Simulating Coupled THM Problem and Wellbore Integrity Analyses with Two-Phase Fluid Flow in Naturally Fractured Reservoirs

Multi-phase fluid flow in naturally fractured reservoirs (NFR) is a complex and important process to be studied in energy and environmental industries. Either during CH4/H2/CO2 injection/sequestration or thermal EOR process, both fluid pressure and temperature changes may take place which will lead to significant thermal-hydraulic-mechanical (THM) responses that will impact both safety and efficiency of the project. Simulating THM responses in NFR, Formulation for an implicit pressure and explicit saturation (IMPES+T) finite element approach with an explicit thermal coupling is developed. Different shape functions, i.e. Petrov-Galerkin method, are applied to the saturation, displacement, fluid pressures, and temperature with convective energy transport calculation. In addition, a simplified coupled hydraulic-mechanical (HM) algorithm is proposed for displacement-pressures calculations, which is also coupled to an independently calculated convective-conductive thermal process. The induced pore pressure and critical stresses are highlighted in a dual-porosity medium, evaluating the wellbore integrity during injection and production in naturally fractured formations. A thermally induced tangential stress increase of 1 MPa on the wellbore wall is equivalent to a formation strength reduction of the same magnitude and yet more than 50% increase of the effective tangential stress, the key stress affecting wellbore integrity, at the wellbore wall in the fracture system may be achievable in the naturally fractured media (NFM). 1. INTRODUCTION To simulate multiphase fluid flow, heat transport and skeleton deformation processes in NFR [Bai et al., 1993; Khalili, and Valliappan, 1996; Khalili, 2008], numerical method such finite element method has been applied to approximate solutions of the Thermal-Hydraulic-Mechanical (THM) responses. Isothermal multi-phase fluid flow and non-isothermal single-phase cases are extensively dealt with, respectively [Lewis and Ghafouri, 1997; Nair et al., 2005; Pao and Lewis, 2002; Gelet et al., 2012; Khoei et al., 2016; Arzanfudi and Al-Khoury, 2017]. Simulating CO2 injection, energy enhanced production and geothermal energy extraction processes in a NFR, a fully coupled THM model with multi-phase fluid flow is required and thus developed in this paper.