Fluid flow in the Earth: the influence of dehydration reactions and stress

Lead Research Organisation: University of Liverpool
Department Name: Earth, Ocean and Ecological Sciences


Rocks may contain holes (pores) which, like those in a wet sponge, may be filled with water or other fluids. The fluid pressure in these pores may not be the same as the solid pressure the rock is supporting. Changes in fluid pressure may cause earthquakes, so it is important to understand what controls it. There are three linked influences on fluid pressure which we will address.
1. Many minerals contain H2O within the solid structure and this is given off as fluid during heating, or under other circumstances including a drop in surrounding pressure: this is called dehydration reaction. The fluid given off may then change the fluid pressure. The fluid pressure influences and is influenced by the reaction rate - this is feedback. But does the solid pressure influence reaction progress as well? This point is the least understood aspect of reaction behaviour, yet relates to a quite general situation in the Earth.
2. Fluid pressure is changed if the rock is compacted or otherwise deformed (like squeezing the sponge). Rock is much stronger than sponge, so we need to understand just how fast it will change shape (changing the volumes of pores and hence the pressure within them).
3. Fluid can flow through connected pores, thus dissipating anomalous pressures. We need to measure and understand the permeability, that is, the ease of flow.

In general the reaction is accompanied by deformation and by fluid flow. Our research will unravel the linked effects of these three processes, using experiments on natural materials as well as theoretical modelling. We will use the minerals gypsum (which undergoes dehydration reactions at quite low temperatures) and serpentine in experiments: as intact masses (actual rock, with fluid flow difficult), and as highly porous powders (high permeability, fluid flow easy). One group of experiments will be run below the reaction temperature, so we can examine the deformation behaviour in the absence of other effects. Other groups will be above the reaction temperature so as to examine the effects of solid pressure, fluid pressure and time on reaction progress, and the consequent feedbacks on fluid pressure. Mathematical modelling is required to extrapolate results to large bodies of rock (too large to run experiments on directly) and geological timescales (years to millions of years); we will conduct this in parallel with experiments.

Our results will inform understanding a great variety of situations in which fluid and rock pressures are different: here are two examples. Earthquakes may be triggered by fluid pressure changes but will themselves change fluid pressure. This in turn may lead to dehydration reactions (or the reverse, rehydration reactions) which again modify the pressure field. In geothermal energy fields, fluids move through porous rock but are often chemically reactive. We will apply our results to understanding how metamorphic changes in geothermal fields occur, and how these modify porosity and fluid flow.

Our objectives are core to NERC Theme "Earth System Science 3.3b: Dynamics of the Earth's Interior and their Manifestation at the Surface", from which we quote:
"Many natural hazards processes depend on material properties and dynamic processes that are poorly characterised or understood. These can be best addressed through laboratory measurements and experiments on natural or analogue materials. Major challenges arise because many hazardous processes involve complex multiphase mixtures (gas, solid, liquid) whose properties are either poorly characterised or understood".
Reactive fluid flow in deforming media is one such dynamic process.

Planned Impact

In addition to the academic beneficiaries, there are 3 groups of non-academics we will address and engage in the work (see separate impact plan for more detail). First, we will target schoolchildren both through training their teachers within a well-established Inset program at the University of Liverpool and direct engagement at the schools themselves. This will generate interest in the Earth System as a whole as our work provides insight into so many crustal processes, such as subduction zone seismicity, mid-crustal rheology and compaction in sedimentary basins.

Second, our work has direct implication for earthquake hazard and we wish to engage with this community to identify how we might apply our work more effectively in the area of policy making in addition to the fundamental science. We will do this by visiting members of the Earthquake Hazards Reduction Program (led currently by Dr. Nick Beeler) at Menlo Park, while attending the AGU Fall meeting.

Finally, we will engage the geothermal energy industry who will have a direct interest in the process that we study. Under the conditions at which geothermal field operate, chemical alteration occurs resulting in the potential weakening and compaction of the porous rocks at depth. Furthermore, deposition of minerals (in particular zeolites) can have an adverse and long-term effect on the productivity of the field. All these processes will be addressed by our work, in a generic way, and we anticipate exploring the ways that our conceptual framework can be applied to geothermal fields. We will visit GNS New Zealand - an organisation with significant interests in the Taupo rift geothermal fields and with particular expertise in the area of geothermal energy.


