Groundwater Flow in Low-Permeability Environments
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WATER RESOURCES RESEARCH, VOL. 22, NO. 8, PAGES 1163-1195, AUGUST 1986 Groundwater Flow in Low-Permeability Environments Water Resources Division, U.S. Geological Survey, Reston, Virginia Certain geologic media are known to have small permeability; subsurface environments composed of these media and lacking well developed secondary permeability have groundwat'er flow sytems with many distinctive characteristics. Moreover, groundwater flow in these environments appears to influence the evolution of certain hydrologic, geologic, and geochemical systems, may affect the accumulation of pertroleum and ores, and probably has a role in the structural evolution of parts of the crust. Such environments are also important in the context of waste disposal. This review attempts to synthesize the diverse contributions of various disciplines to the problem of flow in low-permeability environments. Problems hindering analysis are enumerated together with suggested approaches to overcoming them. A common thread running through the discussion is the significance of size- and time-scale limitations of the ability to directly observe flow behavior and make measurements of parameters. These limitations have resulted in rather distinct small- and large-scale approaches to the problem. The first part of the review considers experimental investigations of low-permeability flow, including in situ testing; these are generally conducted on temporal and spatial scales which are relatively small compared with those of interest. Results from this work have provided increasingly detailed information about many aspects of the flow but leave certain questions unanswered. Recent advances in laboratory and in situ testing techniques have permitted measurements of permeability and storage properties in progressively "tight- er" media and investigation of transient flow under these conditions. However, very large hydraulic gradients are still required for the tests; an observational gap exists for typical in situ gradients. The applicability of Darcy's law in this range is therefore untested, although claims of observed non-Darcian behavior appear flawed. Two important nonhydraulic flow phenomena, osmosis and ultrafiltration, are experimentally well established in prepared clays but have been incompletely investigated, particularly in undisturbed geologic media. Small-scale experimental results form much of the basis for analyses of flow in low-permeability environments which occurs on scales of time and size too large to permit direct observation. Such large-scale flow behavior is the focus of the second part of the review. Extrapolation of small-scale experimental experience becomes an important and sometimes controversial problem in this context. In large flow systems under steady state conditions the regional permeability can sometimes be determined, but systems with transient flow are more difficult to analyze. The complexity of the problem is enhanced by the sensitivity of large-scale flow to the effects of slow geologic processes. One- dimensional studies have begun to elucidate how simple burial or exhumation can generate transient flow conditions by changing the state of stress and temperature and by burial metamorphism. Investi- gation of the more complex problem of the interaction of geologic processes and flow in two and three dimensions is just beginning. Because these transient flow analyses have largely been based on flow in experimental scale systems or in relatively permeable systems, deformation in response to effective stress changes is generally treated as linearly elastic; however, this treatment creates difficulties for the long periods of interest because viscoelastic deformation is probably significant. Also, large-scale flow simula- tions in argillaceous environments generally have neglected osmosis and ultrafiltration, in part because extrapolation of laboratory experience with coupled flow to large scales under in situ conditions is controversial. Nevertheless, the effects are potentially quite important because the coupled flow might cause ultra long lived transient conditions. The difficulties associated with analysis are matched by those of characterizing hydrologic conditions in tight environments; measurements of hydraulic head and sampling of pore fluids have been done only rarely because of the practical difficulties involved. These problems are also discussed in the second part of this paper. Introductiton ..............................................1163 Improved laboratory and in situ testing techniques have The small scale: experimental characterization of flow in demonstrated that the permeability of many geologic materi- tight I.ocks ......................................... 1165 als is very small. At the same time, no properly tested geologic ,rnyaraulic 5 , flow: measurement of parameters .............1165 Nonhydraulic flow phenomena ........................ .I171 media have proved to be entirely impermeable. Large regions Large low-permeability flow systems ......................1175 of low-~ermeabilit~lithologies are often signifi- Steady flow in low-permeability environments ............1177 cantly more permeable than small volumes of the same rock, P~r~islenceof transient flow ........................... 1177 owing to the presence of joints, fractures, and faults. Neverthe- Nonh lydraulic flow in large-scale systems ................1185 less, there is ample evidence that many sizable regions possess The f ield problem .....................................1187 Overvic:w ..............................................1188 low permeability, presumably because fractures either fail to Histo irical bias ........................................1188 develop, are not connected or transmissive, or tend to be Fund amental difficulties and the scientific method ........ 1189 sparse. For example, artesian systems often have extensive Notatio n ............................................... 1189 confining beds of quite low regional permeability. In certain other settings, one finds fluid pressures indicative of transient hydrodynamic conditions which, because they are unrelated to human activity, are presumably long lived. Such conditions This pa per is not subject to U.S. copyright. Published in 1986 by are explicable in the presence of large volumes of low- the Ameri~can Geophysical Union. permeability media but are otherwise difficult to explain. Ex- Paper rlumber 6W4392. amples include some of the "anomalous" pressures known 1163 from drilling in many parts of the world. In still other settings, and lone-term., flow behavior which cannot be observed. This distributions of ions in groundwater suggest the presence of raises the important point that where the permeability is extensive low-permeability bodies which act as semipermeable small, flow experiments encompassing significant volumes are membranes. not practical. Even areally extensive in situ tests, such as Low-permeability environments are most commonly associ- aquifer-confining layer tests, sample the tight confining layer ated with fine-grained sedimentary deposits such as shales and but a short distance in from its boundary. Usually a small clays. These occur in active and ancient sedimentary basins fraction of the confining layer is involved. Tests using bore- and in accretionary wedges at plate boundaries. Crystalline holes in tight rocks likewise involve relatively small volumes rocks generally prove to be relatively permeable at large scale of the rock. Thus, compared with the volumes of interest, because of extensive fracturing. However, at depth they may practicable laboratory and in situ tests are small in scale. Yet, have regionally low permeability because of healing or filling these are often the only source of information on-which to of fractures [Walder and Nur, 19841. Even in shallow crys- base analyses. Only in systems which are at steady state is it talline rocks, fractures may be sparse, leaving relatively large sometimes possible to directly measure the hydraulic conduc- unfractured areas [Balk, 19371, or they may be numerous but tivity of large volumes of tight media. The problem is much unconnected as Marsily [I9851 suggested is the case in the more complex if the flow is in a transient state because the French Massif Central. Certain other terranes may have small changes occur extremely slowly. One is, therefore, unable to permeability, including parts of the oceanic crust at depth observe the large-scale transient response to a known stress [Anderson et al., 19851 and salt and other evaporite deposits which often helps characterize more permeable' formations [Kreitler et al., 19851. Groundwater flow in such environ- [e.g., Randovet al., 19851. ments, which are low in permeability at the scale of interest, is Extrapolation of flow behavior in tight rock from small to the subject of this review. Flow under partially saturated con- large scale is fraught with difficulties. As indicated, the size ditions, occurring near the surface and in soils, will not be scale dependence of permeability is a problem; similar and less considered. The term "low permeability" is here applied to well understood difficulties are associated with the nonhy- media with hydraulic conductivity to water of approximately draulic conductivities governing various types of osmotic flow. 10-%/s and smaller. A less well recognized but highly significant problem concerns Fluid flow in permeable rocks has been extensively studied the time scale dependence of storage properties.