Planning and Analysis of Fractured Rock Injection Tests in the Cerro Brillador Underground Laboratory, Northern Chile

Abstract:

Early theories of fluid migration in unsaturated fractured rock hypothesized that

matrix suction would dominate flow up to the point of matrix saturation. However,

experiments in underground laboratories such as the ESF (Yucca Mountain, NV) have

demonstrated that liquid water can migrate significant distances through fractures in

an unsaturated porous medium, suggesting limited interaction between fractures and

unsaturated matrix blocks and potentially rapid transmission of recharge to the sat-

urated zone. Determining the conditions under which this rapid recharge may take

place is an important factor in understanding deep percolation processes in arid areas

with thick unsaturated zones. As part of an on-going, Fondecyt-funded project (award

11150587) to study mountain block hydrological processes in arid regions, we are plan-

ning a series of in-situ fracture flow injection tests in the Cerro Brillador/Mina Escuela,

an underground laboratory and teaching facility belonging to the Universidad la Serena,

Chile. Planning for the tests is based on an analytical model and curve-matching

method, originally developed to evaluate data from injection tests at Yucca Mountain

(Fairley, J.P., 2010, WRR 46:W08542), that uses a known rate of liquid injection to

a fracture (for example, from a packed-off section of borehole) and the observed rate

of seepage discharging from the fracture to estimate effective fracture aperture, matrix

sorptivity, fracture/matrix flow partitioning, and the wetted fracture/matrix interac-

tion area between the injection and recovery points. We briefly review the analytical

approach and its application to test planning and analysis, and describe the proposed

tests and their goals.