Stochastic modelling of injection-induced seismicity in the Cooper Basin enhanced geothermal system

<p>Fluid-injections under high pressures into deep “hot” rock formations are routinely performed during the development of Enhanced Geothermal Systems (EGS). Such fluid-injections, which aim to enhance the permeability in the targeted rock formation, can induce intense microseismicity and in some cases even larger magnitude earthquakes. A characteristic of injection-induced seismicity is its spatial migration with time, which is considered indicative of pore-pressure diffusion and the geometry of the stimulated volume in which permeability is enhanced. Understanding the details of earthquake migration during stimulation operations is particularly important for the design of EGS, the management of operations, as well as for the mitigation of hazardous induced earthquakes. Herein, we develop a stochastic model to map the spatiotemporal evolution of injection-induced seismicity. The model is based on the well-established Continuous Time Random Walk (CTRW) theory that has widely been applied in nonlinear transport phenomena in complex heterogeneous media. Within this context, we describe the spatiotemporal evolution of injection-induced seismicity with an appropriate master equation and the time-fractional diffusion equation. Application of the model to two stimulation experiments in the Cooper Basin (Australia) EGS shows that induced seismicity migrates slowly with time away from the injection points according to a subdiffusive process, with waiting times between the successive earthquakes drawn from a broad probability density function with asymptotic power-law behavior. Moreover, we show that the solution of the time-fractional diffusion equation adequately describes the propagation of induced seismicity in time and space, showing a peak of earthquake concentration close to the injection point and a stretched exponential decay for the concentration of distant events. The results demonstrate that the CTRW model can efficiently describe nonlinear diffusion of injection-induced seismicity during stimulation operations in EGS.       </p><p><strong>Acknowledgements</strong></p><p>The research project was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “2nd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers” (Project Number: 00256).</p>

High injection rates decrease the probability of creating undesired, far-reaching fluid migration pathways at Cooper Basin geothermal field

<p>Enhanced Geothermal Systems apply the pressurized fluid injection to fracture impermeable rocks to form pathways in which water circulates. The cold water under high pressure is pumped into the hot subsoil, where it heats up and returns to the surface. However, the induced fractures may coalesce into unwanted paths that allow the fluids to reach pre-existing faults, triggering major seismic events.</p><p>This work investigates the relationship between injection and a degree of disordering of sources, ZZ, at Cooper Basin geothermal field in Australia, following the methodology developed and applied to study The Geysers geothermal field case (Lasocki & Orlecka-Sikora, 2020). The parameter ZZ quantifies the potential of seismicity to build pathways for fluid migration. It is the average distance between the seismic events in the eight-dimensional parameter space consisting of three hypocentral coordinates, T- and P-axis plunges, T-axis trend, and polar and azimuthal angles in the spherical system of coordinates beginning at the open hole of an injection well. A decrease of ZZ indicates an increasing hazard of forming far-reaching migration pathways. In The Geysers case, ZZ turned out to be highly correlated with the injection rate.</p><p>Here we focus on the case of Habanero 4 well stimulation from 17 - November 30, 2012 (data access, see: IS EPOS, 2020). We processed 489 seismic events with known focal mechanisms. The events moment magnitude varies between 0.8 and 3.1.  </p><p>Our analysis shows that ZZ is significantly correlated with both the injection rate and the wellhead pressure. The higher the injection rate / the wellhead pressure was, the less probable was the creation of undesired fluid migration pathways. The Cooper Basin’s and The Geyser’s reservoir rocks are vastly different, the former – granite, the latter – greywacke sandstone, likewise the stimulation techniques applied in these two reservoirs. However, in both cases, ZZ was positively correlated with injection rate; thus, the potential to build unwanted paths for fluids was negatively correlated. These results suggest that such correlation may be a global feature of rock fracturing caused by pressurized fluid injections.</p><p><em>This work has been supported by S4CE (Science for Clean Energy) project, funded from the European Union’s Horizon 2020 - Framework Programme, under grant agreement No 764810 and by PRIN-MATISSE (20177EPPN2) project funded by Italian Ministry of Education and Research.</em></p><p> </p><p><strong>References:</strong></p><p>IS EPOS (2020), Episode: COOPER BASIN, https://tcs.ah-epos.eu/#episode:COOPER_BASIN, doi:10.25171/InstGeoph_PAS_ISEPOS-2020-001</p><p>Lasocki, S., & Orlecka-Sikora, B. (2020). High injection rates counteract formation of far-reaching fluid migration pathways at The Geysers geothermal field. Geophysical Research Letters, 47, e2019GL086212. https://doi.org/10.1029/2019GL086212</p>

