Deformation and pore pressure in dehydrating gypsum under transiently drained conditions

Abstract

The development of pore-fluid pressure in dehydrating rocks is a competition between the reaction, which produces water, and the change in permeability caused by the reduction in solid volume. Our study explores the development of pore-fluid pressure and the mechanical behavior of dehydrating rocks under conditions where the relative drainage is transient, i.e. drainage evolves as the reaction progresses. The dehydration system studied was gypsum to bassanite plus water. Hydrostatic pressure and axial compression experiments were performed at 23° to 150°C, confining pressures ( P c) of 0.1 to 200 MPa, pore pressures ( P p) of 10 to 100 MPa, and strain rates of 7 × 10 −7 to 6 × 10 −5 1/s. The volume of water expelled from the sample during dehydration, the differential stress, the reaction ratio, and the porosity distribution were determined as a function of time. The expulsion of water with time is divided into three stages broadly defined as: (I) the formation of a connected pore network; (II) the maximum water expulsion rate; and (III) the completion of the reaction. At high effective pressures ( P e = P c − P p), gypsum deforms by plastic flow and cataclasis at temperatures below that necessary for dehydration. Above the dehydration temperature, samples show weakening and embrittlement, indicative of low effective pressures, when deformed in stage I. in stages II and III, the sample strength increases gradually with time, eventually exceeding the pre-dehydration strength of gypsum. These results suggest that high pore pressures (low effective pressures) are only transient and occur in stage I. The exceptional strengthening in stages II and III occurs because the new phase, bassanite, is stronger than gypsum. The evolution of pore pressure with time in a transiently drained dehydrating system has been modeled by incorporating fluid release and porosity change rates into a hydraulic diffusion equation. The numerical simulations show that initially, pore pressure increases following an undrained path and a pore pressure in excess of hydrostatic is possible. Later, the increase in porosity and permeability makes drainage more efficient and the pore pressure decreases rapidly. The magnitude and duration of the excess pore pressure are sensitively dependent on the dehydration kinetics and the hydro-mechanical properties. The pore pressure peak predicted from the numerical simulations correlates with the experimental results that weakening and embrittlement occur only in stage I. Our results suggest that even under natural conditions where rocks have a finite drainage, high excess pore pressures may occur.