[1] An experiment was designed to observe water flow in a simple analogue fractured rock network. The fracture network was modeled by the gaps between an array of limestone blocks, whose permutations generated different realizations of the network. Three out of five of these realizations displayed flow convergence, in contrast to that predicted by porous continuum models. Such convergence occurs because fracture intersections act as capillary barriers that integrate or focus flow within the network.

tion, 1995 through 1997(Faybishenko et al., 1998, and the Hell's Half Acre BasaltThe behavior of unsaturated flow was investigated in a laboratory Infiltration tests, 1997 through 2000 (Wood and Podgormodel. A constant and uniform supply of chemically equilibrated ney, 1999;Podgorney et al., 2000;Wood et al., 2000b). water was introduced to the upper surface of three artificial fractures These tests were conducted at the 100-, 10-, and 1-m in a surrogate fracture network consisting of a thin wall of uncemented limestone blocks. Water was collected from the lower boundary via scales, respectively. Due to the very different behavior fiberglass wicks placed at the bottom of each artificial fracture. Eight observed at these different scales, it can be concluded experiments were conducted to evaluate the repeatability of flow that fractured basalt vadose zone must be conceptualunder nearly identical conditions and to characterize general patterns ized at a hierarchy of scales (Faybishenko et al., 2001). in flow behavior. Collected data revealed that flow generally con-Although the hierarchy of scales approach describes verged to a single fracture in the bottom row of blocks. Periods of behavior of a fractured basalt vadose zone at multiple pathway switching were observed to be more common than periods scales of observation, it fails to provide a means for with steady, constant flow pathways. We noted the importance of scaling between scales of observation and does not defracture intersections for integrating uniform flow and discharging a scribe the unit processes generating flow dynamics ob-"fluid cascade," where water advances rapidly to the next capillary served during field testing. barrier creating a stop and start advance of water through the network. Under very similar initial moisture and boundary conditions, flow in Common to this conceptualization, and indeed to the system was less repeatable than expected. The results of this other studies conducted on a wide variety of fractured simple experiment suggest that the interaction of multiple fracture rock types, is the suggestion that at large spatial scales intersections in a network creates flow behavior not generally recogand relatively short temporal scales, fracture networks nized in popular conceptual and numerical models, (i.e., convergence can facilitate the deep penetration of dissolved contamiof flow, pathway switching, and fluid cascades).

Deep basalt formations within large igneous provinces have been proposed as target reservoirs for carbon capture and sequestration on the basis of favorable CO 2 -water-rock reaction kinetics that suggest carbonate mineralization rates on the order of 10 2 -10 3 d. Although these results are encouraging, there exists much uncertainty surrounding the infl uence of fracture-controlled reservoir heterogeneity on commercial-scale CO 2 injections in basalt formations. This work investigates the physical response of a low-volume basalt reservoir to commercial-scale CO 2 injections using a Monte Carlo numerical modeling experiment such that model variability is solely a function of spatially distributed reservoir heterogeneity. Fifty equally probable reservoirs are simulated using properties inferred from the deep eastern Snake River Plain aquifer in southeast Idaho, and CO 2 injections are modeled within each reservoir for 20 yr at a constant mass rate of 21.6 kg s -1 . Results from this work suggest that (1) formation injectivity is generally favorable, although injection pressures in excess of the fracture gradient were observed in 4% of the simulations; (2) for an extensional stress regime (as exists within the eastern Snake River Plain), shear failure is theoretically possible for optimally oriented fractures if S h ≤ 0.70S V ; and (3) low-volume basalt reservoirs exhibit sufficient CO 2 confi nement potential over a 20 yr injection program to accommodate mineral trapping rates suggested in the literature.

The behavior of unsaturated flow was investigated in a laboratory model. A constant and uniform supply of chemically equilibrated water was introduced to the upper surface of three artificial fractures in a surrogate fracture network consisting of a thin wall of uncemented limestone blocks. Water was collected from the lower boundary via fiberglass wicks placed at the bottom of each artificial fracture. Eight experiments were conducted to evaluate the repeatability of flow under nearly identical conditions and to characterize general patterns in flow behavior. Collected data revealed that flow generally converged to a single fracture in the bottom row of blocks. Periods of pathway switching were observed to be more common than periods with steady, constant flow pathways. We noted the importance of fracture intersections for integrating uniform flow and discharging a “fluid cascade,” where water advances rapidly to the next capillary barrier creating a stop and start advance of water through the network. Under very similar initial moisture and boundary conditions, flow in the system was less repeatable than expected. The results of this simple experiment suggest that the interaction of multiple fracture intersections in a network creates flow behavior not generally recognized in popular conceptual and numerical models, (i.e., convergence of flow, pathway switching, and fluid cascades).

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