EFFECTS OF REACTION KINETICS AND FLUID DRAINAGE ON THE DEVELOPMENT OF PORE PRESSURE EXCESS IN A DEHYDRATING SYSTEM

Abstract

Fluid is released by dehydration reactions during prograde metamorphism. If the Claperyron slope for the dehydrating reaction is positive, then there is a net decrease in the total solid volume, which implies an irreversible increase in porosity. If the dilation of the pore space is insufficient to provide storage for all the released fluid, then pore pressure excess is generated, and if it becomes sufficiently high, it may lead to brittle fracturing. The time scale for pressure generation and the pore pressure excess can be maintained over long duration hinge on the interplay of reaction kinetics and fluid drainage. Motivated by experimental and microstructural observations, a hydrological model is developed that incorporates dehydration kinetics and its pressure dependence. Analytic solutions were derived for the undrained development of pore pressure. Whether lithostatic pressure may be exceeded hinges on magnitude of the overstep in temperature and corresponding equilibrium pressure. The time scale for development of pore pressure depends on the trade-off between poroelasticity and the pressure sensitivity of reaction rate. A finite difference model was also developed to simulate the progressive development of pore pressure excess, dehydration and porosity development. The model captures the experimental observation in gypsum of a reaction front that progressively propagates from the drained end toward the undrained end of a laboratory sample. It is also in reasonable agreement with experimental data on fluid drainage and porosity production.