore, with prolonged drilling fluid exposure causing further instability in the near-wellbore rock, posing significant drilling challenges. Developing drilling fluids and materials that can cement or consolidate formations is essential for enhancing wellbore stability in broken formations.

1. INTRODUCTION
Under the influence of multiple tectonic movements, the Tarim Basin has developed significant structural broken formations along major fault belts. The discontinuity of rocks in broken formations is significant, with notable differences in mechanical properties between the broken zone and the matrix zone. In recent years, when drilling encounters broken formations, issues such as stuck pipe and drilling fluid losses have frequently arisen, closely correlated with the mechanical characteristics of the broken formations [1].
Conventional rock mechanics analysis, based on continuum assumptions, fails to adequately address the discontinuous nature of deformation and stress in fractured formations. To overcome this limitation, the discrete element method emerges as a promising approach. By treating rock as an assembly of rigid bodies connected by structural planes, the discrete element method can effectively capture the discontinuities in deformation and stress, making it well-suited for modeling broken formations. The discrete element method, also known as the discontinuous element method, was initially proposed by Cundall in 1971 as a numerical modeling technique for discontinuous systems [1]. Santarelli conducted hydraulic numerical simulations of drilling fluids using the discrete element method, demonstrating that enhanced filtration control and rheological properties of drilling fluids could improve wellbore stability [2]. Jamshidi et al. [3-4], considering the presence of fractures, employed a discrete element model (DEM) to numerically analyze the stability of horizontal wellbores. They investigated the effects of drilling fluid circulation, well depth, stress state, and fluid pressure on fracture surfaces. Discrete element numerical simulations related to fractured formations have explored the impact of particle size distribution, drilling fluid density, and fluid infiltration on wellbore instability, further corroborating the positive influence of specific drilling fluid densities on broken formations [5].