FLUID OVERPRESSURES AND PRESSURE SOLUTION IN THE CRUST

In this paper, we examine the coupled problem of fluid overpressure build-up, permeability and local mass transfer in the crust. Fluid flow in the crust is described by means of an advection-diffusion equation derived from the poro-elastic theory. A source term is added to the equation which could result from different processes: i.e., porosity reduction or fluid injection. The capability of the crust to develop and maintain fluid overpressures depends mainly on the magnitude of the source term and on the permeability. We start from a permeability model based on the statistical description of a population of cracks and we assume that permeability variation with depth is controlled both by crack closure and fracture density depth variation. Then, we focus our attention on the effect of porosity reduction. Pressure solution creep occurs on asperities on crack surfaces and this leads to a porosity reduction. The kinetic of porosity reduction is derived from a microstructural description of asperities. We have performed 1-D simulations in order to discuss the conditions under which fluid overpressures are expected. We review the mechanical properties of the upper crust and infer time constant for fracturation within the crust. Consequences for electrical and acoustic properties are also briefly discussed.