The role of stress history on the flow of fluids through fractures

Abstract

Understanding flow along fractures and faults is of importance to the performance assessment (PA) of a geological disposal facility (GDF) for radioactive waste. Flow can occur along pre-existing fractures in the host-rock or along fractures created during the construction of the GDF within the excavation damage zone (EDZ). The complex fracture network will have a range of orientations and will exist within a complex stress regime. Critical stress theory suggests that fractures close to localized shear failure are critically stressed and therefore most conductive to fluid flow. Analysis of fault geometry and stress conditions at Sellafield has revealed that no features were found to be, or even close to being, classified as critically stressed, despite some being conductive. In order to understand the underlying reasons why non-critically stressed fractures were conductive a series of laboratory experiments were performed. A bespoke angled shear rig (ASR) was built in order to study the relationship between fluid flow (water and gas) through a fracture surface as a function of normal load. Fluid flow reduced with an increase in normal load, as expected. During unloading considerable hysteresis was seen in flow and shear stress. Fracture flow was only partially recovered for water injection, whereas gas flow increased remarkably during unloading. The ratio of shear stress to normal stress seems to control the fluid flow properties during the unloading stage of the experiment demonstrating its significance in fracture flow. The exhumation of the Sellafield area during the PalaeogeneÀNeogene resulted in considerable stress relaxation and in fractures becoming non-critically stressed. The hysteresis in shear stress during uplift has resulted in faults remaining, or becoming, conductive. The field and laboratory observations illustrate that understanding the stress-history of a fractured rock mass is essential, and a mere understanding of the current stress regime is insufficient to estimate the flow characteristics of presentday fractures.

Carbon dioxide storage in the Captain Sandstone aquifer: determination of in situ stresses and fault-stability analysis

“…Although the analysis presented assumes that critically stressed faults in the current stress regime will present a risk to storage integrity due to an increase in along-fault permeability during slip, it has been shown that shear-stress hysteresis during uplift and exhumation can also affect the ability of faults to conduct fluids, even if they are not critically stressed in the present stress state (Sathar et al 2012). Experiments on synthetic fault gouge composed of pure kaolinite found that, although fault orientation does have an effect on gas entry pressure, non-optimally orientated faults can also experience flow (Cuss et al 2015).…”

An overview of wellbore methods of investigating stress state of the upper layers of the earth’s crust

“…Тем не менее, существуют примеры регионов, в которых указанная гипотеза не подтверждается. В частности, детальные исследования региона проекта Селлафилд [Sathar et al, 2012] показали, что количественно гипотеза связи между критически напряженными и флюидопроводящими трещинами не подтверждается -ни одна из наблюдаемых трещин не находится в критическом напряженном состоянии, хотя некоторые из них являются флюидопроводящими. Авторы объясняют это несоответствие историей развития напряженного состояния: хотя в текущем поле напряжений флюидопроводящие трещины не являются критически напряженными, они могли быть критически напряженными раньше.…”

Impact of in situ stress and fault reactivation on seal integrity in the East Irish Sea Basin, UK

“…The relationship between in situ stress and fluid flow in faults is well established, with critically stressed faults aligned favourably to the stress tensor perceived as being most likely to be hydraulically conductive and posing a greater risk to seal integrity. While this has been shown not to be true in all cases (Laubach et al 2004;Sathar et al 2012;Cuss et al 2015), a number of studies from a variety of settings worldwide (Barton et al 1995;Wiprut & Zoback 2000;Finkbeiner et al 2001;Hennings et al 2012) indicate that it is useful to adopt as a general rule in assessing seal integrity, at least in the absence of evidence regarding specific fault properties. In a study of fluid flow in naturally occurring fractures at the Soultzsous-Forets Hot Dry Rock geothermal site in France, Evans (2005) showed that while all flowing fractures were observed to be critically stressed, a large number of similarly stressed fractures were not hydraulically conductive.…”