56 Geophysics in Geothermal Exploration For a geophysicist, rock is composed of three elements: the matrix, the porosity, and the fluids. The matrix is the solid part constituted of minerals, each of them characterized by their physical properties such as density, velocity, resistivity, susceptibility, etc. Porosity refers to the amount of empty space within the rock, often described as the fraction of the rock’s total volume that is occupied by voids, cracks, or pores. These pores can vary greatly in size, shape, and connectivity, and they significantly influence the rock’s ability to store fluids. Porosity is expressed as a percentage, with higher values indicating more pore space. The arrangement and distribution of these pores within the matrix are crucial for determining other properties, such as permeability, which describes how easily fluids can move through the rock. Permeability seems to be the most important (and hard to determine) property for all reservoir problems. It controls whether the rock can deliver or transmit fluids or not. The fluids are pore-filling materials: water, oil, gas, air, and pollutants, each of them having specific physical properties. These fluids, with their specific physical properties like viscosity and conductivity, play a major role in the rock’s overall behavior. The physical properties of these elements will condition the physical properties of the rock (Figure 2.1). Figure 2.1 Physical properties of rocks (after D. Chapellier, IFP School course, personal communication).
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