| As is known to all,The resources of shale gas in China is rich and has a broad development potential,but at the same time accompanied with a huge number of borehole instability problems in drilling shale formation.The misunderstanding of the mechanical behavior in shale formation leads to the borehole instability.Shale has the characteristics of low permeability and anisotropy.When drilling in shale formation,it is very important to choose the correct mechanical model for borehole stability analysis.This thesis mainly take consideration in three aspects.(1)Because of the water-sensitivity in shale,the formation parameters are starting to have the characteristics of distribution in time and space.This paper mainly analyzes the experiment for shale hydration swelling parameter in the second chapter.Referring to the linear elastic hydration model proposed by Chenevert and developing it in the third chapter.Considering the heterogeneity of the in-situ stress and dividing the model into two parts which including the axisymmetric and the asymmetric.Because the hydration model is a non-linear model.Using the finite difference method to solve the model;(2)It is more suited to use porelasticity to analyze the borehole stability problem due to the low permeability in shales.Although some scholars have begun to study this theory,most of engineering applications still use the linear elastic model.Thus,The fourth chapter focuses on the fundamental of poroelasticity and gives the analytical solution of pore pressure and wellbore stress in isotropic and transversely isotropic shale formation;(3)Referring to the model(considering the ion transfer)proposed by Heidug and Wong which coupling chemical and hydraulic.This model was derivation by the nonequilibrium thermodynamics and was a Biot-like model.In this thesis,we expressed the chemical potential as a linear function of the solute mass fraction and obtained the analytical solution of pore pressure and wellbore stress in isotropic shale formation. |
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