Summary
Microseismic imaging of a hydraulic-fracture stimulation showed significant
fracture reorientation across a thrust fault. Fracture orientations were
identified through a combination of alignment of event locations, polarization
of the seismic waves, and injection details. Stimulation below the fault
indicated a near-horizontal fracture geometry. Above the fault, a near-vertical
fracture geometry was observed. A change in fault orientation was supported by
differences in the microseismic-signal characteristics and the
treatment-injection data. This difference in fracture geometry was attributed
to rotations in the direction of minimum principal stress, which is consistent
with observed differences in the injection pressures.
Introduction
The effectiveness of hydraulic-fracture stimulations is critical for optimal
economic production of tight gas. Deformation associated with fracturing
results in small-magnitude microearthquakes that can be used to image the
stimulated fracture network. Microseismic images can be used to map the
fracture orientation complexity associated with interaction with pre-existing
fractures and to assess the temporal development of the fracture geometry
(Warpinski et al. 1998; Sleefe et al. 1995; Dobecki 1983). The actual fracture
performance can then be used to better engineer the stimulation to optimize
drainage (Mayerhofer et al. 2005).
Hydraulic fractures are formed generally through tensile fracturing
resulting from injection of pressurized fluids and tend to form orthogonal to
the minimum-principal-stress direction. Knowledge of this fracture orientation,
as well as other aspects of the fracture geometry, is important to optimally
designing the stimulation to maximize the reservoir drainage pattern of the
well. One way to deduce the orientation of a hydraulic fracture is through
alignment of microseismic-event locations. Additionally, microseismic-signal
attributes, such as the polarization direction of S- (shear-) waves and
relative amplitudes of the P- and S-waves, also can be used to deduce the
orientation of the fracture plane associated with each microseism (Zoback and
Zinke 2002). Furthermore, if the amount of seismic energy radiated in different
directions is measured adequately, the so-called focal mechanism or fracture
orientation can be computed directly, assuming that the coseismic deformation
results from a shear failure mechanism (Zoback and Zinke 2002). Additionally,
the relative amount of various failure mechanisms, such as fracture dilation,
also can be extracted through-processing technique known as moment-tensor
inversion (Gibowicz and Kijko 1994). Most hydraulic-fracture microseismic
images are recorded with sensors in a single observation well, such that the
seismic radiation is measured only in a limited direction. This limited
sampling of the radiation pattern certainly limits the uniqueness of the
fracture-mechanism analysis. However, in some projects in which multiple
observation wells are used (Warpinski et al. 2005), the 3D radiation pattern
will be measured better, allowing for the possibility of more-accurate fracture
planes. Nevertheless, the observed seismic radiation can be used to at least
constrain the orientation of the fracture plane in cases where the data cannot
image the failure-plane orientation accurately.
In this paper, we present a case study by use of microseismic imaging to
determine the geometry of a hydraulic fracture. A two-stage fracture treatment
was monitored where a sandstone formation had been stacked vertically by a
regional tectonic thrust fault. The resulting microseismic-event locations and
-signal attributes indicated that there was a significant change in the
fracture orientation across the fault. Analysis of the pumping data was also
used to infer a significant stress change across the fault to validate the
interpretation of a stress rotation to explain the substantially different
fracture geometries.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
2 August 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
15 September 2008
- Manuscript approved:
1 November 2008
- Published online:
1 May 2009
- Version of record:
1 May 2009
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