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Study on Wellbore Stability in Shale Formation

Author: ChenXiuRong
Tutor: YaoAiGuo
School: China University of Geosciences
Course: Geological Engineering
Keywords: Shale Wellbore stability Shale hydration Thermal effects
CLC: TE21
Type: Master's thesis
Year: 2009
Downloads: 842
Quote: 3
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Abstract

Shale in a higher degree of consolidation of sedimentary rock mainly of clay minerals, which not only has the common characteristics of the rock, but also has water resistance. Shale because of its water resistance and become the focus of the study of the stability of the borehole. Shale generally contains 20% to 30% of the clay mineral, if the main component of the clay is montmorillonite, water swelling; if the main component of the kaolinite, illite clay, then the expansion of small, but easy embrittlement; illite - smectite mixed layer, strong bond, ionic bond reduced part than another part of the hydration ability, resulting in non-uniform expansion, a further weakening of the structural strength of the shale. Shale formation is difficult to deal with. This is shale borehole instability become a common problem in drilling engineering, as well as domestic and foreign drilling technology sector has been unremitting efforts to research the reasons. The research work for the the shale borehole wall instability had already started from the middle of the last century, has undergone the pure mechanics research - mud chemical research - mechanical and chemical coupling - mechanical - chemical - thermodynamic coupling studies several research stage. These theoretical models are different degrees of promoting the development of shale wellbore stability studies. However, due to the particularity of the shale, the existing theoretical models are different levels of existence of defects. On the multi-field coupling theory model, the existing theoretical models can be roughly divided into two categories: First, the use of stress superposition method, this method will calculate the in situ stress field, the additional stress of drilling fluid seepage additional stress field generated by the superposition of the stress field of mechanical borehole wall surrounding rock temperature change. This coupling method obviously does not have a strict mechanical sense only in the case of the shale formation as a rigid body to adopt; second is to use the law of conservation of energy, the disadvantage of this coupling method is difficult to determine shale hydration the chemical relationship between the can deformation. Moreover, because many of the parameters in the shale complexity of multi-field coupling, the coupling model it is difficult to be measured in the laboratory, which also affects the usefulness of the coupled model. Shale borehole instability is a very complex issue, and also a large number of factors affecting borehole instability. Wellbore stability analysis, considering too many influencing factors, not only complex, but also affect the usefulness of the model. So starting from the main influencing factors on wellbore stability analysis. The basic starting point of this paper is to study the shale wellbore stability problems in the drilling process. Starting from the wellbore stability, shale wellbore stability principles of mechanics, the hydration of the shale, drilling fluid shale thermal transfer issues. The main contents of this paper include the following aspects: 1. Analysis in the form of shale borehole instability and failure mechanism; based on linear elasticity theory and the principle of effective stress, without considering the of shale drilling fluid interaction the case, the the effective borehole wall surrounding rock stress distribution in the form; Drucker-Prager criteria to judge the borehole wall whether instability; collapse pressure and fracture pressure equivalent density calculation; analysis of the main effects of wellbore stability factors; departure from the composition and physical properties of the shale, analyze the underlying reasons of the shale hydration, further analysis of the shale hydration mechanism; finite difference method for solving the diffusion of adsorbed water shale equation, quantitative analysis of the variation around the shale borehole the strata water content distribution with radial distance and time; quantitative research based on the relationship of the of shale mechanical parameters and water content, hydration of shale mechanical parameters; introduced the problem of the problem of heat conduction and heat - coupled finite element analysis methods; analyze simulation software ANSYS finite element calculation and analysis of the drilling fluid to the formation of heat transfer around the borehole due to formation of the temperature field distribution law; taking into account the distribution of temperature and ground stress case the borehole wall surrounding rock stress, compared with the situation without considering temperature impact, by contrast, analysis of the impact of thermal effects on wellbore stability; comparative analysis using ANSYS simulation software consider shale hydration and thermal effects, and does not consider the difference between quantitative analysis of the influence of both wellbore stability. Shale wellbore stability study is a systematic study of this paper is only a preliminary study of mud shale wellbore stability force - chemical - thermal coupling analysis, there are many other factors not considered, the need for further exploration research.

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