Research On Wellbore Stability In A Transversely Isotropic Formation Considering Thermo-Poroelastic Effects

With the rapid development of shale gas exploration,wellbore stabiliy of stratified formation attracts more attention nowadays.A wellbore model based on fully homogeneous and isotropic hypothesis is no longer appropriate for stratums with distinct bedding structure and anisotropic strength,while the more preferable model is when referring to transversely isotropic assumption adopted in this study.Combined with thermal effect resulting from drilling operation and seepage effect owing to pressure differences between wellbore and formation,a theoretical research on thermo-poroelastic wellbore stabiliy has been conducted in this thesis.The main work undertaken in this paper is as follows(1)Stress distribution around wellbore(or at wellbore wall)is analyzed both in fully homogeneous isotropic formation and transversely isotropic formation.In addition,influences of anisotropy of elastic constants(Yang's modulus and Poisson's ratio)and bedding-plane attitudes are studied for the latter.(2)An anisotropic seepage model as well as seepage stress model aimed at horizontal well sections is derived,and seepage stress distribution is investigated under the condition of underbalanced drilling horizontal wells.(3)Wellbore-stratum temperature field model is established,then a fully implicit finite difference scheme is applied in solution.All the heat sources deriving from pressure losses and mechanical fricition are included.Temperature profiles within wellbore together with that in formation are obtained at same time.Also,thermal stress distribution is uncovered.(4)Thermo-poroelastic wellbore stability for bedding transversely isotropic layers together with homogeneous and isotropic formation is studied considering thermal effect and seepage effect separately or comprehensively.Mohr-Coulomb criterion and Mogi-Coulomb criterion are used parallelly and results of equivalent collapse density related to the two criterions are compared.For transversely isotropic condition,influencing factors,like anisotropy of elastic constants,bedding-plane attitudes,weak plane's effect,the strength of bedding planes,thermal and seepage effects,are considered in analysis procedure.This study builts a theoretical foundation of the more precise analysis of wellbore stability in transversely formation considering thermal and seepage effects,and the conclusions of this study are promising to guide optimal design before drilling at oil field.