HR: 1330h
AN: T43D-07    [Abstracts]
TI: Fracture propagation and fluid flow in fractured reservoirs: field studies and numerical models
AU: * Brenner, S L
EM: Sonja.Brenner@geo.uni-goettingen.de
AF: Geoscience Centre, University of Gottingen, Goldschmidtstr. 3, Gottingen, 37077 Germany
AB: In fractured reservoirs (e.g., for petroleum or geothermal water), fluid flow is largely controlled by the permeability of the fracture network. Together with shear fractures (faults), hydrofractures (extension fractures generated by internal fluid pressure, including mineral veins and joints) contribute considerably to the permeability in fractured reservoirs. The permeability of an individual fracture is proportional to the cube of its aperture. But for fluid flow to occur between two sites in a reservoir, there must be at least one interconnected cluster of fractures that links these sites, that is, the percolation threshold must be reached. Field observations show that in heterogeneous and anisotropic, e.g., layered, rocks many hydrofractures become arrested or offset at layer contacts (become stratabound) and do not form interconnected networks. Here I present results from field studies in layered sedimentary rocks from the Bristol Channel Basin, UK. The Lower Jurassic sections exposed near Kilve, Somerset Coast (Southwest England), and around Nash Point, Glamorgan Coast (South Wales) consist of limestone and shale layers dissected by normal faults (Kilve) or strike-slip faults (Nash Point). Whereas joints occur throughout the study areas, calcite veins occur almost exclusively in the cores and damage zones of the faults. These observations indicate that geothermal water was transported along the then-active faults into the host rocks. Furthermore, there is evidence that the veins were injected as hydrofractures from the fault planes into the limestone layers next to the faults. The most important factors that contribute to hydrofracture arrest or offset are discontinuities, stiffness (Young's modulus) changes between layers, and stress barriers - layers where the local stress field is unfavorable to the propagation of a hydrofracture. Using numerical models I explore the conditions for hydrofracture propagation and conclude that mechanical layering largely controls whether evolving hydrofractures become confined to single layers (stratabound) or not (non-stratabound) in which case a vertically interconnected fracture system may form. These results compare very well with the field results. Non-stratabound fractures often show great variations in fracture orientation and aperture. For vertical hydrofractures, field observations and numerical models indicate that the apertures tend to be greatest in the softest layers. However, the present field observations show that calcite veins and joints mostly follow inclined shear fractures in the soft shale layers but change into extension fractures in the stiff limestone layers. Normally, the inclined shear fractures have much smaller apertures than the vertical extension fractures. Variations of fracture aperture in layered fluid reservoirs are important because of possible channeling of fluid flow along the widest parts of a fracture. These aperture variations of fractures in layered rocks were studied using numerical models. The models indicate that in layered rocks, at the contacts between soft and stiff layers, the stress trajectories (the directions of the principal stresses) often become rotated. This conclusion fits with field observations indicating that not only do fractures change dip, but also strike, between layers, with important implications for reservoir permeability. This follows because fractures of unfavorable strike (with respect to the local stress field) tend to close and fluid flow is inhibited. By contrast, fractures with favorable strikes tend to open up, in which case fluid flow is enhanced.
DE: 3210 Modeling
DE: 5104 Fracture and flow
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2005 Joint Assembly