Fracture (geology) | EPFL Graph Search

A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity. Brittle deformation Fractures are forms of brittle deformation. There are two types of primary brittle deformation processes. Tensile fracturing results in joints. Shear fractures are the first initial breaks resulting from shear forces exceeding the cohesive strength in that plane. After those two initial deformations, several other types of secondary brittle deformation can be observed, such as frictional sliding or cat

Transmissivity of rock fractures: Normal loading and shear reactivation

Monia Woehner

Enhanced Geothermal Systems represent a major field of study in the context of renewable energy resources. To create extractable energy from those reservoirs, a high enough fluid flow rate for production needs to be achieved. This fluid flow rate is directly related to the permeability of the fracture system in the reservoir. Hence, by increasing the reservoir’s permeability the fluid flow increases as well. For this purpose, shear stimulation has proven itself effective but remains a challenging field of study. Many aspects of this mechanism are still unknown and have to be explored further. This study focuses on the fracture transmissivity evolution of low porosity rocks (i.e. granite and marble) with smooth fracture surfaces subjected to progressing shear displacement until several millimetres of offset. The aim of this study was to record fracture transmissivities continuously throughout the experiment without stopping the mechanical loading processes and thereby using the oscillatory pore pressure method. In this context, a new experimental procedure was created making a direct shear configuration using a standard triaxial apparatus possible. Both samples were exposed to a constant confining pressure of 25 MPa which can be directly reflected as normal stress on the fracture. The granite fracture transmissivities were in the range of 6.8317·10−20 to 3.4429·10−19 m3. They first followed a decreasing trend until reaching the minimum value. Then a strong peak could be observed after which transmissivities were increasing again until the end of the experiment. Numerous peaks and deviations from the linear trend could also be observed. The marble fracture transmissivities were in the range of 1.4604 · 10−20 to 2.1110 · 10−20 m3 and were generally lower than the granite’s. Also, the transmissivity evolution followed a different path, continiously and slowly decreasing until the final position. The curve was generally more flattened and did not show any strong peaks. These results showed that rock fractures behave in different manners according to their lithology and more precisely, to the material’s hardness. Rock fractures often form wear products which can either obstruct the flow paths or enhance them as a function of the wear mechanism and wear volume. The granite sample encountered the debris formation mechanism that would transition from mild to severe wear. This transition could be observed in its fracture transmissivity passing from a decreasing to an increasing trend. On the other hand, the marble sample was subjected to a continual asperity smoothing mechanism, thereby slowly closing up the fracture and decreasing its fracture transmissivity. Within the frame of EGS, the results of this work can have interesting implications for production rates. Results showed that not only loading conditions can affect the fluid flow through rock fractures. Rock lithologies and ranges of shear displacement also play an important role and should be explored further.

2020

Laboratory Investigation on Seismic Response of Rock Fractures Filled with Granular Materials

Wei Wu

Filling materials may exist in all scales of rock fractures, not only influencing seismic wave attenuation, but controlling rock mass instability. The objective of this study is to experimentally investigate the seismic response of rock fractures filled with granular materials. This experimental study simplifies the complex interaction between seismic waves and filled rock fractures and considers the seismic-induced compressive responses of a filled single fracture and a set of filled parallel fractures and the seismic-induced frictional slip on a filled fracture. The seismic-induced compressive response of a filled single fracture is investigated by a split Hopkinson rock bar (SHRB) test. The SHRB test simulates the P-wave propagation across the filled fracture and verifies the analytical solutions predicted by the displacement and stress discontinuity model. The seismic wave attenuation across the filled fracture is due to wave reflection at the fracture interfaces and dynamic compaction of the filling materials. The specific fracture stiffness and the specific initial mass of the filling materials are two key parameters in interrelating the physical, mechanical and seismic properties of the single fracture filled with dry granular materials. The SHRB test is modified to study the seismic-induced compressive responses of a set of filled parallel fractures and to verify the analytical solutions predicted by the modified recursive method based on the displacement and stress discontinuity model. The seismic wave attenuation across the filled parallel fractures is due to multiple wave reflections between the filled fractures, as well as wave reflection at the fracture interfaces and dynamic compaction of the filling materials. The dynamic compaction of the filling materials induces the decreases of loading rate and dominant frequency when the P-wave propagates across each filled fracture in a fracture set. The seismic-induced frictional slip on a filled fracture is performed by a dynamic-induced direct-shear (DIDS) test. A generated plane P-wave propagates as a dynamic shear load and induces the frictional slip along a layer of filling materials. The dynamic shear stress is non-uniformly accumulated in the filling materials. The shear stress at the trailing edge controls the dynamic triggering of frictional slip owing to the highly compacted filling materials. The dynamically triggered frictional slip is quickly arrested by the re-connection of sand contacts after the instantaneous disturbance, while the statically triggered frictional slip is unrecoverable during a short duration and contains a main slip and a few short slips before and after the main slip. The seismic response of rock fractures filled with granular materials is decomposed into the seismic-induced compressive responses and the seismic-induced frictional slip in this study. Both seismic-induced responses are strongly related to the dynamic response of the filling materials. The analytical models and the experimental experiences are possible to be numerically combined to study random wave propagation across rock fractures filled with granular materials and to be applied to estimate seismic wave attenuation and rock mass instability.

EPFL2013

Seismic events miss important kinematically governed grain scale mechanisms during shear failure of porous rock

Edward Ando

Catastrophic failure in brittle, porous materials initiates when smaller-scale fractures localise along an emergent fault zone in a transition from stable crack growth to dynamic rupture. Due to the rapid nature of this critical transition, the precise micro-mechanisms involved are poorly understood and difficult to image directly. Here, we observe these micro-mechanisms directly by controlling the microcracking rate to slow down the transition in a unique rock deformation experiment that combines acoustic monitoring (sound) with contemporaneous in-situ x-ray imaging (vision) of the microstructure. We find seismic amplitude is not always correlated with local imaged strain; large local strain often occurs with small acoustic emissions, and vice versa. Local strain is predominantly aseismic, explained in part by grain/crack rotation along an emergent shear zone, and the shear fracture energy calculated from local dilation and shear strain on the fault is half of that inferred from the bulk deformation.

NATURE PORTFOLIO2022