Hydromechanical modeling of pulse tests that measure both fluidpressure and fracture-normal displacement of the Coaraze Laboratory site,France

Journal Article · Sat Apr 22 04:00:00 EDT 2006 · International Journal of Rock Mechanics&MiningSciences

DOI:https://doi.org/10.1016/j.ijrmms.2006.03.006· OSTI ID:910586

Cappa, F; Guglielmi, Y; Rutqvist, J; Tsang, C-F; Thoraval, A

In situ fracture mechanical deformation and fluid flowinteractions are investigated through a series of hydraulic pulseinjection tests, using specialized borehole equipment that cansimultaneously measure fluid pressure and fracture displacements. Thetests were conducted in two horizontal boreholes spaced one meter apartvertically and intersecting a near-vertical highly permeable faultlocated within a shallow fractured carbonate rock. The field data wereevaluated by conducting a series of coupled hydromechanical numericalanalyses, using both distinct-element and finite-element modelingtechniques and both two- and three-dimensional model representations thatcan incorporate various complexities in fracture network geometry. Oneunique feature of these pulse injection experiments is that the entiretest cycle, both the initial pressure increase and subsequent pressurefall-off, is carefully monitored and used for the evaluation of the insitu hydromechanical behavior. Field test data are evaluated by plottingfracture normal displacement as a function of fluid pressure, measured atthe same borehole. The resulting normal displacement-versus-pressurecurves show a characteristic loop, in which the paths for loading(pressure increase) and unloading (pressure decrease) are different. Bymatching this characteristic loop behavior, the fracture normal stiffnessand an equivalent stiffness (Young's modulus) of the surrounding rockmass can be back-calculated. Evaluation of the field tests by couplednumerical hydromechanical modeling shows that initial fracture hydraulicaperture and normal stiffness vary by a factor of 2 to 3 for the twomonitoring points within the same fracture plane. Moreover, the analysesshow that hydraulic aperture and the normal stiffness of the pulse-testedfracture, the stiffness of surrounding rock matrix, and the propertiesand geometry of the surrounding fracture network significantly affectcoupled hydromechanical responses during the pulse injection test. Morespecifically, the pressure-increase path of the normaldisplacement-versus-pressure curve is highly dependent on thehydromechanical parameters of the tested fracture and the stiffness ofthe matrix near the injection point, whereas the pressure-decrease pathis highly influenced by mechanical processes within a larger portion ofthe surrounding fractured rock.

• Research Organization: Ernest Orlando Lawrence Berkeley NationalLaboratory, Berkeley, CA (US)
• Sponsoring Organization: USDOE Director. Office of Science. Office of AdvancedScientific Computing Research. Office of Basic Energy Sciences. ChemicalSciences Geosciences and Biosciences Division
• DOE Contract Number: AC02-05CH11231
• OSTI ID: 910586
• Report Number(s): LBNL--60442
• Journal Information: International Journal of Rock Mechanics&MiningSciences, Journal Name: International Journal of Rock Mechanics&MiningSciences Journal Issue: 7 Vol. 43; ISSN IJRMA2; ISSN 1365-1609
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
APERTURES
BOREHOLES
CARBONATE ROCKS
DEFORMATION
EVALUATION
FIELD TESTS
FLUID FLOW
FRACTURES
GEOMETRY
HYDRAULICS
MONITORING
SIMULATION
UNLOADING
numerical modeling pulse tests fractures networkhydromechanical coupling simultaneous pressure and fracture-normaldisplacement measurements distinct element method finite elementmethod