Fracturing and Porosity Channeling in Fluid Overpressure Zones in the Shallow Earth’s Crust

Abstract

At the time of energy transition, it is important to be able to predict the effects of fluid overpressures in different geological scenarios as these can lead to the development of hydrofractures and dilating high-porosity zones. In order to develop an understanding of the complexity of the resulting effective stress fields, fracture and failure patterns, and potential fluid drainage, we study the process with a dynamic hydromechanical numerical model. The model simulates the evolution of fluid pressure buildup, fracturing, and the dynamic interaction between solid and fluid. Three different scenarios are explored: fluid pressure buildup in a sedimentary basin, in a vertical zone, and in a horizontal layer that may be partly offset by a fault. Our results show that the geometry of the area where fluid pressure is successively increased has a first-order control on the developing pattern of porosity changes, on fracturing, and on the absolute fluid pressures that sustained without failure. If the fluid overpressure develops in the whole model, the effective differential and mean stress approach zero and the vertical and horizontal effective principal stresses flip in orientation. The resulting fractures develop under high lithostatic fluid overpressure and are aligned semihorizontally, and consequently, a hydraulic breccia forms. If the area of high fluid pressure buildup is confined in a vertical zone, the effective mean stress decreases while the differential stress remains almost constant and failure takes place in extensional and shear modes at a much lower fluid overpressure. A horizontal fluid pressurized layer that is offset shows a complex system of effective stress evolution with the layer fracturing initially at the location of the offset followed by hydraulic breccia development within the layer. All simulations show a phase transition in the porosity where an initially random porosity reduces its symmetry and forms a static porosity wave with an internal dilating zone and the presence of dynamic porosity channels within this zone. Our results show that patterns of fractures, hence fluid release, that form due to high fluid overpressures can only be successfully predicted if the geometry of the geological system is known, including the fluid overpressure source and the position of seals and faults that offset source layers and seals.

A review of natural hydrofractures in rocks

“…This can be observed in numerical models (Fig. 7), as well as in experiments with granular media and injecting fluids or air, which in fact is also a fluid (Flekkøy, 2002;Johnsen et al 2006Johnsen et al , 2008aVinningland et al 2007aVinningland et al , b, 2010Vinningland et al , 2012Goren et al 2010Goren et al , 2011Niebling et al 2010a, b;Ghani et al 2013Ghani et al , 2015Aleksans et al 2020;Koehn et al 2020). In the numerical model the fractures are first created and are then followed by the development of fracture channels that form as connected fractures that can open while the surroundings are compacted and thus closed.…”

“…2b). Once the principal stresses reach negative values, the horizontal and vertical stress will become negative, where now the vertical stress is more negative and a horizontal fracture will develop upon failure (Cobbold & Rodrigues, 2007;Koehn et al 2020). This can be illustrated with a very simple experiment (Bons & van Milligen, 2001) of a jar filled with loose sand and a pore fluid that initially consists of water, dissolved sugar and yeast (Fig.…”

Structural Controls on Basin- and Crustal-Scale Fluid Flow and Resulting Mineral Reactions

“…They describe the formation of hydraulic breccias, subhorizontal fractures, and extensional as well as shear-mode fractures depending on the fluid pressure evolution for the different initial settings. The results by Koehn et al [29] reveal a complex porosity evolution for the different systems and highlight the importance of knowing the geometry of the geological system (including porous layers, seals and faults) when predictions of overpressure distribution and thus of hydrofracture formation have to be made.…”

“…Fluid overpressure in different geological settings can lead to the formation and propagation of hydrofractures, as well as dilating high-porosity zones. By means of novel numerical simulations, Koehn et al [29] systematically evaluate the influence of effective stress fields on failure mechanisms, fracture patterns, and fluid drainage. They consider three different scenarios of pressure buildup and hydrofracturing typically encountered in nature, including a sedimentary basin, a vertical zone, and a horizontal layer offset by a fault.…”

The Importance of Physiochemical Processes in Decarbonisation Technology Applications Utilizing the Subsurface: A Review

“…(1) Knowledge transfer from fossil fuel industry and sharing of data publicly (e.g., Erdlac, 2006;Bu et al, 2012;Groff et al, 2016): This should include the re-skilling and repurposing/ deployment of highly skilled and experienced oil and gas professionals, especially engineers and geologists; (2) Knowledge transfer from active decarbonisation plants around the world to allow optimization and sustainable implementation of technologies in other countries: Examples include storing CO 2 in basalt in the CarbFix Pilot Project in Iceland (Matter et al, 2009), geothermal energy plants such as Reykjanes, Krafla (Friðleifsson et al, 2015;Friðleifsson et al, 2019) or Larderello, Italy (Batini et al, 2003), and ATES at Eindhoven University of Technology in Netherlands (Kallesøe and Vangkilde-Pedersen, 2019); (3) Short and long term laboratory experiments: For instance, scaling experiments (e.g., Stáhl et al, 2000), porositypermeability measurements on fault rocks (e.g., Michie et al, 2020a;Michie et al, 2020b) coupled with in-depth microstructural studies (e.g., Kaminskaite et al, 2019;Kaminskaite et al, 2020); (4) Experiments at test sites, such as the UKGEOS coal mine geothermal test site in Glasgow, nuclear waste disposal sites in Olkiluoto, Finland (Siren, 2015), SKB in Sweden (Rosborg and Werme, 2008), Mont-Terri in Switzerland (Tsang et al, 2012), Mol-Dessel in Belgium (Desbois et al, 2010), and Bure and Tournemire in France (Armand et al, 2007;Matray et al, 2007). ( 5) Study of natural geological systems for long term behaviour and comparisons of predictions based on laboratory experiments coupled with numerical simulations: For instance, outcrops and/or core plugs taken out from natural geothermal systems where hydrothermal fluids have been flowing over long timescales (>10 2 -10 4 yrs) or fossil geothermal systems provide us with the examples of how hydrothermal fluids have affected the rocks on a large scale and how long the system has sustained the flow for (e.g., Major et al, 2018); (6) Numerical modelling using sophisticated and continuously improving codes, e.g.,: Microstructural modelling using hybrid approaches e.g., ELLE (Vass et al, 2014;Piazolo et al, 2019;Koehn et al, 2020) or codes f...…”