# Microseismicity Cloud Can Be Substantially Larger Than the Associated Stimulated Fracture Volume: The Case of the Paralana Enhanced Geothermal System - **Authors:** Jeremy Riffault (ORCID 0000-0002-0316-7215); David Dempsey (ORCID 0000-0003-2135-5129); Satish Karra (ORCID 0000-0001-7847-6293); Rosalind Archer - **Journal:** Journal of Geophysical Research: Solid Earth, vol. 123(8), pp. 6845-6870 (2018) - **Publisher:** American Geophysical Union (AGU) - **DOI (version of record):** [https://doi.org/10.1029/2017jb015299](https://doi.org/10.1029/2017jb015299) - **This page:** https://ddempsey.github.io/papers/2018-riffault-microseismicity-cloud-can-substantially-larger/ - **Open PDF:** [https://ddempsey.github.io/papers/2018-riffault-microseismicity-cloud-can-substantially-larger/riffault2018-microseismicity-cloud-can.pdf](https://ddempsey.github.io/papers/2018-riffault-microseismicity-cloud-can-substantially-larger/riffault2018-microseismicity-cloud-can.pdf) (Accepted manuscript (peer-reviewed, author's version)) - **Full text (Markdown):** [https://ddempsey.github.io/papers/2018-riffault-microseismicity-cloud-can-substantially-larger/fulltext.md](https://ddempsey.github.io/papers/2018-riffault-microseismicity-cloud-can-substantially-larger/fulltext.md) - **Notice:** An edited version of this paper was published by AGU. Published 2018 American Geophysical Union. Version of record: https://doi.org/10.1029/2017jb015299 - **Also in:** UC Research Repository, https://hdl.handle.net/10092/105823 - **Explainer:** all explainers on this site are AI-generated. - **BibTeX:** https://ddempsey.github.io/papers/2018-riffault-microseismicity-cloud-can-substantially-larger/citation.bib - **Topics:** Induced seismicity & faults; Geothermal systems - **Keywords:** Geothermal gradient; Geology; Induced seismicity; Microseism; Hydraulic fracturing; Seismology; Pore water pressure; Petrology ## In brief During the 2011 Paralana geothermal stimulation in South Australia, earthquakes occurred up to 400 m from the well. Our inversion of the earthquake and well data suggests that permeability increased only within 40 m of it. For the best-fitting model, the stimulated volume was 0.4% of the seismically active volume. - At Habanero in 2003, the density of earthquake hypocentres near the well rose linearly with injection pressure. This supports its use as a proxy for fluid pressure. - At Paralana, two permeability scenarios, extending 50 m and 400 m from the well, both reproduced the measured rise in injectivity. Only the earthquake data separated them. - Our inversion suggests that permeability at Paralana increased no further than 40 m from the well, while earthquakes were triggered up to 400 m away. - For the best-fitting model, permeability at the well rose by a factor of about 30. The stimulated volume was 0.4% of the seismically active volume. - We suggest that slip on existing fractures was not the main cause of the injectivity increase at Paralana. Acid injection, cooling of the rock, or hydraulic fracturing are more likely causes. ## The problem Temperatures high enough for economic geothermal energy are found almost everywhere, if wells are drilled deep enough (Tester et al., 2007). An enhanced geothermal system (EGS) extracts this heat by circulating water through the rock, from an injection well to a production well (Majer et al., 2007). At those depths, permeability is usually low (Majer et al., 2007). Hydraulic stimulation increases it by injecting cold water at high pressure. The stimulated volume sets the size of the heat resource. However, it cannot be observed directly. Stimulation nearly always triggers small earthquakes (Majer et al., 2007; Zang et al., 2014). These are usually attributed to hydroshearing, in which raised fluid pressure lets existing fractures slip (Pearson, 1981) and remain more permeable (Chen et al., 2000). Several studies have linked the growing earthquake volume to the growing stimulated volume (Baisch et al., 2004; Bendall et al., 2014; Rothert and Baisch, 2011). Our goal was to test whether the earthquakes and the permeability increase occupied the same volume during the 2011 stimulation at Paralana. ## What we did We used earthquake catalogues from two stimulations about 4 km deep in South Australia. At Habanero in 2003, 20,000 m³ of water, a cube about 27 m across, was injected over 30 days and 10,436 earthquakes were located. At Paralana in 2011, 2,200 m³ of water, a cube about 13 m across, and 110 m³ of acid were injected over 3.4 days, and 3,809 earthquakes were located. Injectivity, the injection rate divided by the wellhead pressure, remained almost constant at Habanero and increased at Paralana. Whether a single fracture slips depends on its unknown strength and stress. Counting many earthquakes averages out these variations. We therefore counted hypocentres per unit area of the plane of injection, the gently dipping fracture zone outlined by the earthquakes, in rings 50 m wide around the well. We assumed that a fracture slips once fluid pressure rises by a critical amount, then slips again with each further rise. Hypocentre density should then increase in proportion to the pressure rise beyond that critical amount. For Paralana, we simulated flow from the well with the FEHM simulator (Zyvoloski, 2007). Permeability in the simulation grew with time and fell with distance from the well, following a six-parameter formula that assumes no particular physical cause. We inverted for these and three other parameters. A Markov chain Monte Carlo method ran 300,000 simulations to find values that reproduce both the injectivity and the hypocentre density. ## What we found At Habanero, hypocentre density was roughly uniform within 300 m of the well. It rose linearly with injection pressure. The best fit gives about 280 earthquakes per square kilometre for each megapascal of pressure above a critical pressure rise of 4.6 MPa. Beyond 300 m, density fell with distance, quickly at first and then more slowly. This pattern is expected for pressure spreading from the well. We suggest that hypocentre density is a reasonable proxy for fluid pressure. However, it will be less accurate where other processes, such as thermal contraction, trigger most earthquakes.
