Analysis of the hydromechanical behavior of a flat-lying fracture during a slug test

doi:10.1016/j.jhydrol.2007.08.020

Journal of Hydrology

Volume 347, Issues 1–2, 15 December 2007, Pages 35-47

https://doi.org/10.1016/j.jhydrol.2007.08.020 Get rights and content

Summary

Slug tests dilate and contract fractures in aquifers. We developed a coupled hydromechanical analysis of an idealized fracture to evaluate effects of these deformations in fractured crystalline rock aquifers. The analysis shows the abrupt increase in pressure at the beginning of a slug test causes a slight opening of a fracture, but the fracture continues to open as the pressure falls in the wellbore. This happens because the pressure increases over an ever-widening area even though it decreases in the wellbore. Eventually decreasing pressure in the vicinity of the wellbore offsets the spreading pressure and the fracture starts to close, ultimately returning to its initial aperture when the pressure equilibrates. The displacement in the wellbore when the pressure head is rising is always smaller than when the head is falling, and this produces a loop-like, hysteresis when displacement is plotted as a function of pressure. A sensitivity analysis shows the displacement at the wellbore is sensitive to the magnitude and distribution of aquifer properties, suggesting that the displacement signal could be combined with pressure measurements to improve the interpretation of slug tests during aquifer characterization.

Section snippets

Conceptual model

Sub-horizontal fractures can be particularly important in rock aquifers because they provide a permeable framework that extends the influence of a well in all directions. Accordingly, we will idealize a fractured rock aquifer by considering only a single, flat-lying fracture embedded in a matrix of low permeability crystalline rock. The spacing between opposing fracture walls varies from as much as a few millimeters down to negligible values. Fluid can be transmitted through interconnected void

Theoretical analysis

Processes inferred to occur during a slug test in fractured rock will be represented by considering fluid flow and normal deformation within an axisymmetric fracture of radius, a, and aperture, δ. Fluid flow and normal deformation are coupled during the solution.

Results

A baseline analysis was developed using dimensions and parameters that approximate conditions at our field site in fractured biotite gneiss underlying western South Carolina (Table 1). The baseline grid consists of 100 blocks along the fracture. Six evenly spaced blocks were adjacent to the wellbore, and the grid spacing increased thereafter as a geometric progression using a 1.15 multiplier. The baseline case used 40 time steps, with Δt = 0.2 s during the first 6 steps. The wellbore was assumed

Discussion

The basic hydromechanical response of a horizontal fracture to a slug test came as a surprise to us when preliminary results from the analysis described in this paper were first available. We expected that the displacements would track the wellbore pressure, probably because this is the response that would be predicted from a porous media model where displacement is proportional to pressure change. The analysis shows, however, that the pressure and displacement signals can be markedly different

Conclusion

Slug tests will dilate fractures in aquifers, according to results of hydromechanical analyses. The abrupt increase in pressure at the beginning of the test causes a slight displacement, but the fracture continues to open even as the wellbore pressure head drops. Continued opening occurs as the pressure disturbance propagates over an ever widening area of the fracture face. Maximum displacements of microns are expected to accompany head changes of meters, but the displacements may be larger for

Acknowledgement

The authors would like to thank the National Science Foundation, grant number EAR0001146 for funding the research.

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Analysis of the hydromechanical behavior of a flat-lying fracture during a slug test

Summary

Section snippets

Conceptual model

Theoretical analysis

Results

Discussion

Conclusion

Acknowledgement

International Journal of Rock Mechanics Mining Sciences & Geomechanics Abstracts

International Journal of Rock Mechanics & Mining Sciences

International Journal of Rock Mechanics & Mining Sciences

Journal of Contaminant Hydrology

International Journal of Rock Mechanics Mining Sciences & Geomechanics Abstracts

International Journal of Rock Mechanica Mining Science Geomechanica Abstract

International Journal of Rock Mechanics Mining Sciences & Geomechanics Abstracts

Slug tests in fissured aquifers

Water Resources Research

The interpretation of slug tests in fissured rocks

Quarterly Journal of Engineering Geology

Steady shape analysis of tomographic pumping tests for characterization of aquifer heterogeneities

Water Resources Research

A slug test for determining hydraulic conductivity of unconfined aquifers with completely or partially penetrating wells

Water Resources Research

A travel time based hydraulic tomographic approach

Water Resources Research

The Design Performance and Analysis of Slug Tests

Pumping tests in networks of multilevel sampling wells; motivation of methodology

Water Resources Journal

Response of a finite-diameter well to an instantaneous charge of water

Water Resources Research

Combined interpretation of radar, hydraulic, and tracer data from a fractured-rock aquifer near Mirror Lake, New Hampshire, USA

Hydrogeology Journal

Flow to a partially penetrating well in a double-porosity reservoir

Water Resources Research

Slug test analysis in dual porosity reservoirs

SPE Formation Evaluation

Slug tests in partially penetrating wells

Water Resources Research

Estimating lithologic and transport properties in three dimensions using seismic and tracer data: The Kesterson aquifer

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A new analytical model for fracture-dominated reservoirs

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Estimation of field-scale soil hydraulic and dielectric parameters through joint inversion of GPR and hydrological data

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