ABSTRACT

Simultaneous measurements of fluid pressure and displacement normal to a fracture were performed for a series of seventy hydraulic pulse injection tests. The field tests were conducted in two horizontal boreholes spaced one meter vertically and intersecting a high-permeable vertical fault located within a 13 500 m3 fractured natural reservoir. Pulses tests were simulated with a coupled hydromechanical three-dimensional discrete model of the fractured rock. Field measurement and numerical modeling show that the hydraulic aperture and normal stiffness strongly vary along the fault plane. From the normal displacement versus fluid pressure relationship, modeling shows that the in situ intrinsic properties of the pressurized fault and the stiffness of the surrounding rock explain the fault hydromechanical behavior during the pulse pressure increase. During the pulse pressure decrease, the hydromechanical response of the pressurized fault is strongly influenced by the hydromechanical behavior of the other surrounding fractures of the network.

INTRODUCTION

Hydraulic pulse injection testing in single borehole has previously been applied to determine hydraulic properties of rock fractures, including permeability and storativity (Rutqvist 1996). Using specialized equipment for short duration pulse, the method has been proven to be useful for measuring hydraulic properties in fractures located several meters depth with hydraulic aperture values as small as a few microns (Rutqvist 1996, Rutqvist et al. 1997). Conventionally, interpretation of a pulse test is made by fitting the field results to type curves of pressure change with time without consideration of hydromechanical mechanisms (Cooper et al. 1967, Bredehoeft & Papadopulos 1980, Wang et at. 1977). However, it has been shown that the deformation of a fracture and of the surrounding rock during a pulse injection test can induce misinterpretations leading to an error of several orders of magnitude in the determination of fracture permeability and storativity (Rutqvist 1996). To improve accuracy in estimating the hydromechanical properties of fractures, it has been recommended combining hydraulic field tests and fractures mechanical normal displacement measurements (Rutqvist et at. 1998). Mechanical measurements of transient aperture change during a pulse injection test can provide a substantially improved estimate of fracture storativity, which implies that fracture permeability can be determined more accurately from the pressure transient test. Nevertheless, hydraulic field tests on single fractures or within a fractures network that measure simultaneously both hydraulic and mechanical responses are rare (Myer 1991), although such tests can be useful for in situ characterization of fractures mechanical parameters.

In this paper, in situ simultaneous measurements of fluid pressure and displacement normal to a near-vertical fault for a series of seventy pulse tests are presented. Fault hydraulic aperture and normal stiffness are back-calculated with a coupled hydrornechanical three-dimensional discrete model featuring the fracture inside the surrounding fracture network. To investigate the sensitivity of the hydromechanical response of the pressurized fault to the geometry of the fracture network, a parametric analysis is conducted.

FIELD EXPERIMENTS

The hydraulic pulse tests were conducted on the Coaraze Laboratory Site in France. The description of the site is presented in detail in Cappa et al. (2004).

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