Introduction to Hydromechanical Well Tests in Fractured Rock Aquifers

Abstract

This article introduces hydromechanical well tests as a viable field method for characterizing fractured rock aquifers. These tests involve measuring and analyzing small displacements along with pressure transients. Recent developments in equipment and analyses have simplified hydromechanical well tests, and this article describes initial field results and interpretations during slug and constant-rate pumping tests conducted at a site underlain by fractured biotite gneiss in South Carolina. The field data are characterized by displacements of 0.3 microm to more than 10 microm during head changes up to 10 m. Displacements are a hysteretic function of hydraulic head in the wellbore, with displacements late in a well test always exceeding those at similar wellbore pressures early in the test. Displacement measurements show that hydraulic aperture changes during well tests, and both scaling analyses and field data suggest that T changed by a few percent per meter of drawdown during slug and pumping tests at our field site. Preliminary analyses suggest that displacement data can be used to improve estimates of storativity and to reduce nonuniqueness during hydraulic well tests involving single wells.

Combining periodic hydraulic tests and surface tilt measurements to explore in situ fracture hydromechanics

“…The term a C w is generally negligible compared to C f , and with ρ w g = 10 4 N/m 3 , we obtain S ≃10 −5 (dimensionless). This order of magnitude lies in the lower end of estimates from cross‐borehole flowmeter test performed in the area by Le Borgne et al [] but is higher than typical results from comparable hydromechanical field experiments [ Burbey et al , ; Schweisinger et al , ; Svenson et al , ].…”

“…For instance, Schweisinger et al [, ] have evidenced that in the field, the aperture change in a flat horizontal fracture is a hysteretic function of fluid pressure, consistent with results obtained earlier in laboratory stress‐strain experiments on rock samples [e.g., Bandis et al , ; Barton et al , ]. In addition, Schweisinger et al [] were able to verify that in situ fracture transmissivities T h typically decrease when pressure increases during a hydraulic test. Such hydromechanical monitoring complements more classical well tests to enhance constraints on fracture storage [ Burbey et al , ; Rutqvist et al , ; Schweisinger et al , ; Svenson et al , ] by reducing nonuniqueness, even if the nonstationarity of storativity S during transient pressure conditions is a matter of debate [ Murdoch and Germanovich , ].…”

“…In addition, Schweisinger et al [] were able to verify that in situ fracture transmissivities T h typically decrease when pressure increases during a hydraulic test. Such hydromechanical monitoring complements more classical well tests to enhance constraints on fracture storage [ Burbey et al , ; Rutqvist et al , ; Schweisinger et al , ; Svenson et al , ] by reducing nonuniqueness, even if the nonstationarity of storativity S during transient pressure conditions is a matter of debate [ Murdoch and Germanovich , ]. Despite these undisputable advances, fracture deformation measurements in these experiments are often performed at the injection source and the fracture inlet itself.…”

Validation of non-Darcian flow effects in slug tests conducted in fractured rock boreholes

“…The data in Fig. 11 also indicate that even if fracture dilation/contraction effects (e.g., Svenson et al, 2007;Schweisinger et al, 2009) occur during either type of hydraulic test (causing more flow per unit dH), they are masked by the effects of non-Darcian flow (causing less flow per unit dH). It is likely that this type of non-ideality cannot be identified through hydraulic tests alone.…”

The seismo-hydromechanical behavior during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in situ stimulation experiment

“…Evans and Wyatt (1984) estimated the closure of a fracture zone from observed surface deformations induced by drilling-related drainage of fluid pressure within the structure. Similarly, Gale (1975), Jung (1989), Martin et al (1990), Guglielmi et al (2006) and Schweisinger et al (2009) used borehole caliper sondes to monitor changes in fracture aperture and pressure during hydraulic jacking tests. The resulting displacements and the flow and pressure responses allowed relationships between mechanical and hydraulic aperture changes to be established and helped to constrain the fracture/fault normal compliance on larger scales.…”