Bidirectional and unidirectional fracture growth during hydrofracturing: Role of driving stress gradients

Abstract

[1] An unexpected observation from induced seismicity during stimulation experiments was the identification of asymmetric bidirectional and unidirectional growth of the seismic front and back front, indicating asymmetric growth of the hydrofracture itself. We develop and analyze a new analytical hydrofracture model that considers for the first time the effect of stress and pore pressure gradients on growth. It is based on plane strain linear elastic fracture mechanics and further considers 1-D laminar flow, the opening shape of the fracture, and a Griffith fracture criterion. The model explains asymmetric bidirectional growth during the injection and bidirectional and unidirectional growth during the postinjection phase. Analytical relations are derived for both cases to estimate the front and back front of the seismicity as a function of injection pressure, volume rate, stress gradients, viscosity, and elastic modules of the rock. Interestingly, the postinjection phase can be described by self-similar solutions, which depend only on the stress gradient and the injection pressure and which predict a parameter-independent length increase of the fracture after the injection stops. We use the theoretical opening shape of the fracture to calculate time-and space-dependent Coulomb stress changes in the rock in order to predict the patterns of induced seismicity in the neighborhood of the fracture. The model explains in detail the patterns of earthquakes induced during hydrofracturing stimulation experiments in a low-permeable gas field sandstone in west Texas, and we estimate a lateral stress or pore pressure gradient of more than 0.8 MPa km −1 . If the downhole net pressure during the experiments was 1 MPa, the gradient is constrained at about 10 MPa km −1 .Citation: Dahm, T., S. Hainzl, and T. Fischer (2010), Bidirectional and unidirectional fracture growth during hydrofracturing: Role of driving stress gradients,

A multidisciplinary study of the final episode of the Manda Hararo dyke sequence, Ethiopia, and implications for trends in volcanism during the rifting cycle

“…The initial vertical propagation of the dyke involves some seismicity in an otherwise aseismic region (as noted for other intrusions, e.g. Belachew et al 2011), and the seismicity in this region during the May 2010 dyke intrusion is (1) only during the initial injection, and (2) only tracking the vertical motion at the very centre, which is consistent with the seismicity tracking the propagation of the tip during the initial stages of dyke propagation Dahm et al 2010;Belachew et al 2011). Seismicity propagates north and south from the segment centre, emplacing two dyke segments of approximately 8 and 10 km length, which show differing final geometries and variations in seismicity.…”

On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity

“…We also observe unidirectional rather than bidirectional growth. Thus, we argue that the mechanism proposed by Dahm et al (2010) might not be sufficient to explain asymmetric fracture growth in our case. To date, it is not entirely clear why fractures grew in such a unidirectional manner.…”

“…Dahm et al (2010) suggested that asymmetric bidirectional fracture growth during injection and bidirectional to unidirectional growth after shut-in may be driven by gradients of in situ stress or pore pressure. In our case, fractures grow upwards in the case of HF1 and HF2 and downwards in the case of HF3, which would imply that a presumed stress or pressure gradient would change direction between 13 and 8 m borehole depths.…”

Effects of mechanical layering on hydrofracture emplacement and fluid transport in reservoirs

“…The term "overpressure" in the sense used here is well-established in the technical literature (Heimpel and Olson, 1994;Bonafede and Rivalta, 1999a,b). However, p o is also referred to as driving pressure or driving stress (Pollard and Segall, 1987;Vermilye and Scholz, 1995;Dahm et al, 2010), or as net pressure (Valko and Economides, 1995). Fluid overpressure, as defined here, is not to be confused with an abnormal pore formation pressure.…”