TIME DEPENDENT FLUID FLOW IN NEAR-CRITICAL PERMEABILITY MODELS

Several indications are being collected all over the world that fluids released from the mantle have important effects on seismic and volcanic activity. The upward migration of fluids extracted in this way from the asthenosphere is expected to take place at lithostatic pressure, up to the bottom of the brittle crust, where deviatoric stresses induced by tectonic motions become significant with respect to the gravity load. The steady or, maybe episodic, migration of these high pressure fluids, from below the brittle-ductile transition up to the hydrostatic aquifers, has significant effects on rock strength, according to the Coulomb failure criterion, and on the flow of resident fluids. Fluid flow within rocks is described in terms of rock permeability, according to Darcy law, and permeability is a complex function of the distribution of cavities in the rock, depending on their density, shape and degree of connectivity. Therefore, permeability varies considerably even among different samples of the same rock. Poro-elasticity is the discipline taking into account simultaneously the elastic property of the rock and the flow of compressible fluids filling its pores. Several conceptual models of a permeable rock have been proposed, which usually provide a proportionality between porosity and permeability, but the constant of proportionality may differ considerably among different models. Poro-elasticity allows for porosity variations due to changes of confining pressure and pore pressure, but permeability is usually assumed to be a constant rock property, which is clearly inappropriate. “Dual porosity models” may account for the very different permeabilities related to grain size pores and to macroscopic fractures in a rock, but model parameters are again assumed to be constant. However, dual porosity models have the important merit of distinguishing between a pervasive small scale permeability network and a larger scale network of fractures along which enhanced fluid motion takes place. A dislocation model is presented here which describes a pervious rock in terms of an “intrinsic permeability” (e.g. related to connected intergranular porosity) and a pressure dependent permeability, related to the opening of cracks when the pore pressure exceeds the confining pressure. The resulting “effective permeability” is found to be extremely sensitive to the parameters describing the distribution of cracks and to the pore pressure, which is, in turn, strongly controlled by variations of the effective permeability. If only the crack opening depends on pressure (while its length and density remain constant), the effective permeability has lower and upper bounds, which may easily differ by a few orders of magnitude. Moreover, in a near-critical stress state, when the stress intensity factor is close to failure conditions, crack propagation takes place, which is accompanied by pore pressure drop within the crack. The small scale pressure transients, taking place in the surrounding grain size porosity, are studied. An important result is that crack extension takes place in “jerks”, delayed by pore-pressure drop and suction takes place of fluids resident within the inter-granular pore space, toward the crack network, where their mobility is greatly enhanced. We show that the present permeability model provides a useful unifying tool for the understanding and the interpretation of several observations (seismic, geochemical and hydraulic) commonly made in tectonically active regions.