POROTHERMOELASTIC MODEL FOR ANALYSIS OF STRESS STATE AROUND WELLBORES IN ANISOTROPIC ROCK

Abstract

Analyzing the stress state around wellbores, and thereby the safe mud pressure window, is an indispensable task in the design as well as assessment of wellbore stability. As considering a realistic behavior model of materials, distributions in the literature mainly referred to the development of analytical or numerical calculation models without mentioning the stress state around the wellbore related to the form of well wall instability. This study aims to analyze the stress state of the rock mass around the wellbore in a homogenous porothermoelastic anisotropic rock based on the finite element method. Two scenarios involving thermal conditions at the well wall are taken into account, i.e. cases of “cooling” and “heating” the wellbore. The coupled thermo-hydro-mechanical behavior model of the rock is used to describe instantaneously the thermic, hydraulic, and mechanical processes and their reciprocal interactions. The results showed that the temperature conditions on the well wall strongly affect the stress distribution around the wellbore, and thereby the stability of the well wall. For the cooling case, the compressive stress around the well wall is not large, but high tensile stress can appear, especially for the axial stress component which can reach 23 MPa. For the heating case, both the compressive tangential and axial stresses with very high values occur around the wellbore ranging from 70-75 MPa. The parametric study showed that the degree of anisotropy of Young’s modulus, thermal expansion coefficient, as well as bedding angle of the transversely isotropic rock, strongly affect the stress distribution around the wellbore