Dipping permeable sandstone bodies encased in overpressured low permeability mudstone have a characteristic pressure field: sandstone pressures follow the hydrostatic gradient while mudstone pressures have a steeper (often lithostatic) gradient. This pressure distribution drives fluid into the base of the sandstone and expels it at the crest. We use mudstone pressures predicted from porosity and measured sandstone pressures to describe the spatial variation in pressure in two Eugene Island 330 reservoirs (Gulf of Mexico). In one severely overpressured reservoir, bounding mudstones are less compacted at the reservoir crest than at the reservoir base, and we interpret that flow is focused along the reservoir and expelled at the crest. In the second reservoir, mudstone is compacted around the base of the sandstone, and we interpret pore fluids were drawn into the sandstone. Dipping sandstone bodies encased in overpressured mudstone regulate hydrocarbon migration, affect borehole stability, and impact slope stability.

We present an equilibrium model of methane venting through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon. Free gas supplied from below forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation. This system self-generates local three-phase equilibrium and allows free gas migration to the seafloor. Log and core data from Ocean Drilling Program (ODP) Site 1249 show that from the seafloor to 50 m below seafloor (mbsf), pore water salinity is elevated to the point where liquid water, hydrate and free gas coexist. The elevated pore water salinity provides a mechanism for vertical migration of free gas through the regional hydrate stability zone (RHSZ). This process may drive gas venting through hydrate stability zones around the world. Significant amount of gaseous methane can bypass the RHSZ by shifting local thermodynamic conditions.

Estimated pressures in overpressured Plio-Pleistocene muds of the Eugene Island 330 (E.I. 330) field (offshore Louisiana) differ from pressure measurements in adjacent sands in a consistent manner. At structural highs, fluid pressure in sand exceeds pressure in the adjacent mud; at structural lows, the relationship is reversed. Two physical models describe the origin of these pressure differences: rapid loading and steady-flow. Observation and theory suggest that stratigraphic layering focuses fluid flow toward structural highs. � 2000 Elsevier Science B.V. All rights reserved.

. The transport of reacting solutes in an equilibrium dissolution reaction leads to a moving boundary problem. Dependent on the boundary conditions we consider a two or three phase free boundary problem for a diffusion-convection and reaction process in a bounded domain. The system is driven out of equilibrium by a forced exchange of solutes with the exterior of the domain and is characterized by the dynamic interaction between flow conditions at the boundaries and a reaction-induced dissolution. Extending previous results, we consider arbitrary chemical equilibrium reactions with general equilibriumconditions. We prove well-posedness of the non-stationary problem and present a new numerical approximation by finite differences on an adaptive time dependent grid. Key words. free boundary problems, convection-diffusion-reaction, adaptive methods AMS subject classifications. 35R35, 76S05, 76T05 1. Introduction. Transport and chemical reactions have been extensively studied in recent yea...

This study explores the hydraulic response to pumping of leaky perched aquifers that receive continuous recharge. We explore whether distinct drawdown curves result that might reveal boundary effects or the position of the production well within the aquifer. Three-dimensional saturated-unsaturated simulations were developed to generate both circular (point-source recharge) and rectangular (line-source recharge) perched aquifer systems. Once the simulations stabilized at quasi steady-state in response to recharge, pumping at a constant-rate was imposed on the perched aquifer system. Diagnostic plots of the simulated log drawdown \ log time, as well as time-derivative curves, were analyzed. Results showed a repeatable, distinctive pattern of late-time negative slope-derivative curves that result from the size of the perched aquifer decreasing due to pumping and from continuing leakage through the aquitard as the system returns to steady-state. Because leakage decreases as the size and saturated thickness of the perched aquifer diminishes, additional water is available to meet pump demands and the rate of drawdown in the well decreases. This behavior is due to the small and constrained geometry of the aquifer. A lower pump rate required less reduction