Injectivity alone did not constrain permeability at Paralana. Two scenarios, with permeability increases extending 400 m and 50 m from the well, both reproduced the rise in injectivity. Only the change in hypocentre density over time distinguished them. In our inversion, only a narrow range of scenarios fitted both data sets. In all of them, permeability increased no further than 40 m from the well, and in most no further than 25 m. Earthquakes occurred up to 400 m away. For the best-fitting scenario, the volume where permeability rose by more than 1% was 0.4% of the seismically active volume, roughly one part in 250. In that scenario, permeability at the well rose by a factor of about 30. This seems reasonable in comparison with factors of 50 at Desert Peak (Dempsey et al., 2015) and 400 at Basel (Häring et al., 2008). The inversion assumes flow spreads equally in all directions, although the Paralana earthquakes did not. In two synthetic tests that break this symmetry, the inversion appeared to recover the maximum extent of the permeability increase. If hydroshearing had increased permeability substantially, the stimulated and seismically active volumes should have agreed more closely. We therefore suggest that hydroshearing was not the main cause of the injectivity increase at Paralana. At Habanero, more than 10,000 earthquakes occurred without an increase in injectivity. Hydroshearing may not have increased permeability there either. At Paralana, we consider acid, cooling of the rock, and hydraulic fracturing, alone or together, more likely causes. Acid can dissolve minerals in fractures near the well and widen them. No acid was injected at Habanero, and injectivity there did not rise. Cooling can make rock contract and widen fractures. In our model, cold water reached no further than 60 m from the well, similar to the extent of the permeability increase. Fluid pressure may also have exceeded the minimum stress in the rock and opened a hydraulic fracture. ## Limitations Our inversion gives a radially averaged permeability and cannot represent flow channels. The six-parameter formula also excludes permeability increases that reverse, for example when cooled rock warms again. Our results come from one stimulation. We therefore conclude only that permeability increase and induced earthquakes can be decoupled in some cases. Where they are, the volume of earthquakes is a poor indicator of the stimulated volume. Measuring stimulation success by the extent of the earthquakes alone can then be misleading. ## Glossary An **enhanced geothermal system (EGS)** produces heat from hot rock whose permeability may have been increased by injecting fluid. **Permeability** measures how easily fluid flows through rock. **Hydraulic stimulation** is the injection of cold water at high pressure to increase the permeability of rock around a well. **Injectivity** is the injection rate divided by the wellhead pressure. A rising injectivity indicates that water enters the rock more easily. **Hydroshearing** is slip of existing fractures caused by raised fluid pressure. It can leave the fractures more permeable. A **hypocentre** is the point where an earthquake starts. **Hypocentre density** is the number of hypocentres per unit area of the plane of injection, counted up to a given time. A **megapascal (MPa)** is a unit of pressure, about ten times atmospheric pressure. The **critical pressure rise** is the increase in fluid pressure needed before a fracture first slips. The **seismically active volume** is the volume of rock in which induced earthquakes occurred. The **stimulated volume** is the volume of rock whose permeability was increased by stimulation. **Inversion** estimates the inputs of a model, here the permeability, from observations of its outputs. **Markov chain Monte Carlo** is a method that runs a model many times to find the range of parameter values consistent with the data. A **hydraulic fracture** is a new tensile crack opened when fluid pressure exceeds the smallest compressive stress in the rock. ## 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. ## Cite Riffault, Jeremy, Dempsey, David, Karra, Satish, Archer, Rosalind (2018). Microseismicity Cloud Can Be Substantially Larger Than the Associated Stimulated Fracture Volume: The Case of the Paralana Enhanced Geothermal System. Journal of Geophysical Research: Solid Earth, 123(8), 6845-6870. https://doi.org/10.1029/2017jb015299 ```bibtex @article{riffault2018microseismicity, title = {Microseismicity Cloud Can Be Substantially Larger Than the Associated Stimulated Fracture Volume: The Case of the Paralana Enhanced Geothermal System}, author = {Riffault, Jeremy and Dempsey, David and Karra, Satish and Archer, Rosalind}, journal = {Journal of Geophysical Research: Solid Earth}, year = {2018}, volume = {123}, number = {8}, pages = {6845-6870}, publisher = {American Geophysical Union (AGU)}, doi = {10.1029/2017jb015299}, url = {https://ddempsey.github.io/papers/2018-riffault-microseismicity-cloud-can-substantially-larger/} } ```