Fluid Flow In Rock Fractures Research Articles

Non-linear regimes of fluid flow in rock fractures

Non-linear regimes of fluid flow in rock fractures

Stress-controlled localization of deformation and fluid flow in fractured rocks

Abstract The discrete-element method (UDEC — Universal Distinct Element Code) was used to numerically model the deformation and fluid flow in fracture networks under a range of loading conditions. A series of simulated fracture networks were generated to evaluate the effects of a range of geometrical parameters, such as fracture density, fracture length and anisotropy. Deformation and fluid flow do not change progressively with increasing stress. Instability occurs at a critical stress and is charzacterized by the localization of deformation and fluid flow usually within intensively deformed zones that develop by shearing and opening along some of the fractures. The critical stress state may be described in terms of a driving stress ratio, R = (fluid pressure — mean stress)/1/2 (differential stress). Instability occurs where the R ratio exceeds some critical value, R C , in the range – 1 to −2. At the critical stress state, the vertical flow rates are characterized by a large increase in both their overal magnitude and degree of localization. This localization of deformation and fluid flow develops just prior to the critical stress state and may be characterized by means of multifractals. The stress-induced criticality and localization displayed by the models is an important phenomenon, which may help in the understanding of deformation-enhanced fluid flow in fractured rock masses.

Detecting Sub-Surface Groundwater Flow in Fractured Rock Using Self-Potential (SP) Methods

In the wine district of Clare Valley in South Australia, the natural-voltage SP-signal generated by fluid flow in fractured rock during a pumping test was carefully monitored over time. From ten electrode-locations surrounding the pumping well logged every 15 s, the drawdown cone produced by pumping was determined on basis of the SP measurements together with laboratory measurements of the grain-boundary ? (zeta) potential. Such measurements allowed calculation of the fractured-rock aquifer?s transmissivity and average permeability. Results were confirmed by piezometer measurements to the extent that data were available. The study has revealed that SP-signals generated during pumping tests are of complex nature. However, if the pumping test is sufficiently long to allow the signal to stabilise, and careful field procedures are in place, then SP-measurements have the potential to add significant hydrogeological information. SP-measurements are relatively easy and cheap, and are, contrary to traditional hydrogeological methods, not restricted to the locations of existing piezometers, which is particularly useful in fractured-rock aquifers.

Detecting subsurface groundwater flow in fractured rock using self-potential (SP) methods

In the wine district of Clare Valley in South Australia, the natural-voltage SP signal generated by fluid flow in fractured rock during a pumping test was carefully monitored by time-continuous measurements. From ten electrode locations surrounding the pumping well, the drawdown cone produced by pumping was determined on the basis of the SP measurements together with laboratory measurements of the grain-boundary ζ (zeta)-potential. Such measurements allowed calculation of the fractured-rock aquifer's transmissivity and average permeability. Results were confirmed by piezometer measurements to the extent that data were available. The study has revealed that SP signals generated during pumping tests are of a complex nature. However, if the pumping test is sufficiently long to allow the signal to stabilise, and careful field procedures are in place, then SP measurements have the potential to add significant hydrogeological information. SP measurements are relatively easy and cheap, and are, contrary to traditional methods, not restricted to the locations of existing piezometers, which is particularly useful in fractured-rock aquifers.

Flow and Transport Through Unsaturated Fractured Rock—Second Edition.

Edited by DANIEL D. EVANS, THOMAS J. NICHOLSON, and TODD C. RASMUSSEN. Geophysical Monograph 42. American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. 2001. 196 p. $38.50 (AGU Member) $55.00 (Nonmember). ISBN 0-87590-983-3. The subject of fluid flow in fractured rock has long

Non‐linear flow in fractured rock

This paper deals with theory and computation of fluid flow in fractured rock. Non‐Darcian flow behavior was observed in pumping tests at the geothermal research site at Soultz‐sous‐Forêts (France). Examples are examined to demonstrate the influence of fracture roughness and pressure‐gradient dependent permeability on pressure build‐up. A number of test examples based on classical models are investigated, which may be suited as benchmarks for non‐linear flow. This is a prelude of application of the non‐linear flow model to real pumping test data. Frequently, conceptual models based on simplified geometric approaches are used. Here, a realistic fracture network model based on borehole data is applied for the numerical simulations. The obtained data fit of the pumping test shows the capability of fracture network models to explain observed hydraulic behavior of fractured rock systems.

Effect of contact obstacles on fluid flow in rock fractures

Rock fractures have both conductive areas and impermeable obstacles, where the two rock surfaces are in contact. This paper aims at a better understanding of the effect of contact obstacles on fluid flow. Finite element simulations through rock fractures with conductive and contact areas were conducted to test validity of the theoretical equation. The contact correction term (1–2c) was modified to (1–2.4c), yielding better estimates when compared with measured hydraulic apertures, especially at a high fractional contact area. The modified correction term (1–2.4c) reflects a strong impact on fluid flow at high fractional contact areas. As some void areas surrounded by contact regions do not contribute to fluid flow, incorporation of these areas into contact areas leads to the better calculation of fluid flow through rough fractures with contact obstacles.

