Evidence of a Horizontal Hydraulic Fracture From Stress Rotations Across a Thrust Fault
- Shawn C. Maxwell (Schlumberger) | Ulrich Zimmer (Pinnacle Technologies) | Ronald W. Gusek (Trican Well Service) | David J. Quirk (Pinnacle Technologies)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- May 2009
- Document Type
- Journal Paper
- 312 - 319
- 2009. Society of Petroleum Engineers
- 2.2.2 Perforating, 2.4.3 Sand/Solids Control, 5.8.1 Tight Gas, 4.1.2 Separation and Treating, 5.1.2 Faults and Fracture Characterisation, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.8.2 Shale Gas, 1.10 Drilling Equipment, 2.5.2 Fracturing Materials (Fluids, Proppant), 3 Production and Well Operations, 5.1.8 Seismic Modelling
- stimulation, microseismic, hydraulic fracture, fault
- 3 in the last 30 days
- 736 since 2007
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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.
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.
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