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.
Highlights
Fracturing and the development of mineralized veins and breccia are an important process in the Earth’s crust linked to fluid flow and mineral deposits and have important applications related to the energy transition with reactive flow in geothermal systems and carbon capture and storage (CCS) in aquifers and decommissioned oil and gas fields and energy storage in sedimentary basins [1,2,3,4,5,6]
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
Our results show that patterns of fractures, 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
Summary
Fracturing and the development of mineralized veins and breccia are an important process in the Earth’s crust linked to fluid flow and mineral deposits and have important applications related to the energy transition with reactive flow in geothermal systems and carbon capture and storage (CCS) in aquifers and decommissioned oil and gas fields and energy storage in sedimentary basins [1,2,3,4,5,6]. A rise in the local fluid pressure in a rock pore leads to an outward directed force that works against the solid and can reduce the solid stress to an effective stress σ′ij according to Terzaghi’s law [9]: σ′ij = σij − Pf δij, ð1Þ with σij being the total normal stress, Pf the pore fluid pressure, and δij the Kronecker delta with the sign convention of positive compressive stress This effect is often visualized in the Mohr diagram of effective shear to normal stress where a rise in fluid pressure leads to a reduction of the mean stress and eventually to failure if the fluid pressure is high enough. This compressibility is not necessarily important for the initial fracturing process but becomes important once the system evolves and the solid and fluid interact dynamically leading to opening and closing channels, for example, in the fluidization of sediments [16,17,18,19]
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