Summary
Microseismic mapping is extensively used in the Barnett Shale to map
hydraulic fracture complexity associated with interactions of the stimulation
with pre-existing fractures (fracs). Previous studies have indicated a fair
correlation between the well performance and extent of the seismically active
volume. However, in addition to this measure of the extent of the stimulated
fracture network, the characteristics of this fracture network is also expected
to impact the well performance. In particular, the fracture spacing is believed
to be an important factor controlling the potential gas flow. In this paper, we
use the density of the total seismic moment release (a robust measure of the
microseism strength) as an indication of the seismic deformation that may
correlate to the fracture density. The study uses a set of microseismic maps of
hydraulic fracture stimulations, including cases in which the stimulated
reservoir volume measured by the extent of the seismically active region poorly
correlated with the well performance. Incorporating the seismic moment density
to assess the fracture density with the network extent, an improved correlation
with the well performance was observed.
Introduction
Microseismic mapping of hydraulic fracture stimulations has become a common
technique to map the fracture growth and geometry (Warpinski et al. 2004;
Fisher et al. 2002; Maxwell et al. 2002; Fisher et al. 2004; Rutledge et al.
2004; Shapiro et al. 2004; Chambers et al. 2008; Lu et al. 2008; Warpinski et
al. 2005). Microseismic images provide details of the fracture azimuth, height,
length, and complexity resulting from interaction with pre-existing fratures.
The resulting images can be used to calibrate numerical simulations of the
fracture growth, allowing more confident modeling of other stimulations in the
field, and a better identification of the stimulated region that may ultimately
be drained by the well.
Arguably, the Barnett Shale is the field that has had the most fracs mapped
over the last several years. Microseismic mapping in the Barnett Shale has
repeatedly demonstrated extreme fracture complexity resulting from interaction
between the injection and a pre-existing fracture network (Fisher et al. 2002;
Maxwell et al. 2002; Fisher et al. 2004; Rutledge et al. 2004; Shapiro et al.
2004; Chambers et al. 2008; Lu et al. 2008; Warpinski et al. 2005; Mayerhofer
et al. 2006). Even between neighboring wells, the geometry of the stimulated
fracture network shows a high degree of variability caused by localized
differences in the fracture network (Fisher et al. 2002). The Barnett Shale has
a low-intrinsic matrix permeability, and the permeability enhancement
associated with the fracture stimulation results in permeable fracture networks
sufficient for economic gas recovery in the field. Previous studies have shown
a correlation between the volume of the reservoir stimulated as measured by the
volume of the reservoir that emits microseisms during the stimulation, and the
production ultimately realized from the well (Fisher et al. 2002; Fisher et al.
2004; Mayerhofer et al. 2006). The correlation is attributed to larger fracture
networks being stimulated in wells in which a large microseismically active
volume of the reservoir has been realized, resulting in more permeable fracture
pathways connected to the well and therefore a higher potential for gas flow to
the well. Recently, many operators in the Barnett Shale have attempted
horizontal completions, which have allowed large volumes of the reservoir to be
stimulated with large fracture networks. Many of these completions use
perforated, cemented liners, and the microseismic images allow for
indentification of improved perforation staging to maximize the stimulated
reservoir volume (SRV) (Fisher et al. 2004).
Many of the Barnett Shale stimulations are water fracs in which large
volumes of water are injected at a high rate (Mayerhofer et al. 1997). One
possible mechanism for the success of waterfracs is that increased fluid
pressure in natural fractures induced shear failure, resulting in fracture
dilation associated with mismatched surfaces on opposite sides of the fracture.
Within this conceptual framework, the microseismic events correspond to the
actual fracture movement. The earlier investigations of the SRV measured the
total volume of the microseismically active region. However, this measure of
the stimulated volume does not take into account the properties of the fracture
network, which has also been indicated to impact well performance (Mayerhofer
et al. 2006). Furthermore, the permeability enhancement of the fracture may be
related to deformation associated with fracturing. Beyond the basic hypocentral
locations of the microseisms used to calculate the SRV, additional seismic
signal characteristics allow investigation of the source of the mechanical
deformation resulting in the microseisms. In particular, the seismic moment
(Aki and Richards 1980), a robust measure of the strength of an earthquake or
microearthquake can be used to quantify the seismic deformation (Maxwell et al.
2003).
In this paper, we examine several published microseismic projects in the
Barnett Shale formation for correlation between the production and
seismic-deformation attributes. In the next section, we describe seismic
moments and the calculation of seismic deformation. We illustrate how a seismic
moment can be used to remove a recording bias present in most microseismic
monitoring applications and the importance for calculating the seismic
deformation. Finally, we present the comparison between production, seismic
deformation, and SRV for several published datasets.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
27 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
12 September 2008
- Manuscript approved:
22 September 2008
- Published online:
2 March 2009
- Version of record:
26 February 2009
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