Geological and Bioregional Assessments: a tale of two basins

The Australian Government’s $35.4 million Geological and Bioregional Assessment (GBA) Program is assessing the potential impacts of shale, tight and deep coal gas development on water and the environment in the Beetaloo, Isa and Cooper GBA regions. This paper compares the outcomes of impact assessments for the Beetaloo and Cooper GBA regions, highlighting the role that local geology, hydrogeology, ecology and regulatory regimes play when assessing potential impacts of unconventional gas development. Unconventional gas development activities between basins are broadly consistent, involving drilling, stimulation of the reservoir (typically through hydraulic fracturing), production and processing of hydrocarbons, export to market and decommissioning and rehabilitation. The characteristics of these activities and their potential impacts are strongly influenced by local factors including the geology, environment, industry practices and regulatory regimes. While subsurface impacts associated with hydraulic fracturing and well integrity are considered unlikely in both regions, regional geology means there is greater stratigraphic separation between target resources and overlying aquifers in the Beetaloo Sub-basin than in the Cooper Basin. Local ecological conditions and species influence the nature of potential impacts on protected matters in the two basins, which are mostly associated with surface disturbance and spills or accidental release of fluids. A key similarity between the two regions is the broadly consistent regulation and management of potential impacts in the two basins. Preliminary results of the causal network analysis indicate that mitigation measures are available for all pathways in which unconventional gas resource development activities may have the potential to impact on endpoints.

Geological and Bioregional Assessments: a program to encourage industry development and improve regulatory efficiency while maintaining the highest possible environmental standards

The Australian Government’s $35.4 million Geological and Bioregional Assessment (GBA) Program provides independent scientific information and baseline data to governments, the community and regional industries on the potential impacts of shale, tight and deep coal gas development on water and the environment. The program aims to encourage industry development and growth by improving the understanding of upstream operations and their potential impacts to drive regulatory efficiency while maintaining the highest environmental standards. The GBA program comprises a series of independent scientific studies undertaken by CSIRO and Geoscience Australia, supported by the Bureau of Meteorology and managed by the Department of Agriculture, Water and the Environment. The program was conducted in three stages and focuses on where industry is currently exploring, conducting assessments across three regions; the Cooper Basin, Beetaloo Sub-basin and the Isa Superbasin, each with potential to supply gas to the East Coast Gas Market. The GBA program brings together a range of disciplines to collect, aggregate and analyse environmental baseline data to conceptualise the geologic, hydrologic, ecologic and anthropogenic features of these regions. This robust ‘conceptual’ understanding of the regions combined with rigorous hazard identification enables the program to prioritise potential impacts on water and the environment, improving regulatory efficiency by focusing regulators towards managing those activities where the potential impacts can’t be avoided. Additional papers and presentations from our partners in the GBA program tell the story of how the Program was developed and delivered. The Program leaves a legacy of publicly available baseline data, information and assessment tools that will make regulation of the industry more efficient in the regions assessed.

Geological and Bioregional Assessments: assessing the prospectivity for tight, shale and deep-coal resources in the Cooper Basin, Beetaloo Subbasin and Isa Superbasin

The Geological and Bioregional Assessment Program is a series of independent scientific studies undertaken by Geoscience Australia and the CSIRO, supported by the Bureau of Meteorology, and managed by the Department of Agriculture, Water and the Environment. The program consists of three stages across three regions with potential to deliver gas to the East Coast Gas Market. Stage 1 was a rapid regional prioritisation conducted by Geoscience Australia, to identify those sedimentary basins with the greatest potential to deliver shale and/or tight gas to the East Coast Gas Market within the next 5–10 years. This prioritisation process assessed 27 onshore eastern and northern Australian basins with shale and/or tight gas potential. Further screening reduced this to a shortlist of nine basins where exploration was underway. The shortlisted basins were ranked on a number of criteria. The Cooper Basin, the Beetaloo Subbasin and the Isa Superbasin were selected for more detailed assessment. Stage 2 of the program involved establishing a baseline understanding of the identified regions. Geoscience Australia produced regional geological evaluations and conceptualisations that informed the assessment of shale and/or tight gas prospectivity, ground- and surface-water impacts and hydraulic fracturing models. Geoscience Australia’s relative prospectivity assessments provide an indication of where viable petroleum plays are most likely to be present. These data indicate areal and stratigraphic constraints that support the program’s further work in Stage 3, on understanding likely development scenarios, impact assessments and causal pathways.