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Description The grant aim is to understand how deformation in the Earth, fluid flow and metamorphism (growth of new minerals) influence each other. Gypsum was used in experiments since it reacts faster than, but in an analogous fashion to, the silicates which prevail in the Earth. As of March 2020 there are X key findings.
1. (2014). Stress (different force per unit area in different directions) was previously thought to have a minor effect on mineral growth. A new theory now shows that the effects are ten times bigger than previously envisaged and this prediction may in the future change the way we interpret the minerals seen in deformed rocks
2. (2015-16). From experiments involving shearing material whilst heating we find that the two processes (involving in addition fluid flow) help trigger earthquakes under certain circumstances but not always (2016).
3. (2016-17) Synchrotron experiments (where a sample is X rayed as it changes chemically) show that gypsum breakdown is controlled by diffusion and that porosity (holes) develop in a simple pattern around each new mineral grain. We have the first movie of metamorphism occurring as heating triggers the release of water from rock. This work provides direct insights into how such dehydration creates connected fluid pathways allowing the release of fluid. This has general significance for how volatiles such as water are moved around in the Earth.
4. (2017-18) Rocks with holes (pores) collapse as they are put under pressure. That sort of collapse can drive out fluid which alters how, for example, hydrocarbon reservoirs evolve; fluid expulsion may in other settings trigger earthquakes. Our new study shows how the forces acting on an experimentally produced porous rock cause collapse but not in the way described by a widely adopted model. The response may be shared by other rock types with similar grain arrangements so our new model for collapse may have wide significance
5. (2017-18) Such dehydration reactions often develop across moving "reaction fronts" which separate unreacted material from that which has undergone substantial change. These are preserved in nature but their origin and rate of development are poorly understood. New experiments show how the width and movement rate of such fronts depend on temperature and imposed stresses; the experimental results are described by a simple mathematical theory which allows prediction of reaction front behaviour for any dehydration reaction.
6. (2018-20) We have shown how a new approach to understanding compaction in porous rocks (published in relation to bassanite) also applies to sandstone compaction, published in 2019 (paper submitted).
7. We have also found surprising localized behaviour during hydrostatic compaction of bassanite (paper in preparation).
Exploitation Route By the usual academic methods of dissemination, our findings can assist in understanding earthquake triggering and in interpreting mineral assemblages in deformed rocks. The reaction we study, that is gypsum breaking down to bassanite (plaster of Paris) during heating, is of commercial value :the plaster industry is worth many billions of pounds worldwide. There is potential to liaise with industrial researchers for mutual benefit.
Sectors Chemicals,Construction,Environment,Other

Description Discussion with British Gypsum has led to a joint PhD proposal, addressing issues of industrial interest as well as blue-skies science
First Year Of Impact 2018
Sector Chemicals
Title New hydrostatic rig 
Description This rig was built for research on this grant, to allow long duration experiments which could not otherwise be conducted. The rig allows for independent control of confining pressure and fluid pressure at elevated temperatures. 
Type Of Material Improvements to research infrastructure 
Provided To Others? No  
Impact Data from long duration experiments are currently being analysed 
Title New method for permeability calculations 
Description Our research involves measuring rock permeability by transmitting oscillatory pressure waves. This tool is new software to enable accurate and precise determination of permeability from the experimental data 
Type Of Material Improvements to research infrastructure 
Year Produced 2016 
Provided To Others? Yes  
Impact Work is ongoing using this tool to interpret experimental results, in Liverpool and in UCL. 
Description Florian 
Organisation University of Edinburgh
Department Earth Science
Country United Kingdom 
Sector Academic/University 
PI Contribution Conducting synchrotron experiments and interpreting results
Collaborator Contribution Design of cell for use in synchrotron; conducting synchrotron experiments and interpreting results
Impact Bedford et al. accepted subject to minor revision
Start Year 2015