Wellbore Stability: A Critical Review and Introduction to DEM

Abstract

Borehole stability issues are always a spotlight in drilling activities because of their costly consequences, including borehole collapse, lost circulation, stuck pipe etc. Wellbore instability is primarily a function of how rocks respond to the induced stress concentration around the wellbore during various drilling activities. By considering different failure mechanisms between the formation and drilling fluid interaction, several major wellbore models have been presented over the last seventy years. Those models take into account the mechanical, chemical, hydraulic, or thermal effects between the drilling fluid and the formation, or couple two or more effects in a model. The time effect is also taken into account in some of the models. In most of the models,the rock is treated as a continuous materialborehole failure is normally based on single initial failure point. However, rocks are discontinuous materials formed under an environment of complex stresses. Also, it is likely an overstatement to say that instability occurs when only one point fails. Even if a small group of grains disconnects from wellbore, the well can still be stable. While somewhat irregular (noncircular) the wellbore can still accommodate casing and installation of down-hole equipment. Therefore, this paper also introduces a new approach based on grain-scale discrete element modeling (DEM) to mimic the realistic rock condition. The rock is modeled as an assembly of numerous grains bonded by cement-like materials, and pore spaces are formed between the small grains. The dynamics of rock grains is simulated and tracked on a computer. Micro cracks (because of tensile or shear failure) occurring at stress-concentrated zones and their coalescence to form macro fractures are tracked. The borehole shape and size are tracked with time. This paper is useful to those who wish to understand the main limitations of the conventional models and the potential usefulness of a new approach based on a discrete element method (DEM). The approach presented in this paper can also help engineers understand how wellbore instability (post-initial failure) develops with time. Introduction Subsurface rocks are under a balanced stress condition before a well is drilled. Such equilibrium will be disturbed when a well is drilled. Although drilling fluid can partially support the wellbore surface, the presence of a wellbore can cause the redistribution of stresses around the borehole. If the stress concentration exceeds the strength of the rock, failure in the nearwellbore region occurs. Wellbore stability is a major concern in drilling operations. It is the major cause of nonproductive time during drilling operation and costs the oil and gas industry more than $6 billion USD worldwide annually (SPE review, SPE-UK, 2005). With the increasing demand of energy (oil and gas), drilling operations move to the direction where more and more harsh environments are encountered. With more wells to be drilled under high pressure and high temperature conditions, the industry expects more severe wellbore stability problems to occur. Although wellbore stability has been studied (experimentally and theoretically) for many years, it remains one of the major challenges for the oil and gas industry due to the complex nature of the drilled formations. Therefore a better understanding of wellbore stability is of imperative importance for the oil and gas industry. In this paper, the discrete element method (DEM) is adopted in this study to get a better understanding of time-dependent transient wellbore instability that takes place in a realistic rock condition.

Study on the Mechanisms and Evaluation Methods of Borehole Instabilities in Gas Drilling

“…Some weak rocks or crumbliness rocks (Fig 1), even if without influence coming from fluid, can't also keep borehole stability in gas drilling. According to Coulomb-Mohr criterion, when shear stress of shear plane is greater than the sum of inherent shear strength of rock and cohesion on the shear plane, failure will occurred [11][12][13] , as follow: The crumbliness rocks mainly display spallation, chip off-falling and a large area of hole enlargement.…”

Large‐scale experiments of the borehole instability on shale formation influenced by drill pipe rotation

“…The research method developed from linear elasticity to poroelasticity, from elastic analysis to plastic analysis, and from single field analysis to multi‐field coupling analysis . The solving method developed from analytical, semi‐analytical methods to numerical simulation methods including finite difference, finite element, boundary element, and discrete element, etc . Both isotropic and anisotropic of shale rock were considered.…”

The effect of Non-Darcy Flow on Induced Stresses Around a Wellbore in an anisotropic in-situ stress field

“…This will result in different incidences; with wellbore instability and sand production being only two examples of its many. The financial loss resulted from these incidences are significant and reported to be over six billion dollars in the case of wellbore instability [1], and tens of billion dollars due to sand production annually [2].…”