Fluid flow in deep carbonates: Assessing the geothermal potential of carbonate rocks through field work, experiments and numerical modelling

Fluid flow in deep carbonates: Assessing the geothermal potential of carbonate rocks through field work, experiments and numerical modelling

Prof Sandra Piazolo (SEE), Prof Quentin Fisher (SEE), CASE Partner (to be announced)

In this exciting project, you will use targeted field work, microscopic analysis, flow through experiments and numerical modelling to constrain the evolution of fluid flow in carbonate rocks. Results from your research will have direct implications for assessing the geothermal potential of carbonate rocks.Image of travertine steps from Italy, close to proposed field area.

At a time when zero carbon technologies are urgently needed, assessment of the potential of the subsurface for geothermal heat and energy generation is critical. This project aims to utilize natural geothermal systems in combination with experiments and numerical models to learn how best to assess the geothermal potential of the subsurface. The focus will be given to carbonate hosted geothermal systems as these represent a high potential for deep geothermal energy both locally and internationally.

The yield and long-term production of geothermal resources is highly dependent on the status and evolution of fluid flow through the rock through time and space. Fluid flow is controlled by the permeability of the rock, in carbonates this permeability can be changed by fluid-rock interaction which in turn is highly dependent on chemistry and temperature of percolating fluids and hosting rocks (Brogi and Capezzuoli, 2014; Brogi et al., 2016). Carbonate rocks are highly reactive and their geothermal productivity may decrease over time due to mineral precipitation. However, the timing of self-sealing in carbonate geothermal systems has not been well studied. The controls of fault-fracture interaction are also not well understood both in terms of geomechanical controls and its impact on fluid flow.

Research

To address the question of potential and longevity of a geothermal system, the project will study natural examples of long-lived natural geothermal systems. This will help evaluate and predict the reservoir productivity and injectivity at various geothermal system temperatures. The student will study the influence of low-temperature (T<220°C) hydrothermal flow on the permeability of fault and fracture conduit systems using outcrops associated with hot springs, which have been producing over long timescales (>102-105 yrs) (Major et al., 2018, Fig. 1). These outcrops may be marked by travertine deposits on the top of the strata due to the hydrothermal origin of travertines. The student will study high-temperature (T>220°C) hydrothermal systems near magma intrusions, where the adjacent altered rock has been uplifted to the surface. These rocks are often metamorphosed due to the very high temperatures and contain porphyry and epithermal ore deposits (e.g., Bogie et al., 2005; Bella et al., 2016; Zucchi, 2020, Fig. 1).

Image showing the two end member hydrothermal systems , fault related and magma driven
Figure 1. Schematics illustrating natural geothermal systems occurring at low (T<220°C) and high (T>220°C) temperatures and driven by different processes: faulting (left; from Brogi and Capezzuoli, 2014) and magma intrusion (right; from Bogie et al., 2005).

Objectives

Five main questions will be looked at in detail:

  • How fast do existing high fluid pathways such as host rock porosity and fractures heal?
  • What is the rate of self-heal of high flow conduits associated with faulting?
  • How do these cemented faults and fractures interact with the injected geothermal waste water? Does the hydro-fracturing become easier, when the reservoir already contains a pre-existing, but cemented fracture network?
  • How does mineralogy (i.e. calcite vs dolomite) impact the evolution of permeability in faults and fractures?
  • How do the mechanisms and their rates related to Question 1-3 change with temperature?

Methods

This project is highly innovative as it brings together several fields of research. It will combine outcrop, laboratory and simulation modelling work packages.

Field work and sample analysis:

Samples will be collected from both low-temperature fault-driven and high-temperature magma-driven geothermal systems and compared with the host rock samples unaffected by these systems (Figure 1). Fieldwork will be carried out both in the UK (outcrops and borehole drill samples) and in Italy. 1-2 outcrops will be selected from each system for detailed analysis. The Italian locations have a long history in geothermal energy exploitation. International potential exposures include:

  1. Fault-driven geothermal systems: Rapolano fault (Brogi et al., 2017); Boccheggiano fault (Brogi et al., 2016); Monte Amiata (Brogi et al., 2016); the Larderello geothermal area in the Lago Basin (Liotta and Brogi, 2020).
  2. Magma-driven geothermal systems: SE Elba Island (Acquarilli and Norsi beaches) (Zucchi, 2020).

Field relationships and quantitative microstructural and microchemical analysis – to analyse generation of cementation and growth structures will be performed (e.g. delle Piane et al., 2018, Gardner et al. 2020). This may also include dating of the healed fractures.

Experimental work will combine several techniques:

  1. Mechanical tests – to determine the strength and failure patterns of samples altered and unaltered by the geothermal systems.
  2. Flow through experiments investigating the rate of closure of fractures using a unique in-situ apparatus available at the Experimental Petrology Laboratory, University of Leeds and the Wolfson multiphase flow laboratory.
  3. Scaling (i.e. crystal growth in fractures) experiments utilizing the experimentation available at Civil Engineering, University of Leeds.

