Naturally fractured reservoirs are considered extremely challenging in terms of accurate recovery prediction because of their complexity and heterogeneity. Conventional simulators only consider pore compressibility as a geomechanical parameter for simulation and assume permeability and porosity as static or pressure-dependent variables. These assumptions are insufficient from a physics point of view because porosity and permeability are functions of effective stress and temperature as well as pressure. Thus, parameter impact on both reservoir characterization and simulation processes should be considered via a thermohydro- mechanically (THM) coupled approach for a more precise simulation. In this paper, a review of THM coupling methods in the petroleum industry is presented, with emphasis on naturally fractured reservoirs. The governing equations for a thermohydro- mechanical coupling are introduced in a fully coupled formulation, and applications of this approach are discussed.
In the last two decades, there has been a strong emphasis on the importance of geomechanics in petroleum engineering. In reservoir management, geomechanics plays a role as a multidisciplinary aspect among the various other engineering specialities (geology, fluid flow, and thermodynamics). In fact, the term "geomechanics" is often applied very broadly to describe a wide range of reservoir phenomena. The origins of formal geomechanics are based on the concept of effective stress and consolidation for incompressible solid grains formulated by Terzaghi in 1936. Based on the concept of Terzaghi's effective stress, Biot investigated the coupling between stress and pore pressure in a porous medium and developed a generalized three-dimensional theory of consolidation with the basic principles of continuum mechanics [1]. He also to some degree extended poroelastic theory to anisotropic and nonlinear materials. Biot's theory and published applications are oriented more toward rock mechanics than fluid flow. Because of this, Biot's theory is less compatible with the conventional fluid-flow models (without geomechanics consideration) in terms of concept understanding, physical interpretation of parameters (e.g., rock compressibilities), and computer code development [2]. Skempton (1960) derived a relationship between the total stress and fluid pore pressure under undrained initial loading through the so-called Skempton pore pressure parameters A and B. Geerstma (1957) gave a better insight of the relationship among pressure, stress and volume, clarifying concept of compressibility in a porous medium. He also explained calculation of reservoir porosity using volumetric strain. Van der Knaap (1959) extended Geertsma's work to nonlinear but elastic geomaterials, such as dense but uncemented sands. Nur and Byerlee (1971) proved that the effective stress law proposed by Biot is more general and physically sensible than that proposed by Terzaghi. In other developments that are relevant to coupled flow-stress problems, Ghaboussi and Wilson (1973) introduced fluid compressibility into classic soil mechanics consolidation theory. Rice and Cleary (1976) showed how to solve poroelastic problems by assuming pore pressure and stress as primary variables instead of displacements as employed by Biot.
