Faulting Induced by Forced Fluid Injection and Fluid Flow Forced by Faulting: An Interpretation of Hydraulic-Fracture Microseismicity, Carthage Cotton Valley Gas Field, Texas

Abstract

We analyzed precisely located microearthquake data detected during five hydraulic fracture treatments in the Carthage gas field of east Texas. The treatments were conducted in two adjacent boreholes within interbedded sands and shales of the Upper Cotton Valley formation. The microearthquakes were induced within narrow horizontal bands that correspond to the targeted sandstone layers. Events throughout all the treatments show strike-slip faulting occurring uniformly along vertical fractures trending close to maximum horizontal stress direction. These events are consistent with the reservoir's prevalent natural fractures, known to be isolated within the sands and trending subparallel to the expected hydraulic fracture orientation. When this uniform fracture system was activated exclusively, the detected shear deformation, measured as the moment release per unit volume of fluid injected, was constant, independent of various fluid viscosities and flow rates used. Within the base of the Upper Cotton Valley formation, anomalous event counts and moment release occurred within dense clusters that delineate bends or jogs in the fracture zones. The mechanisms are also strike-slip, but the fault planes are more favorably oriented for failure. The dense clusters show location patterns diverging in time, suggesting the expulsion of fluids from compressive fault jogs. Fluid flow forced by the adjacent slip-induced loading appears to initially extend the treatments, but the loading also tends to lock up and concentrate stress at the jogs, as evident by fewer but larger events populating the structures as treatments progress. As a result, effective drainage lengths from the boreholes may be shorter than would be inferred from the seismicity extending past the jogs. These high-moment asperities are similar to dense patches of seismicity observed along creeping sections of the San Andreas fault, where they have been attributed to localized zones of strength or stress concentration, surrounded by larger regions undergoing stable, aseismic slip. This similarity, plus large moment deficits in terms of volume injected, suggests a large component of aseismic slip is induced by the Cotton Valley treatments.

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.…”

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

“…At near isotropic stress conditions the fractures branch more strongly and without a preferred propagation direction, a phenomenom often referred to as high fracture complexity (e.g., Katsaga et al, 2015). During large-scale stimulations, there is a tendency for seismic clouds to develop perpendicular to the minimum principal stress direction σ 3 (Häring et al, 2008;Evans et al, 2005), particularly for HF operations (e.g., Rutledge et al, 2004), although for HS stimulations in crystalline rocks there are many examples in which the seismicity cloud is oblique to the σ 3 direction (e.g., Block et al, 2015;Murphy and Fehler, 1986;Pine and Batchelor, 1984), presumably reflecting the complex interplay between stress and the pre-existing fracture population that is suitably oriented for slip reactivation. Furthermore, individual seismicity clusters within the overall seismicity cloud often strike oblique to the maximum principal stress (Eaton and Caffagni, 2015;Deichmann et al, 2014).…”

Time–quefrency analysis of overlapping similar microseismic events

“…natural gas reservoirs (Rutledge and Phillips, 2003;Rutledge et al, 2004) and geothermal reservoirs (Moriya et al, 2002;Phillips, 2000). Eisner et al (2006) reported similar microseismic events that were monitored during hydraulic fracturing in oil and gas fields.…”