Induced Microearthquake Patterns in Hydrocarbon and Geothermal Reservoirs: Six Case Studies

Abstract

The injection or production of fluids can induce microseismic events in hydrocarbon and geothermal reservoirs. By deploying sensors downhole, data sets have been collected that consist of a few hundred to well over 10,000 induced events. We find that most induced events cluster into well-defined geometrical patterns. In many cases, we must apply highprecision, relative location techniques to observe these patterns. At three sedimentary sites, thin horizontal strands of activity are commonly found within the location patterns. We believe this reflects fracture containment between stratigraphic layers of differing mechanical properties or states of stress. At a massive carbonate and two crystalline sites, combinations of linear and planar features indicate networks of intersecting fractures and allow us to infer positions of aseismic fractures through their influence on the location patterns. In addition, the fine-scale seismicity patterns often evolve systematically with time.At sedimentary sites, migration of seismicity toward the injection point has been observed and may result from slip-induced stress along fractures that initially have little resolved shear. In such cases, triggering events may be critical to generate high levels of seismic activity. At one crystalline site, the early occurrence of linear features that traverse planes of activity indicate permeable zones and possible flow paths within fractures. We hope the continued development of microseismic techniques and refinement of conceptual models will further increase our understanding of fluid behavior and lead to improved resource management in fractured reservoirs.Running Head: Microearthquake Patterns in Reservoirs

The Dallas-Fort Worth Earthquake Sequence: October 2008 through May 2009

“…In contrast, studies of hydrofracture-induced microearthquakes, especially in tight-gas environments, typically find b values of 2.0 and higher; indeed, Dinske (2009a, 2009b) report b of 2.50 for microearthquakes produced by hydrofracturing the Barnett shale. Second, the cumulative number and cumulative moment of microearthquakes induced by hydrofracture tends to increase linearly with the volume of fluid injected (e.g., Phillips et al, 2002;Rutledge et al, 2004;Shapiro and Dinske, 2009b), rather than occurring in bursts or clusters as is observed for DFW in Figures 4 and 13. Finally, the largest DFW earthquakes have similar magnitudes (M 3.3) to historical natural earthquakes in northeast Texas.…”

“…In this paper we investigate which activities related to natural gas recovery might possibly have induced the earthquakes, that is, drilling, hydrofracture, production, or fluid waste disposal. We will also compare the properties of the DFW sequence to natural and human-induced earthquake sequences in Texas and elsewhere (e.g., see Phillips et al, 2002;Majer et al, 2007;Suckale, 2009Suckale, , 2010.…”

Tritium Transport at the Rulison Site, a Nuclear-stimulated Low-permeability Natural Gas Reservoir

“…They also identify a trailing aseismic zone, which they suggest may be related to the fracture length that is maintained open and thus available for propant placement. The aseismic zone attained lengths of about one-third to one-half of total seismic length, suggesting potentially conductive hydrofrac lengths of 61 to 198 m. Long seismic lengths (over 760 m) were also identified for an injection in the Austin Chalk (Phillips et al, 2002), though again, a lack of conductivity was observed based on production performance. Work in other environments has found much shorter lengths.…”

“…In practice, this appears to often work well in sedimentary environments. Phillips et al (2002) observed a strong tendency for microseismicity induced by fluid injection to organize into thin, roughly horizontal strands of activity in clastic sedimentary environments. The near-horizontal orientations led them to suspect that stratigraphy controls fracture behavior, with stratigraphic changes limiting fracture growth through an increase in ductility or an increase or decrease in mechanical strength and supported stress at a layer boundary.…”

“…Phillips et al (2002) emphasize that although the presence of seismicity is taken to indicate a pressure connection, it is not necessarily a high-permeability connection between that point and an injection well. They note that some of the most conductive flowpaths are likely to be in seismically inactive regions.…”