Study On Wellbore Stability In Shale Formation

Shale is one sedimentary rock which is clay mineral-based and higher degree consolidated. Shale not only has the common characteristics of rock, but also hydration. For this, shale becomes the focus of the well-bore stability study. Generally, shale contains 20% ~ 30% of the clay minerals. If the main component of the clay is Montmorillonite, the rock is easy water swelling; if the main component of clay is Kaolinite and Illite, while the expansion is small, but easy to brittle or crack. And to Eamonn mixed-layer, the strong electrovalent bond reduces. The hydration of one part being greater than another results in non-uniform expansion, and further weakens the structural strength of shale. Therefore, it is very difficult to deal with shale formation. This is the reason why the shale borehole instability becomes widespread in drilling projects, as well as domestic and foreign drilling technology sector has been committing themselves to resolving it.Started from the 40's in last century, the research work of shale borehole instability has gone through several stages: the study of pure mechanics, mud chemical research, coupled mechanical and chemical research, coupled mechanical - chemical - thermodynamics. These theoretical models have promoted the research of shale borehole stability in varying degrees. However, due to the specificity of shale, the existing theoretical models exposure sundry defects. In terms of multi-field coupling theoretical models, the existing theoretical models can be roughly divided into two categories: one category uses stress superposition method, which is the mechanically superposition of the calculated in-situ stress field, additional stress field generated by drilling fluid's flowing, additional stress field caused by the rock temperature changes of borehole. Obviously, this coupling method is out of strict mechanical significance. It can only be used under the circumstances that the shale formation be regarded as a rigid body. The other category uses energy conservation law. The shortcoming of this coupling method is that it's very difficult to determine the relationship between its chemical energy caused by hydration of shale and deformation. Moreover, as a result of the complexity of coupling several fields in shale, many parameters in coupled models are very difficult to measure in the laboratory, which also affect the practicality of the coupled model. For shale instability is a very complex problem, as well as too many factors affecting the instability, the problems of coupled model on borehole stability analysis are inevitable. Since the method of multi-field coupling is complex and unpractical, still with some problems, a simple, practical approach should be adopted to research borehole stability. Based on this, this article will just consider the most important factors with simplifying the analysis of borehole stability.In this paper, the basic starting point is studying shale borehole stability in the process of drilling. Based on the main factors impacting on borehole stability, the mechanical principle of borehole stability, the impact factors, the issue of shale hydration, drilling fluid and heat transfer problem of shale are studied respectively. The main content of this article are as follows:1. The forms of shale borehole instability and the mechanism of instability are analyzed; Based on the line elastic theory and the effective stress principle, an effective form of stress distribution surrounding the borehole is established without taking the interaction between shale and drilling fluid into account.; Drucker-Prager failure rule are used to determine whether the borehole is instable or not. Simultaneously, the equivalent drilling fluid density calculations of the collapse pressure and rupture pressure are introduced. The main factors which affect the borehole stability are analyzed;2. Based on composition and physical properties of shale, internal factors of the shale hydration, and then the shale hydration mechanism are analyzed.; By means of solving equation of shale water absorption diffusion, the moisture content distribution of rock around the borehole along with radial distance and time changes are quantitative analyzed; And in line with the relationship between the mechanical parameters and water content of shale, the impact on the mechanical parameters from shale hydration is studied;3. The finite element method of thermal conduction and coupled mechanical-thermodynamics are introduced. And then the distribution of temperature field around the borehole caused by the heat transfer from drilling fluid to the formation is calculated by the software of ANSYS finite element; furthermore, the distribution of borehole stress around the borehole is analyzed taking the temperature into account and compared with the condition not considering the impact of temperature; Under three conditions: only stress, stress and shale hydration, stress, shale hydration and thermal effects, comparative analysis of stability is done through using the software of ANSYS finite element. By comparing them, the shale hydration and thermal effects on the stability of the borehole is quantitative analyzed.As the study of borehole stability is a system study, this paper is a preliminary study only considering stress - chemical - thermal coupling analysis of borehole stability. Further research needs to be explored for that there are many factors not taken into consideration.