Abnormal fluid pressures caused by deposition and erosion of sedimentary basins
Gravitational loading or unloading due to deposition or erosion in sedimentary basins is one of the most common mechanisms responsible for abnormal fluid pressures in geologic formations. This paper examines the integrated response of fluid pressures to both deposition and erosion using vertical one-dimensional numerical models. The modeling results show that vertical loading and unloading of sedimentary basins containing thick interbeds of low permeability can give rise to abnormal fluid pressure-depth trends that closely resemble those observed in the field. On a geological time scale, an under-pressured formation may become normally pressured or over-pressured and vice versa because of transient flow induced by vertical geologic movements. A case study in the Guymon area of Texas County, Oklahoma, shows that there are two underpressured reservoirs in this area. The burial curve constructed from geologic information indicates that this area has undergone multiple subsidence and uplift events. Based on stratigraphic information, a vertical geologic section of this area is represented by a numerical model consisting of two reservoir formations and two shale (confining) units. The numerical model is able to reproduce the observed pressure abnormalities based on the drill-stem test (DST) data. The hydraulic conductivities of the two shale formations are estimated to be 4.2 × 10 and 5.1× 10 m s from model calibration.q 1998 Elsevier Science B.V.
Uncertainty Quantification of geochemical and mechanical compaction in layered sedimentary basins
Abstract In this work we propose an Uncertainty Quantification methodology for sedimentary basins evolution under mechanical and geochemical compaction processes, which we model as a coupled, time-dependent, non-linear, monodimensional (depth-only) system of PDEs with uncertain parameters. While in previous works (Formaggia et al. 2013, Porta et al. 2014) we assumed a simplified depositional history with only one material, in this work we consider multi-layered basins, in which each layer is characterized by a different material, and hence by different properties. This setting requires several improvements with respect to our earlier works, both concerning the deterministic solver and the stochastic discretization. On the deterministic side, we replace the previous fixed-point iterative solver with a more efficient Newton solver at each step of the time-discretization. On the stochastic side, the multi-layered structure gives rise to discontinuities in the dependence of the state variables on the uncertain parameters, that need an appropriate treatment for surrogate modeling techniques, such as sparse grids, to be effective. We propose an innovative methodology to this end which relies on a change of coordinate system to align the discontinuities of the target function within the random parameter space. The reference coordinate system is built upon exploiting physical features of the problem at hand. We employ the fact that the locations of the material interfaces display a smooth dependence on the random parameters and are therefore amenable to sparse grid polynomial approximations. We showcase the capabilities of our numerical methodologies through two synthetic test cases. In particular, we show that our methodology reproduces with high accuracy multi-modal probability density functions displayed by target state variables (e.g., porosity).
Geologic Field Evidence Suggesting Membrane Properties of Shales: ABSTRACT
Some anomalous pressure and salinity data observed within oil- and water-bearing reservoir rocks can not be explained by prior theories in hydrodynamics and geochemistry. Laboratory evidence has shown that compacted clay minerals act as semipermeable membranes and thereby exhibit osmotic-pressure and salt-filtration effects. Osmotically induced pressure and salt filtering occur in reservoir rocks adjacent to shales presumably serving as semipermeable membranes. Osmotic conditions might result from differences across a shale of salt concentration. Pressure would tend to increase in the reservoir rock on the emergent side of the shale membrane and decrease on the influx side under osmotic conditions. Cross-formational flow through a shale membrane may also cause salt filter ng and thereby increase the salinity on the influx side of the membrane. End_Page 953------------------------------ Three widely separated areas in North America (central Alberta, Canada; San Juan Basin, New Mexico and Colorado; and Wheeler Ridge anticline, San Joaquin Valley, California) have anomalous potentials and salinities that may be explained by the movement of water cross-formationally through shales acting as semipermeable membranes. Pressure and salinity anomalies from other areas possibly may be explained by shale-membrane phenomena. End_of_Article - Last_Page 954------------