The visualization of flow paths in experimental studies of clay-rich materials

One of the most challenging aspects of understanding the flow of gas and water during testing in clay-rich low-permeability materials is the difficulty in visualizing localized flow. Whilst understanding has been increased using X-ray Computed-tomography (CT) scanning, synchrotron X-ray imaging and Nuclear Magnetic Resonance (NMR) imaging, real-time testing is problematic under realistic in situ conditions confining pressures, which require steel pressure vessels. These methods tend not to have the nano-metre scale resolution necessary for clay mineral visualization, and are generally not compatible with the long duration necessary to investigate flow in such materials. Therefore other methods are necessary to visualize flow paths during post-mortem analysis of test samples. Several methodologies have been established at the British Geological Survey (BGS), in order to visualize flow paths both directly and indirectly. These include: (1) the injection of fluorescein-stained water or deuterium oxide; (2) the introduction of nano-particles that are transported by carrier gas; (3) the use of radiologically tagged gas; and (4) the development of apparatus for the direct visualization of clay. These methodologies have greatly increased our understanding of the transport of water and gas through intact and fractured clay-rich materials. The body of evidence for gas transport through the formation of dilatant pathways is now considerable. This study presents observations using a new apparatus to directly visualize the flow of gas in a kaolinite paste. The results presented provide an insight into the flow of gas in clay-rich rocks. The flow of gas through dilatant pathways has been shown in a number of argillaceous materials (Angeli et al., 2009; Autio et al., 2006; Cuss et al., 2014; Harrington et al., 2012). These pathways are pressure induced and an increase in gas pressure leads to the dilation of pathways. Once the gas breakthrough occurs, pressure decreases and pathways begin to close. This new approach is providing a unique insight into the complex processes involved during the onset, development and closure of these dilatant gas pathways

Bentonite homogenisation during the closure of void spaces

In a geological repository, the disposal of radioactive waste will result in the creation of engineering voids. Bentonite is commonly proposed as a sealing material as a result of its high swelling capacity. As the bentonite expands, the non-uniform development of porewater pressure and its coupling to total stress within the bentonite, may impair homogenisation. In this study we present results from five laboratory tests performed on sodium- and calcium-based bentonites to examine their swelling potential and capacity to homogenise over extreme bentonite-to-void ratios. Results demonstrate that even under these extreme ratios, the bentonite is able to swell and ultimately fill each void, creating a small swelling pressure. The swelling pressure development is spatially complex and time-consuming, and does not appear to be influenced by friction. Instead, it is characterised by plastic yielding of the clay with 70%–80% of the volume change associated with clay expansion adjacent to the void. This leads to heterogeneity illustrated by the presence of persistent differential stresses and the non-uniform distribution of moisture contents. Increases in the moisture content were measured but did not always correlate with the development of swelling pressure. This disequilibrium of the system is likely a reflection of the test durations and the slow evolution in the rates of change in swelling and porewater pressure beyond 130 days. Given the length of the experimental tests presented here, the time required to achieve full homogenisation of the clay is likely to be many years, if it occurs at all. Gravity segregation was also present in horizontal tests, further impairing clay homogenisation. However, as presented in this paper, it is possible to define functional relationships describing the bentonite swelling potential across engineering voids of differing size. This information will assist in establishing a safety case for bentonite usage in geological radioactive waste disposal

The role of fault gouge properties on fault reactivation during hydraulic stimulation; an experimental study using analogue faults

During the hydraulic stimulation of shale gas reservoirs the pore pressure on pre-existing faults/fractures can be raised sufficiently to cause reactivation/slip. There is some discrepancy in the literature over whether this interaction is beneficial or not to hydrocarbon extraction. Some state that the interaction will enhance the connectivity of fractures and also increase the Stimulated Reservoir Volume. However, other research states that natural fractures may cause leak-off of fracturing fluid away from the target zone, therefore reducing the amount of hydrocarbons extracted. Furthermore, at a larger scale there is potential for the reactivation of larger faults, this has the potential to harm the well integrity or cause leakage of fracturing fluid to overlying aquifers. In order to understand fault reactivation potential during hydraulic stimulation a series of analogue tests have been performed. These tests were conducted using a Bowland Shale gouge in the Angled Shear Rig (ASR). Firstly, the gouge was sheared until critically stressed. Water was then injected into the gouge to simulate pore fluid increase as a response to hydraulic stimulation. A number of experimental parameters were monitored to identify fracture reactivation. This study examined the effect of stress state, moisture content, and mineralogy on the fault properties. The mechanical strength of a gouge increases with stress and therefore depth. As expected, a reduction of moisture content also resulted in a small increase in mechanical strength. Results were compared with tests previously performed using the ASR apparatus, these showed that mineralogy will also affect the mechanical strength of the gouge. However, further work is required to investigate the roles of specific minerals, e.g. quartz content. During the reactivation phase of testing all tests reactivated, releasing small amounts of energy. This indicates that in these basic conditions natural fractures and faults will reactivate during the hydraulic stimulation if critically stressed. Furthermore, more variables should be investigated in the future, such as the effect of fluid injection rate and type of fluid

