MATHEMATICAL-MODELING OF GRAVITATIONAL COMPACTION AND CLAY DEHYDRATION IN THICK SEDIMENT LAYERS

Gravitational compaction is an important process in sedimentary basins which controls the reduction of porosity with burial depth and the development of high pore fluid pressures. Often, sediments contain hydrated, expandable clay (smectite) that can undergo a transition to a dehydrated, non-expandable clay (illite). During dehydration, structural water bound within the sheet layers of smectite is released into the pore space, which can increase the pore pressure and influence geological processes such as solute transport, hydrocarbon migration and hydrothermal fracturing. A multicomponent continuum mechanics model that accounts for Darcy's Law, Terzaghi's principle of effective stress and a thermally activated dehydration reaction is derived and solved numerically. A closed-form solution is available in the limiting case of hydrostatic pore pressure and no dehydration. The results show that excess pore-pressure development is controlled by the sedimentation parameter, the dimensionless ratio of the hydraulic conductivity to the sedimentation rate. For relatively impermeable sediments, chemically released water can increase the excess pore pressure by as much as 30 per cent, and the excess pressure persists over geological time-scales. The pressure contribution due to the excess pore water is important provided that the dehydration goes to completion at sufficient burial depth, which depends in part on the activation energy of the reaction. If sediments overlie a permeable basement, fluid can flow out of the sediments and relieve pore pressure throughout the sedimentary column.