Microseismicity Cloud Can Be Substantially Larger Than the Associated Stimulated Fracture Volume: The Case of the Paralana Enhanced Geothermal System

Abstract

The goal of hydraulic stimulation is to increase formation permeability in the near vicinity of a well. However, there remain technical challenges around measuring the outcome of this operation. During two enhanced geothermal system stimulations in South Australia, Paralana in 2011 and Habanero in 2003, extensive catalogs of microseismicity were recovered. It is often assumed that shear failure of existing fractures is the main mechanism behind both the induced earthquakes and any permeability enhancement. This underpins a common notion that the seismically active volume is also the stimulated reservoir. Here we compute the density of earthquake hypocenters and provide evidence that, under certain conditions, this spatiotemporal quantity is a reasonable proxy for pore pressure increase. We then apply an inverse modeling approach that uses the earthquake observations and a modified reservoir simulator to estimate the parameters of a permeability evolution relation. The regime implied by the data indicates that most permeability enhancement occurred very near to the wellbore and was not coincident with the bulk of the seismicity, whose volume was about 2 orders of magnitude larger. Thus, we conclude that, in some cases, it is possible for permeability enhancement and induced seismicity to be decoupled, in which case the seismically active volume is a poor indicator of the stimulated reservoir. Our results raise serious questions about the effectiveness of hydroshearing as a stimulation mechanism in enhanced geothermal system. This study extends our understanding of the complex processes linking earthquakes, fluid pressure, and permeability in both natural and engineered settings.

Seismic Response to Injection Well Stimulation in a High‐Temperature, High‐Permeability Reservoir

“…Other recent studies have raised the possibility that permeability and seismicity need not be related for cases of induced seismicity (e.g., Guglielmi et al, 2015;Riffault et al, 2018;Rinaldi & Rutqvist, 2019), and the data set we present here contributes to the documentation and understanding of this discrepancy. Other recent studies have raised the possibility that permeability and seismicity need not be related for cases of induced seismicity (e.g., Guglielmi et al, 2015;Riffault et al, 2018;Rinaldi & Rutqvist, 2019), and the data set we present here contributes to the documentation and understanding of this discrepancy.…”

“…Further evidence of aseismic slip has been inferred at the high-temperature Salton Sea geothermal field in southern California from a combined geodetic and seismic data set (Wei et al, 2015). Riffault et al (2018) have also demonstrated that, in some cases, permeability enhancement may not be coupled to detectable seismic slip at all. In the context of high-temperature geothermal reservoirs, thermal stresses induced near the wellbore may govern most of the permeability enhancement observed during well stimulation through the expansion of permeable zones (Grant et al, 2013;Siega et al, 2014), a process that may be aseismic.…”

“…While these temperatures could be interpreted to be near the brittle-ductile transition for quartz-bearing rock (e.g., Scholz, 1988), this is unlikely given that seismicity at the nearby (and lithologically similar) Rotokawa geothermal field occurs in host rock with measured temperatures in excess of 330 • C . In addition, recent results have shown that a significant portion of induced seismicity may be triggered not by pore pressure perturbation, implying hydraulic connection to a wellbore, but by static and dynamic stress transfer onto suitably oriented fractures outside the reservoir (e.g., Cappa et al, 2019;Riffault et al, 2018). In addition, recent results have shown that a significant portion of induced seismicity may be triggered not by pore pressure perturbation, implying hydraulic connection to a wellbore, but by static and dynamic stress transfer onto suitably oriented fractures outside the reservoir (e.g., Cappa et al, 2019;Riffault et al, 2018).…”

“…The advance of the induced pressure front is controlled by the hydraulic diffusivity of the reservoir (also referred to as transmissivity, the permeability thickness product: kh), which is closely linked to the reservoir permeability. However, a variety of mechanisms are responsible for triggering seismicity, many of which are unrelated to the hydraulic properties of the reservoir, leading to speculation as to the utility of SBRC (Cappa et al, 2019;Riffault et al, 2018). At Ngatamariki, and at most other naturally fractured reservoirs, flow is strongly influenced by the spacing and permeability of fractures (Grant & Bixley, 2011).…”

“…We also caution against the use of microseismic hypocenters in defining the depth of significant permeability for constraining the base of reservoir models, partly due to uncertainties in the hypocentral depth (∼200 m). In addition, recent results have shown that a significant portion of induced seismicity may be triggered not by pore pressure perturbation, implying hydraulic connection to a wellbore, but by static and dynamic stress transfer onto suitably oriented fractures outside the reservoir (e.g., Cappa et al, 2019;Riffault et al, 2018). Numerical modeling of the temperature, pressure, and stress changes associated with injection will be required to better understand the effect of injection on fracture stability and the role of seismicity in increasing reservoir permeability (e.g., Ghassemi et al, 2007;Riffault et al, 2018).…”