The importance of experimental thermoporoelastic coefficients for the complete characterization of geothermal reservoirs from low enthalpy to supercritical systems

Abstract

Under isothermal conditions, classical elasticity of solids without pores requires only two moduli to describe the mechanical relationship between stresses and strains, which can be obtained in a simple 1-dimensional experiment. Thermoporoelasticity of hydrothermal rocks requires around 20 experimental coefficients, due to porosity and different measurement conditions which can be only done in 2-dimensional experiments, under distinct pressures and temperature variations. The main geomechanical parameters to study the behavior of rocks in any geothermal system are: porosity ?, absolute permeability k, variation of the fluid contained in the pores ?, density ?, Young's modulus E, global compressibility CB, Poisson's modulus ?, Lamé coefficient ?, shear modulus G, volumetric modulus KB, reverse poroelastic expansion H, restricted and unrestricted reverse specific storage M and R, respectively, Skempton's coefficient B and Biot's modulus b. Petrophysical properties are measured for each specific geothermal system, their values are unique, heterogeneous and temperature dependent. In addition to their geomechanical behavior, porous rocks also have a natural thermal response characterized by other types of parameters such as their volumetric thermal expansivity ?B, and ??, the thermal expansion of the pores.
Due to the natural porosity of geothermal rocks, thermoporoelastic coefficients can be measured experimentally under different conditions: Drained conditions, when the system is open, the fluid can leave the pores, and the solid skeleton fully supports the total compression. Undrained conditions, the system is closed and the fluid in the pores remains constant. There are also different types of compressibility: solid skeleton, pore, volumetric, jacketed, non-jacketed, etc. There is great complexity in the amount and way of measuring thermoporoelastic coefficients whose values explicitly depend on both the confining pressure and the value of the temperature under which the measurement is made.
The central purpose of this article is to define the main thermoporoelastic coefficients, describe how they are measured experimentally, and how they are derived algebraically from a base of only five arbitrary independent coefficients. The existing experimental gaps are clearly pointed out, showing the importance of the coefficients in the correct characterization of all types of geothermal reservoirs, from low enthalpy to supercritical systems.