Development Of Fracture Network Research Articles

Evolution of fracture networks in shear zones: Insights from see-through experiments on biphenyl aggregates

Evolution of fracture porosity in mid-crustal shear zones can be simulated in in-situ experiments. Evolution of fracture networks was monitored during simple shearing of 2-mm-wide zones in wet and dry aggregates of polycrystalline biphenyl (C6H5C6H5) in a Urai–Means see-through deformation apparatus. At low effective confining pressures in wet samples, mixed brittle-viscous deformation occurred at all strain rates (5.6×10−4–5.8×10−6s−1) at 94–97% of the absolute melting temperature. At the fastest strain rate, progressive shearing is rapidly localized to produce a narrow fault zone along a shear zone boundary. In contrast, at the slowest strain rate, fractures develop throughout the shear zone and connect to form continuous fracture systems at low shear strains (γ≈2). These fracture systems accommodate most of the subsequent displacement in contrast to little fracturing and predominantly viscous deformation in a nominally dry experiment. Jogs, as parts of stairstepping fracture networks in wet samples, resemble in shape and distribution veins found in mid- to lower crustal shear zones. The experiments indicate that, from low strains onwards, the presence of fluids close to confining pressure in creeping shear zones can lead to the development of connected fracture networks that also localize most of the displacement.

Growth of geologic fractures into large-strain populations: review of nomenclature, subcritical crack growth, and some implications for rock engineering

Several aspects of fracture arrays are reviewed briefly and discussed. The terminology applied to progressive or multi-stage brittle deformation in rock masses is improved by noting fundamental mechanical differences in fracture type and the kinematic coupling between dilatant mixed-mode crack displacements and wing cracks developed at the fracture tips . An array of initially mixed-mode (I–II) cracks will evolve under remote tensile least principal stress and with increasing strain to a dilatant, mode-I crack array oriented approximately perpendicular to the remote tensile stress. This progressive fracture growth thus defeats predictions of fracture-set orientation and displacement based only on a Mohr circle estimate of initial elastic stress (valid in the rock mass only at the earliest stages of fracture nucleation). Slow, subcritical crack growth in rock is associated with distinctive changes in fracture population geometry, as shown by published numerical simulations of fracture–network evolution. An increase in the stress corrosion index promotes joint clustering and significant changes in joint length–frequency that may lead to characteristic differences in the statistics of large-strain fracture populations. These geometric clues can be used to refine estimates of strength and deformability of rock masses and to infer classes of physico-chemical processes acting at the fracture tips during the development of the fracture population.

Geomechanics

This article discusses special challenges posed for solid mechanics by problems in geotechnology and geophysics and recent advances made by applications of solid mechanics to these areas. In addition, the article discusses selected promising areas of current research, focussing on results and needs for further research in the following areas: multi-axial constitutive relations; the coupling of mechanical response with fluid flow and chemistry; fracture growth, interaction and network development; and earthquake dynamics.

Wave propagation, damage evolution, and dynamic fracture extension. Part I. Percussion drilling

Percussion drilling, wave propagation in damageable media, and the evolution of damage caused by repeated impacts during percussion drilling in brittle disordered materials are investigated. Analytic, numerical, and experimental works complementing each other in clarifying the complex dynamic process of damage evolution and fracture network development in polycrystalline brittle materials are presented. Rock samples from field tests were sectioned and a microscopic investigation of the damage and fracture zones at the bottom and circumference of the borehole was performed. On the basis of experimental findings, we develop a three-dimensional numerical simulation scheme which incorporates wave propagation in dissimilar granular arrangements, dynamic impact phenomena, and accumulation of brittle damage to the rock grains. The combination of brittle damage continuum mechanics with the theory of propagation of elastic wave proves to be a very suitable approach for the description of impact destruction of rock and rocklike materials showing brittle behavior. The developed 3D finite-block numerical code enables one to simulate single and multiple impacts on personal computers. Further development will include the impact of a sequence of bits and the implementation of time-varying elastic properties in the wave propagation model. The results of experimental field tests are in good agreement with numerical model simulations.

Fracture Networks in Rotliegend Gas Reservoirs of the Dutch Offshore and their Impact on Field Development Planning and Infill Drilling: ABSTRACT

The influence of fractures upon the production behaviour of the Rotliegend (early Permian) gas fields in the Dutch offshore has been investigated. Small scale fracture networks can have a significant influence (positive or negative) on the production performance of wells or on the amount of reserves a wall cm drain. Fracture data collected from core and aged logs were corrected to account for the bias related to one dimensional sampling methods. Fracture systems have been described in terms of orientation, frequency, origin, type and in relation to larger-scale structures. In addition, integration of these data with fracture cements and diagenetic data resulted in the timing of the fracture network development. Two fracture systems can be distinguished on basis of their regional extent: (a) Fault related consist of shear fractures which occur close to and parallel with larger-scale faults. These fractures trend NE-SW and NW-SE and locally an N-S oriented set is developed. (b) Regional fractures preferentially consist of tensional fractures oriented around E-W. Fault-related fractures formed during the mid/late Kimmerian rifting phase. In the Jurassic rift basins the regional fractures appear to have formed during the late Cretaceous inversion events whereas outside these basins the regional fractures probably developed duringmore » late Jurassic times. The NE-94 and N-S fault-related shear fractures tend to compartmentalise reservoirs. The NW-SE fault-related shear fractures and the regional extensional fractures tend to enhance reservoir flow properties, especially in areas of poor reservoir quality. The results of such regional fracture studies we being incorporated in the planning of infill drilling and field development of the Rotliegend gas fields in the Dutch Offshore.« less