Jane C. S. Long

Conceptual model of the geometry and physics of water flow in a fractured basalt vadose zone

A conceptual model of the geometry and physics of water flow in a fractured basalt vadose zone was developed based on the results of lithological studies and a series of ponded infiltration tests conducted at the Box Canyon site near the Idaho National Engineering and Environmental Laboratory. The infiltration tests included one 2-week test in 1996, three 2-day tests in 1997, and one 4-day test in 1997. For the various tests, initial infiltration rates ranged from 4.1 cm/d (4.75 ×10−7 m/s) to 17.7 cm/d (2.05×10−7 m/s) and then decreased with time, presumably because of mechanical or microbiological clogging of fractures and esicular basalt in the near-surface zone, as well as the effect of entrapped air. The subsurface moisture redistribution was monitored with tensiometers, neutron logging, time domain reflectrometry, and ground-penetrating radar. A conservative tracer, potassium bromide, was added to the pond water at a concentration of 3 g/L to monitor water flow with electrical resistivity probes and water sampling. Analysis of the data shows evidence of preferential flow rather than the propagation of a uniform wetting front. We propose a conceptual model describing the saturation-desaturation behavior of the basalt, in which rapid preferential flow occurs through the largest vertical fractures, followed by a gradual wetting of other fractures and the basalt matrix. Fractures that are saturated early in the tests may become desaturated thereafter, which we attribute to the redistribution of water between fractures and matrix. Lateral movement of water takes place within horizontal fracture and rubble zones, enabling development of perched water bodies.

Resolution and uncertainty in hydrologic characterization

Hydrologists have applied inverse techniques to obtain estimates of subsurface permeability and porosity variations and their associated uncertainties. Although inverse methods are now well established in hydrology, important aspects of inverse theory, the analysis of resolution, and the trade-off between model parameter resolution and model parameter uncertainty have not been utilized. In this paper the concept of model parameter resolution is incorporated into the analysis of hydrological experiments. Model parameter resolution is a measure of the spatial averaging implicit in estimates of a distributed hydrological property such as permeability. There are two important uses of resolution and uncertainty estimates in hydrology. The first use is to plan a hydrologic testing program. Resolution matrices can be developed for proposed well tests in a variety of synthetic media. Then the effectiveness of the test design can be evaluated in terms of model parameter resolution and uncertainty. Secondly, when real data are available and used in an inversion determining the distribution of hydrologic parameters, estimates of model parameter resolution and uncertainty analysis can indicate the reliability of the solution. For synthetic tests in which the hydraulic conductivity varies and porosity does not, it is found that tracer data can provide better spatial resolution of subsurface hydraulic conductivity variations than transient pressure data. Pressure data are most sensitive to hydraulic conductivity variations immediately surrounding the well. Both pressure and tracer data better determine barriers to flow rather than channels to flow. The methodology is applied to a set of transient pressure data gathered at the Grimsel Rock Laboratory of the Swiss National Cooperative for the Storage of Radioactive Waste. In the fracture under study a low hydraulic conductivity region appears to partition the fracture plane into two distinct zones.

An inverse technique for developing models for fluid flow in fracture systems using simulated annealing

One of the characteristics of flow and transport in fractured rock is that the flow may be largely confined to a poorly connected network of fractures. In order to represent this condition, Lawrence Berkeley Laboratory has been developing a new type of fracture hydrology model called an “equivalent discontinuum” model. In this model we represent the discontinuous nature of the problem through flow on a partially filled lattice. This is done through a statistical inverse technique called “simulated annealing.” The fracture network model is “annealed” by continually modifying a base model, or “template,” so that with each modification, the model behaves more and more like the observed system. This template is constructed using geological and geophysical data to identify the regions that possibly conduct fluid and the probable orientations of channels that conduct fluid. In order to see how the simulated annealing algorithm works, we have developed a synthetic case. In this case, the geometry of the fracture network is completely known, so that the results of annealing to steady state data can be evaluated absolutely. We also analyze field data from the Migration Experiment at the Grimsel Rock Laboratory in Switzerland.

An inverse approach to the construction of fracture hydrology models conditioned by geophysical data: An example from the validation exercises at the Stripa Mine

Abstract One approach for the construction of fracture flow models is to collect statistical data about the geometry and hydraulic apertures of the fractures and use this data to construct statistically identical realizations of the fracture network for fluid flow analysis. We have found that this approach has two major problems. One is that an extremely small percentage of visible fractures may be hydrologically active. The other is that on any scale you are interested in characterizing usually a small number of large features dominate the behaviour ([1] Transport Processes in Porous Media. Kluwer Academic, The Netherlands, 1989). To overcome these problems we are proposing an approach in which the model is strongly conditioned by geology and geophysics. Tomography is used to identify the large features. The hydraulic behaviour of these features is then obtained using an inverse technique called “simulated annealing.” The first application of this approach has been at the Stripa mine in Sweden as part of the Stripa Project. Within this effort, we built a model to predict the inflow to the Simulated Drift Experiment (SDE), i.e. inflow to six parallel, closely-spaced holes, the N- and W-holes. We predict a mean total flow of approx. 3.1 (l/min) into the six holes (two-holes) with a coefficient of variation near unity and a prediction error of about 4.6l/min. The actual measured inflow is close to 2l/min.

Large-scale hydraulic conductivity measurements in fractured granite

The large-scale hydraulic conductivity experiment at Stripa, Sweden, was an attempt to produce a macromeasurement of the average hydraulic conductivity of approximately 200,000 m/sup 3/ of low-permeability fractured granite. Groundwater seepage into a 33 m long, 5 m dia drift was measured as the net moisture pickup of a ventilation system. Water pressures were monitored at 90 locations in the rock mass. The experiment was designed to treat the rock as a porous medium. Analysis of test results indicates a behavior approximating radial flow in a porous medium. Tests made at three different drift air temperatures yielded very similar results. Computations indicate that the average hydraulic conductivity of the monitored rock mass, exclusive of a zone of lower conductivity immediately surrounding the drift, is approximately 9.8 x 10/sup -11/ m/sec. 7 references, 8 figures.

Joint seismic, hydrogeological, and geomechanical investigations of a fracture zone in the Grimsel Rock Laboratory, Switzerland

Author(s): Majer, E.L.; Myer, L.R.; Peterson, J.E.; Karasaki, K.; Long, J.C.S.; Martel, S.J.; Blumling, P.; Vomvoris, S.

An integrated approach for characterizing fractured reservoirs

Abstract Experience has shown that fractures and faults within a given array are not all equally conductive or well-connected. To investigate new techniques for locating conductive fracture flow paths, a series of high resolution (1 to 10 kHz) crosswell and single well seismic surveys and interference tests were conducted in a shallow five spot well array penetrating a fractured limestone formation. Two inverse approaches for constructing fracture flow models were applied to the interference test data. Both approaches successfully reproduced the transient pressure behaviour at the pumping and observation wells and indicated a preferential fracture flow path between two wells aligned in an east-northeast direction, the dominant direction of fracturing mapped in the area. Crosswell and single well seismic experiments were performed before and after air injection designed to displace water from the fracture flow path and increase seismic visibility. The crosswell experiments showed that replacement of water with gas produces significant changes in the seismic signal. The single well reflection surveys were able to precisely locate the position of the fracture flow path. This location was confirmed by core from a slant well which intersected a single open fracture at the targeted depth.