Fluid Flow In Rocks Research Articles

P‐wave velocity and permeability distribution of sandstones from a fractured tight gas reservoir

Fractures are an important fabric element in many tight gas reservoirs because they provide the necessary channels for fluid flow in rocks which usually have low matrix permeabilities. Several sandstone samples of such a reservoir type were chosen for a combined study of rock fabric elements and petrophysical properties. Geological investigations of the distribution and orientation of the fractures and sedimentary layering were performed. In addition, laboratory measurements were carried out to determine the directional dependence of the permeability and P‐wave velocities. Higher permeability values are generally in the plane of the nearly horizontal sedimentary layering with regard to the core axis. With the occurrence of subvertical fractures, however, the highest permeabilities were determined to be parallel to the core axis.Compressional wave velocities were measured on spherical samples in more than 100 directions to get the VPsymmetry without prior assumptions. Below 50 MPa confining pressure, all samples show a monoclinic symmetry of the P wave velocity distribution, caused by sedimentary layering, fractures, and crossbedding. At higher confining pressure, sedimentary layering is approximately the only effective fabric element, resulting in a more transverse isotropic VPsymmetry. Using the geological‐petrophysical model introduced here, the complex symmetry of the VPdistributions can only be explained by the rock fabric elements. Furthermore, water saturation increases the velocities and decreases the anisotropy but does not change VPsymmetry. This indicates that at this state, all fabric elements, including the fractures, have an influence on P‐wave velocity distribution.

Electrokinetic in rocks: Laboratory measurements in sandstone and volcanic samples

The streaming potential due to fluid flow in rocks was measured on saturated sandstones. During triaxial failure test, the electrokinetic coupling coefficient was increased at about 72–86% of the failure stress. This increase is thought to be due to an increase of the macroscopic zeta potential in the shear zone where new surfaces are created and connected, allowing fluid to flow through these new cracks. The streaming potential variations could therefore be used as a precursor of the rupture. Measurements were also performed on volcanic samples, with the aim of using electrokinetic phenomena to monitor volcanic activity. Streaming potential was found to depend on the transport properties i. e., permeability and formation factor. Theoretical consideration suggest that such a dependence should be caused by surface conductivity, but this is not the case here.

Deformation-enhanced fluid flow in rocks

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The relative permeabilities of quartzites and schists during active metamorphism at mid‐crustal levels

The ‘hot spot’ near Bristol, New Hampshire was an area of high fluid flow during metamorphism. In order to determine the mechanisms of fluid flow through the area, we made a detailed study of the δ18O values of quartz from an outcrop of interlayered quartzites and schists. Results indicate that the permeability of the quartzite was very low in comparison to that of the schist.The δ18qtz values from schist layers are remarkably homogenous (15.5–15.8), in contrast to the δ18qtz values of schist units outside of the hot spot, which vary widely. Quartz veins that cut the quartzites and pinch out in the schists have the same δ18O values (15.7) as quartz from the schists. In the quartzites, δ18qtz values immediately adjacent to the veins and schists are similar to those in the veins and schists, but decrease by ∼1 permil in the centers of the layer.We interpret these results to indicate that fluid flow in the hot spot was mainly through the schists and fractures in the quartzites. The oxygen isotope exchange fronts in the quartzites prove that there was some flow from the the schists and the fractures into the quartzites. Comparisons of the shapes of these exchange fronts with exchange fronts predicted by two different models of fluid flow in rocks indicate that fluid transport in the quartzite was predominantly by advection and that the kinetic rates of isotope exchange were fast relative to the advective velocity of the fluid infiltrating the system. Considering the slow rate of oxygen isotope exchange between quartz and a fluid, this implies that fluid flow from the schist and vein fracture into the quartzite was extremely limited.

Innovative methods for quantifying poorly known parameters necessary for numerical basin modeling.

Deterministic models, in which mathematical equations are used to predict the occurrence of various phenomena in the subsurface, are widely used today in basin modeling. However, these models suffer from a number of weaknesses, including incorrect or incomplete equations, incorrect constants in the equations, and errors in input variables. Several alternatives to present deterministic basin models are illustrated here by showing their application in describing permeability.Self similarity, the basis of fractal theory, states that many patterns observed on one scale are also present on other scales. Measured permeability data from hand samples of reservoir rocks must be scaled to adequately describe effective formation permeabilities for carrier beds. Self similarity may also be valuable in evaluating the contribution of fracture permeability to fluid flow. A self-similarity approach may be useful even where self similarity is not strictly valid.A new method for predicting the occurrence of overpressuring and the volume of fluid expelled from both normally pressured and overpressured rocks is based on acquisition of empirical data on the maximum sedimentation rates at which normal pressuring can be maintained under well-constrained conditions. The parameters required to use this method (original and present porosity, viscosity of water, compressibility, present thickness, and recent sedimentation rate) can be obtained more accurately from empirical data than the quantities required for calculation of fluid flow from fine-grained rocks using Darcy's law.Tensional fractures can play a dominant role controlling fluid flow in rocks with low matrix permeabilities. The occurrence and intensity of tensile fracturing can be predicted from stress-strain considerations. It may also prove possible to estimate the degree of fracturing by applying self-similarity concepts to fault patterns observed from seismic data, or from empirical data on the occurrence of overpressuring.Great differences exist between human time scales, which govern reservoir engineering, and geologic time scales, which govern fluid movement in basins. Differences that are of acute importance to a reservoir engineer are often of no practical importance for basin modeling. In basin modeling it is most important to specify permeabilities of seals and of fine-grained rocks as they expell water and hydrocarbons. It is also important to determine the minimum permeabilities capable of permitting long-distance lateral fluid flow. Obtaining reliable data for such rocks should be a main objective of research on basin modeling.

Review of the chromatographic “plate” theory with reference to fluid flow in rocks and sediments

The classical theories of Martin and Synge are used to demonstrate the possibility of Chromatographic flow in sedimentary rocks. A simple experiment is described which defines some of the limits of chromatography in quartz sand and in aqueous media.