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

Wellbore integrity is a key concern during injection and production of CH4/H2/CO2. A depleted porous and fractured reservoir with saline is normally selected for these gases storage. Multi-phase fluid flow in naturally fractured reservoirs (NFR) is a complex and important process affecting the injectivity, productivity and fractured skeleton deformation. Either during CH4/H2/CO2 injection/sequestration or extracting the gas process, induced deformation with non-isothermal fluid flow may occur, leading to significant thermal-hydraulic-mechanical (THM) responses of NFR which may generate substantial impact on the safety and efficiency of the project. Numerical methods such as finite element method is commonly applied in simulating THM responses in NFR. The advantage over the finite different method which is popular in reservoirs simulation is that implicit coupled formulation between solid skeleton deformation and fluid flow are much more efficient computationally. 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 effective stresses in vicinity of a wellbore subject to a non-hydrostatic in-situ stresses and non-isothermal condition are highlighted, Dual-porosity model is proposed. Both the hydraulic fracturing risk during injection and wellbore collapse during production are studied. For every 10 °C temperature rise at a wellbore may generate a tangential stress increase of 1 MPa which is equivalent to a formation strength reduction of the same magnitude. Similarly a tangential stress reduction of 1 MPa at a wellbore may delay a hydraulic fracturing due to such a temperature change. Accurate calculations of the THM induced effective stresses and temperature perturbations at a wellbore and inside the NFR are the focus in this paper. The model proposed and investigation in this study may be applied for CH4/H2/CO2 injection or sequestration design in NFR so that safe range of permissible surface pressure and temperature can be maintained.