Hydraulic fracturing: a review of theory and field experience

This report summarises the current state-of-the-art knowledge of the hydraulic fracturing process used by the shale gas/oil industry using open peer-reviewed literature and from government commissioned research reports. This report has been written to make statements on our knowledge of the following questions: • How do hydrofractures form? • How far do hydrofractures extend during stimulation? • What dictates where hydrofractures propagate? • How do hydrofractures interact with the existing fracture network? • Can the size and distribution of hydrofractures be controlled? Gaps in our knowledge have been highlighted, with the largest of these resulting from differences between North American and European shale rocks

Scoping study examining the behaviour of Boom Clay at disposal depths investigated in OPERA

The Onderzoeks Programma Eindberging Radioactief Afval (OPERA) is the third national research programme for the geological disposal of radioactive waste in the Netherlands, operating during the period 2011 to 2017. The rock types to host a geological disposal facility that are currently being considered in the Netherlands are salt and clay. Earlier Dutch work focussed mostly on salt, but the present programme OPERA is focused on a specific clay formation the Boom Clay. Previous studies have primarily focused on examining behaviour at the Belgium reference depth (~220m). The work presented in this study extends this knowledge-base to repository depths of potential interest in the Netherlands (~500m). In this report, results from a scoping study examining the hydromechanical properties of Boom Clay are presented, including investigations on consolidation, swelling, hydraulic, gas and deformation behaviour. These were performed on preserved core material retrieved from the HADES underground research laboratory, Belgium. Sections of core were then consolidated to a depth representative of the Netherlands. Permeability was sensitive to stress state and thermal load, though incremental changes in NaCl concentration had minimal impact. Hysteresis was observed in thermally induced changes in permeability. Gas entry was closely linked to the minimum principal stress component, with mass and volume changes of samples observed as a result of gas migration. Stress, porewater pressure and gas flow were integrally linked with pathways evolving temporally and spatially. A transition from brittle to ductile deformation was noted with increasing stress in both compression and shear. Boom Clay is both complex and anisotropic in its behaviour

CONTAIN D11 : integrated final results and conclusions

Carbon capture and storage is a technology capable of reducing CO2 outputs on a large scale; the concept usually requires CO2 to be removed from post-combustion flue gases and sequestered in geological formations. Depleted gas fields constitute “the most important storage type for the UK” and will provide a large and important potential future offshore storage capacity (DECC, 2012). Over the last 4 years, the CONTAIN research project has focussed on the geomechanical behaviour of depleted hydrocarbon fields in response to injection with CO2, combining a modelling and experimental approach with the public perceptions of CCS into three work packages. The project has provided a better understanding of the hydromechanical impacts of depletion on caprocks and the effect of subsequent CO2 injection, in order to assist with the implementation of CCS in this type of reservoir. Work package 1 outlined a phenomenological approach to assessing possible deformation during operation. Focus was placed on rock mechanics and transport experiments on material from the geologies of target formations in the North Sea, providing information that could be incorporated into numerical simulations. Work package 2 expanded this understanding by considering fractured caprock. Numerical modelling was used to study the deformation of an initially intact caprock caused by the depletion of an underlying reservoir during oil extraction. Deformation and flow were geomechanically modelled in three dimensions using a fully coupled poroelastic model, incorporating discrete fractures and faults into the caprock. Work package 3 offered new and valuable insight on future public awareness campaigns aimed at gaining acceptance of CCS. Qualitative expert interviews have been used, a CCS expert survey and a public survey across four countries to gain an understanding of perceptions of CCS risks and benefits, and has allowed for comparison of views on CCS between experts and public. In addition, the work package has explored the impact of different message framings on CCS attitudes. The findings of each work package are summarised in this report, with each work package represented by a report chapter. A synthesis of the findings and discussion of the work as a whole follows