Study On Horizontal Borehole Stability In Shale Formation

Currently, the study on borehole stability in shale formation has not only been involved with mechanism issue, but also relatively perfected with coupled mechanical-chemical models or coupled mechanical-chemical-thermal models. However, they share with the similar approach that analyzing the period problem of borehole collapse in virtue of assuming the initial condition and boundary condition for given model and utilizing the partially decoupled manner has remained to be studied on the level of theory, moreover, stimulating the drilling fluid temperature distribution changes in pore space will differ with the applied approaches regarding steady-state and transient state.This paper continues to deepen the analysis of borehole instability in shale formation based on linear poroelastic mechanism theory, and the major research includes as follows:In term of the calculation of the horizontal in situ stresses, a proposed formula is applicable to study the in situ stresses of laminated shale formation on account of the common expression concerning generalized Hooke’s law and assuming the dip laminated shale formation as transverse isotropic rock body; moreover, the calculating method of in situ stresses regarding orthotropic shale formation is proposed; and quantitatively studying the effect of the anisotropy of Poisson’s ratio and Young’s modulus, dip angle on the predicted values of horizontal in situ stresses by comparing with the common isotropic in situ stress calculation model.It presents a convenient numerical analysis method to obtain the angular positions of borehole collapse and fracture without involving with the common complicate iterative method based on the shear failure and tensile fracture condition, and a analytical formula of the optimum safe drilling azimuth angle is derived with analyzing the variation ruling associated with the angular position of borehole collapse in strike-slip faulting stress regime.Employing the shear failure strength criteria such as Mohr-Coulomb﹑Drucker-Prager and Mogi-Coulomb as well to develop borehole stability model regarding horizontal wells in different faulting stress regimes, and determining the upper limit concerning safe drilling mud density with regard to taking into consideration the influence of shear fracture phenomenon. It has optimized the borehole stability model according to Mogi-Coulomb criterion is in line with practical field condition by virtue of comparing with three failure strength criteria with drilled cases, and quantitatively analyzing the effect of the anisotropy of in situ stress, Poisson’s ratio, cohesion, internal friction angle and pore pressure on safe drilling mud density window.It systemically analyzes the optimum drilling well trajectory with linear programming optimal solution theory on the condition that the borehole wall takes place shear failure in different faulting stress regime so as to efficiently guide the horizontal wells construction in shale formation.