Hydro-mechanical controls on fracture permeability in carbonate reservoirs for geothermal energy production and carbon storage
Rationale
Fracture closure with the increasing stress presents a key mechanical process in geoscience. Therefore, it influences the hydraulic properties of fractured carbonates that host critical energy resources and are potential carbon storage sites. Fracture apertures control their permeability and hence ease of geothermal fluid extraction, and injection of carbon dioxide for Carbon Capture and Storage (CCS) (Medici et al. 2019). However, the ease of fluid migration reduces with increasing depth and lithostatic load, until fractures become inefficient flow pathways.
The relation between fracture closure and depth is complex as it is dependant on fracture stiffness and orientation relative to the in situ stress field (Hillis 1998). Furthermore, natural fractures have asperities on their surfaces, and their roughness characteristics determine the sensitivity of fracture permeability to loading (Pyrak-Nolte and Morris 2000; Berkowitz 2002). Because of the morphological complexity of fracture surfaces, the degree that fracture permeability reduces with increasing stress is a function of rock type and fracture mode of origin, for example, extensional versus shear failure (faulting). Also, fractures in stronger rocks such as mechanically resistant carbonates may have persistent permeability at high depth where propped open by mineral precipitates (Hillis 1998).
Project Area
The student will develop the above concepts for the specific case of fractured carbonates forming a potential medium enthalpy geothermal reservoir at ~2.5 km depth in Val d’Agri (southern Apennines, Italy). This area is suitable for geothermal use due to the relatively high heat flow (~60 mW/m2) arising from proximity to the active rift system of the Tyrrhenian Sea (Fig. 1). In the Val d’Agri (Figs. 1, 2), the carbonate reservoir system consists of mechanically resistant and thickly layered shallow water carbonates of Middle Jurassic and Lower Cretaceous age. Val d’Agri hosts the largest onshore hydrocarbon reservoir in Europe (Fig. 2), and is extensively drilled: cores can be accessed upon request (Hager et al. 2021).
Approach
Samples of fault and non-fault related fractures will be extracted from core; further samples will be collected from the outcrop analogue of the sedimentary succession (at Monte Alpi). Both non-fault related and fault related fractures will be characterized in the laboratory to determine the effects of different (i) patterns of asperities (or roughness) on (ii) stiffness response and hence (iii) permeability. Samples will be tested in the Rock Engineering Laboratory at University of Leeds to determine the relationship between normal and shear stiffness (sensitivity of the mechanical aperture to load) and roughness characteristics. The project may include measurements of fracture aperture via X-ray tomography and resin injection, fracture roughness, and characterisation of mineral precipitates using X-ray diffraction/electron microscopy.
External Supervisor
The project is in collaboration with Dr. Giacomo Medici from the Sapienza University of Rome who will facilitate visits to field outcrops, and access to cores in both Val d’Agri or other areas in central and southern Italy. The external supervisor will also provide access to well hydraulic test data and in situ stress measurements from the sequence (including information on hydraulic conductivity response versus depth from pumping tests for the fractured carbonates in the Val d’Agri, published by Vadacca et al. 2021).
Summary
Overall, the project will aim to establish the hydro-mechanical depth response of fractured carbonates at a specific site using a combination of traditional and novel experimental techniques in the fields of hydrogeology and rock mechanics. This holistic approach can be exported to other mechanically resistant and thickly layered carbonates that represent potential geothermal reservoirs and targets for CCS worldwide.
Key References
Beaubien SE, Spagnolo GS, Ridolfi RM, Aldega L, Antoncecchi I, Bigi S, Billi A, Carminati E 2023. Structural control of gas migration pathways in the hydrocarbon-rich Val d’Agri basin (Southern Apennines, Italy). Marine and Petroleum Geology 154, 106339.
Berkowitz, B 2002. Characterizing flow and transport in fractured geological media: A review. Advances in Water Resources 25, 861-884.
Hager BH, Dieterich J, Frohlich C, Juanes R, Mantica S, Shaw JH, Plesch A 2021. A process-based approach to understanding and managing triggered seismicity. Nature 595, 684-689.
Hillis, RR 1998. The influence of fracture stiffness and the in situ stress field on the closure of natural fractures. Petroleum Geoscience 4(1), 57-65.
Medici G, West LJ, Mountney NP, Welch M 2019. Permeability of rock discontinuities and faults in the Triassic Sherwood Sandstone Group (UK): insights for management of fluvio-aeolian aquifers worldwide. Hydrogeology Journal 27, 2835-2855.
Pyrak-Nolte LJ, Morris, JP 2000. Single fractures under normal stress: The relation between fracture specific stiffness and fluid flow. International Journal of Rock Mechanics and Mining Sciences 37, 245-262.
Vadacca L, Rossi D, Scotti A, Buttinelli M 2021. Slip tendency analysis, fault reactivation potential and induced seismicity in the Val d’Agri oilfield (Italy). Journal of Geophysical Research: Solid Earth, 126(1), 2019JB019185.