Hydraulic well testing inversion for modeling fluid flow in fractured rocks using simulated annealing: a case study at Raymond field site, California

Cluster variable aperture (CVA) simulated annealing (SA) is an inversion technique to construct fluid flow models in fractured rocks based on the transient pressure data from hydraulic tests. A two-dimensional fracture network system is represented as a filled regular lattice of fracture elements. The algorithm iteratively changes element apertures for a cluster of fracture elements in order to improve the match to observed pressure transients. This inversion technique has been applied to hydraulic data collected at the Raymond field site, CA to examine the spatial characteristics of the flow properties in a fractured rock mass. Two major conductive zones have been detected by various geophysical logs, geophysical imaging techniques and hydraulic tests; one occurring near a depth of 30 m and the other near a depth of 60 m. Our inversion results show that the practical range of spatial correlation for transmissivity distribution is estimated to be approximately 5 m in the upper zone and less than 2.5 m in the lower zones. From the televiewer and other fracture imaging logs it was surmised that the lower conductive zone is associated with an anomalous single open fracture as compared to the upper zone, which is an extensive fracture zone. This would explain the difference in the estimated practical range of the spatial correlation for transmissivity.

Assessment of an equivalent porous medium for coupled stress and fluid flow in fractured rock

The equivalent porous medium (EPM) for coupled stress and fluid flow in fractured rock was assessed. The assessment was focused on the contributions of the seepage-induced force to the system's equilibrium and deformation as well as fluid flux through the fractures, with the special emphasis on the inter-relations among these three aspects. Seepage-induced average stress was calculated for the EPM. And it was proved, with respect to the fluid flow system (FFS) proposed by Long et al. [Long JCS, Remer JC, Wilson CR, Witherspoon PA. Porous medium equivalents for networks of discontinuous fractures. Water Resour Res 1982;18:645–658], that the equivalence of the equilibrium contribution of the seepage-induced force is unconditionally satisfied in the EPM. The equivalence of fluid flux and seepage-induced deformation was evaluated using numerical techniques based on the FFS, in which the seepage-induced deformation was considered as ‘tensile’ deformation under fluid pressures. The numerical simulation results of the two cases suggest that the EPM established in terms of fluid flux based on the FFS guarantees equivalence of coupled stress and fluid flow. Also, the numerical simulation results indicate that the percolation theory can not be applied to a system with fractures of remarkably different size and hydraulic conductivity.

Numerical simulation of radon transport from subsurface to buildings

The finite element code FRACTure was conceived for the stimulation of forced fluid flow in fractured rock. For the treatment of radon transport through the subsurface only minor changes were necessary in this code (extension by a radioactive decay term). The calculations performed so far simulate steady state pressure and radon concentration fields in the ground surrounding a cylindrical building. Comparisons of numerical6and analytic calculations for a simple geometry show excellent agreement. Successive simulations demonstrate the significance of individual transport mechanisms. All models assume constant underpressure in the building and the validity of Darcy's law for mass transport in the underground as well as Fick's law for molecular dispersion. The results show that the radon transport by advection and by diffusion strongly depends on the gas permeability of the underground. The source region of indoor radon extends over a limited volume of a few meters only. In soils with low permeability the diffusive flux is dominating even at high pressure differences between the building interior and the sub-surface. In these cases the radiation risk due to radon entry is small. On the other hand a high soil gas permeability leads to a strong increase in radon entry into the building. For these advective dominated regimes even small pressure changes produce large changes in the indoor radon content.

A double medium model for diffusion in fluid-bearing rock

The concept of a double porosity medium to model fluid flow in fractured rock has been applied to model diffusion in rock containing a small amount of a continuous fluid phase that surrounds small volume elements of the solid matrix. The model quantifies the relative role of diffusion in the fluid and solid phases of the rock. The fluid is the fast diffusion path, but the solid contains the volumetrically significant amount of the diffusing species. The double medium model consists of two coupled differential equations. One equation is the diffusion equation for the fluid concentration; it contains a source term for change in the average concentration of the diffusing species in the solid matrix. The second equation represents the assumption that the change in average concentration in a solid element is proportional to the difference between the average concentration in the solid and the concentration in the fluid times the solid-fluid partition coefficient. The double medium model is shown to apply to laboratory data on iron diffusion in fluid-bearing dunite and to measured oxygen isotope ratios at marble-metagranite contacts. In both examples, concentration profiles are calculated for diffusion taking place at constant temperature, where a boundary value changes suddenly and is subsequently held constant. Knowledge of solid diffusivities can set a lower bound to the length of time over which diffusion occurs, but only the product of effective fluid diffusivity and time is constrained for times longer than the characteristic solid diffusion time. The double medium results approach a local, grain-scale equilibrium model for times that are large relative to the time constant for solid diffusion.