Abstract:
The present paper develop an integrated geomechanical wellbore stability model that incorporates the theoretical derivation regarding Mogi-Coulomb strength criterion with the favor of Lode's parameter and stress invariants, the anisotropic rock strength characteristic associated with weak plane, the coordinates transformations pertaining to borehole and weak plane systems as well as chemo-poro-thermoelastic solutions around a pressurized inclined borehole to analyze the extent of shear failure and tensile fracture regions in horizontal boreholes. The results regarding a horizontal wellbore drilled along the direction of minimum horizontal in situ stress from present paper show that increasing the mud salinity could contribute to consolidating the formation and alleviating the wellbore instability with respect to shear failure where thermal osmosis intensifying the chemical osmosis effect. Moreover, the drilling mud with higher salinity where thermal osmosis weakening the chemical osmosis effect boosts the favorable conditions required for concurrence associated with longitudinal and transverse fractures in the top and low sides of the horizontal wellbore while the corresponding combined tensile fracture regions progressively aggrandize with time progress.
Introduction
Wellbore instability issues are often encountered when drilling the inert to generating mud cake due to low permeability regarding shale formation with the laminated heterogeneity and chemically active characteristics in deep high temperature environment by horizontal borehole. Accordingly, it is necessary and urgent to understand the complex wellbore instability mechanism with reference to time dependent reestablishment of stresses and pore pressure around ther wellbore determined by poroelastic effect associated with undrained loading after instantaneous bored (Detournay and Cheng, 1988), chemical effect related to the chemical osmotic flow resulting from solute transfer (Heidug and Wong, 1996; Yu et al.,2003; Ghassemi and Diek, 2003; 2009; Zhou and Ghassemi, 2009) as well as water activity difference (Chen et al., 2003) between drilling mud and pore fluid in shale formation, thermal effects arising from heat transport (McTigue, 1986; Chen and Ewy, 2005; Zhou and Ghassemi, 2009) and thermal osmotic (Ghassemi and Diek, 2002; Ghassemi and Diek, 2009).
Shale formations are characterized by parallel discontinuous planes of weaknesses configurations of which may exhibit pronouncedly more potential to fail along the slip surfaces as compared to through compact rock matrix. Accordingly, the strength criteria pertaining to shale rock may be determined by different controlling factors associated with dominated failure behaviors. In regard to intact rock failure, poly-axial shear failure strength criteria, such as: Modified Lade (Ewy,1999) and Mogi-Coulomb (Al-Ajmi and Zimmerman, 2005) appropriately evaluate the strengthening effect concerning the intermediate principle stress such that efficiently guide the practical drilling designs, however, Mohr-Coulomb criterion is expected to be significantly conservative in estimating the minimum mud pressure required to ensure the wellbore stability, since the influence of intermediate principal stress on rock strength is disregarded. In another aspect, the single plane of weakness theory is expressed in the form of Mohr-Coulomb differentiating failure modes of the intact rock and weak plane by the angle between major principal stress and the normal of isotropic planes of laminated medium (Jaeger et al.2007).