Numerical modelling:

A series of numerical experiments will be performed based on the field and experimental results enabling assessment of long-term sustainability and safety of geothermal systems.

This may include microstructural modelling using the system Elle (www.microstructure.info, e.g. Piazolo et al. 2019) and fracture and healing pattern modelling (Vass et al. 2014, Koehn et al. 2020).

Impact

This project is highly innovative as it brings together several fields of research. Project outcomes will represent an advance our knowledge and understanding of the physicochemical processes occurring in natural geothermal systems and will give direction in designing sustainable and long-lived geothermal systems in deep and shallow carbonate reservoirs. This project promises to produce cutting-edge research in fluid flow dynamics in carbonates. This knowledge is urgently needed to increase our ability to assess the geothermal potential geothermal heat generation.

Student profile

The project would suit a numerate student with a background in earth sciences, geology, or geophysics. The student should have a strong interest in petrological challenges, a desire to undertake laboratory and fieldwork overseas, and a strong background in a quantitative science. Willingness and excitement for taking up the challenge to work at the boundary of structural geology, geochemistry, geomechanics and microstructural analysis utilizing a combination of techniques (field work, in-depth microstructural analysis, experiments and numerical modelling) is a prerequisite.

The student will be provided with training in state-of-the-art geological, petrophysical and geomechanical methods, including microstructural and large geological data analysis, as well as structural field mapping and numerical modelling.

References

Andre, B.J., Rajaram, H., 2005. Dissolution of limestone fractures by cooling waters: early development of hypogene karst systems. Water Resour Res 41:W01015.

Bella, P., Gaál, Ľ., Šucha, V., Koděra, P. and Milovský, R., 2015. Hydrothermal speleogenesis in carbonates and metasomatic silicites induced by subvolcanic intrusions: a case study from the Štiavnické vrchy Mountains, Slovakia. International Journal of Speleology, 45(1), p.2.

Bogie, I., Lawless, J.V., Rychagov, S. and Belousov, V., 2005. Magmatic-related hydrothermal systems: Classification of the types of geothermal systems and their ore mineralization. Proceedings of Geoconference in Russia, Kuril.

Brogi, A., Capezzuoli, E., Kele, S., Baykara, M.O. and Shen, C.C., 2017. Key travertine tectofacies for neotectonics and palaeoseismicity reconstruction: effects of hydrothermal overpressured fluid injection. Journal of the Geological Society, 174(4), pp.679-699.

Brogi, A., Liotta, D., Ruggieri, G., Capezzuoli, E., Meccheri, M. and Dini, A., 2016. An overview on the characteristics of geothermal carbonate reservoirs in southern Tuscany. Italian Journal of Geosciences, 135(1), pp.17-29.

Delle Piane C, Piazolo S, Timms NE, Luzin V, Saunders M, Bourdet J, Giwelli A, Ben Clennell M, Kong C, Rickard WDA, Verrall M. 2018. Generation of amorphous carbon and crystallographic texture during low-temperature subseismic slip in calcite fault gouge. Geology. 46(2), pp. 163-166

Gardner RL, Piazolo S, Daczko NR, Trimby P. 2020. Microstructures reveal multistage melt present strain localisation in mid-ocean gabbros. Lithos. 366-367

Koehn D, Piazolo S, Sachau T, Toussaint R. 2020. Fracturing and Porosity Channeling in Fluid Overpressure Zones in the Shallow Earth’s Crust. Geofluids Benedicto A (eds.). 2020

Liotta, D. and Brogi, A., 2020. Pliocene-Quaternary fault kinematics in the Larderello geothermal area (Italy): Insights for the interpretation of the present stress field. Geothermics, 83, p.101714.

Major, J.R., Eichhubl, P., Dewers, T.A. and Olson, J.E., 2018. Effect of CO2–brine–rock interaction on fracture mechanical properties of CO2 reservoirs and seals. Earth and Planetary Science Letters, 499, pp.37-47.

Piazolo S, Bons PD, Griera A, Llorens M-G, Gomez-Rivas E, Koehn D, Wheeler J, Gardner R, Godinho JRA, Evans L, Lebensohn RA, Jessell MW. 2019. A review of numerical modelling of the dynamics of microstructural development in rocks and ice: Past, present and future. Journal of Structural Geology. 125, pp. 111-123

Schmoker, J.W. and Halley, R.B., 1982. Carbonate porosity versus depth: a predictable relation for south Florida. AAPG bulletin, 66(12), pp.2561-2570.

Vass A, Koehn D, Toussaint R, Ghani I, Piazolo S. 2014. The importance of fracture-healing on the deformation of fluid-filled layered systems. Journal of Structural Geology. 67, pp. 94-106

Zucchi, M., 2020. Faults controlling geothermal fluid flow in low permeability rock volumes: An example from the exhumed geothermal system of eastern Elba Island (northern Tyrrhenian Sea, Italy). Geothermics, 85, p.101765.