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

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.

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.