Fluid-rock interaction

Neutron imaging provides direct observation of evolving water–air and deuterated water–normal water exchanges in flow experiments performed on a laboratory-deformed, microporous, extremely fine-grained laminated limestone. First D2O, then H2O, are injected into the sample base, and the liquids' positions and concentrations are monitored by a combination of neutron radiography and tomography. The pre- neutron beam experimental deformation, at a confining pressure of 20MPa and with axial shortening, produced arrays of fractures and deformation bands superposed onto the undeformed, complex rock texture. The neutron images document significant, evolving, water speed and flow-direction variability in both D2O (figure 1a) and H2O (figure 1b) at the deci-micron scale, and spatially complex patterns of both increasing and decreasing water saturation in the matrix and in the induced features. We infer that what would normally be referred to as capillarity-driven and pressure-driven water movement occur concurrently, in close proximity and in competition, that fluid movement paths are very different between the base-to top matrix frontal advancement and in movement up semi-open fractures. It appears that as local and global water saturations evolve as these two drivers can change their dominance in both matrix and deformed components.
Thin sections have been used to obtain sub-micron resolution SEM images that provide multi-scale information on the textural features' spatial arrangements. The textural characteristics are consistent with the inferences made from the coarser-scale flow imaging. Alternating lamina types provide the primary lithological heterogeneity, while the experimentally created deformations lead to quasi-planar zones of highly comminuted matrix and fracture-like voids, each with characteristic scales ranging from sub-mm to cm. Together, the induced deformation features delineate a partially connected array. The interplay between fluid movement through deformation features, and flow into (and out of) the laminae, implies near-equivalence of local driving pressure and capillary related energies, with subtle shifts in this balance as water saturation increases. The insights gained invite a re-examination of common rules-of-thumb for multi-phase fluid flow often adopted in fractured, low-permeability microporous rocks.

The rocks forming a volcanic edifice or dome are typically saturated or partially- saturated with water. However, most experiments aimed at better understanding the mechanical behaviour of volcanic rocks have been performed on dry samples, and therefore most large-scale models designed to explore volcano stability have used parameters representative for dry rock. Here, we present a combined laboratory and modelling study in which we (1) quantified the influence of water-saturation on the mechanical behaviour of variably hydrothermally altered dome rocks from La Soufrière de Guadeloupe (Eastern Caribbean) and (2) used these new data to investigate the influence of water on dome stability. Our laboratory data show that the ratio of wet to dry uniaxial compressive strength (UCS) and Young's modulus are ~0.30–0.95 and ~0.10–1.00, respectively. In other words, the dome rocks were all mechanically weaker when water- saturated. Further, the ratio of wet to dry UCS decreased with increasing alteration. Micromechanical modelling suggests that the observed water-weakening is the result of a decrease in fracture toughness (KIC) in the presence of water. The ratio of wet to dry KIC also decreases with increasing alteration, explaining why water-weakening increased as a function of alteration. To explore the influence of water saturation on lava dome stability, we numerically generated lava domes in Particle Flow Code using the experimental data corresponding to unaltered and altered rock under dry conditions. The strength of the dome-forming rocks was then reduced to values corresponding to wet conditions. Our modelling shows that, although the stability of the unaltered dome was not influenced by water-saturation, larger displacements were observed for the wet altered dome. Additional simulations in which we modelled a buried alteration zone within an otherwise unaltered dome showed that higher displacements were observed when the dome was water-saturated. We conclude that (1) the water-saturation reduces the UCS and Young's modulus of volcanic rock, (2) larger decreases in UCS in the presence of water are observed for altered rocks, and (3) the stability of a dome can be compromised by the presence of water if the dome is altered, or contains an altered zone or low-strength layer within which water can circulate preferentially. These conclusions highlight that the degree of alteration and water saturation should be mapped and monitored at active volcanoes worldwide, and that large-scale models should use values for water-saturated rocks when appropriate.

Damage zones commonly exist alongside crustal faults, where a combination of micro- and macrocracks form intricate pathways for fluid flow. The permeability of these zones can exhibit rapid changes as the crack network evolves over time and space. Accurate measurements of permeability under high pressure and high temperature conditions are imperative for modeling and comprehending the behavior and evolution of geothermal systems within crustal fault zones. To facilitate such measurements and provide valuable insights into the permeability of crustal rocks, we have devised and tested a new experimental apparatus.
Our high-pressure, high-temperature permeameter comprises three independent components: the permeant circuit, the confining pressure circuit, and the heating elements. A confining pressure of up to 50 MPa can be applied (using silicon oil as the confining fluid), to which a temperature of up to 150°C can be superimposed. Once the desired confining pressure and temperature are attained, we conduct permeability measurements by flowing nitrogen (as the permeant gas) through the sample, simultaneously monitoring the pressure differential between the upstream pressure transducer and the atmospheric pressure downstream of the sample. Permeability measurements are conducted using either the steady-state method, with varying flow rates monitored by the downstream flowmeter, or the pulse decay method, by analyzing the pressure decay following a sudden pressure pulse. 
Using our experimental apparatus, we determined the permeability of microcracked Lanhélin granite samples as well as that of samples with both microcracks and a single cross-cutting macrofracture. Permeability measurements were performed under confining pressures ranging from 2 to 50 MPa at room temperature, 75 °C, and 150 °C. At room temperature, the permeability of microcracked granite decreases by over two orders of magnitude between 2 and 50 MPa of confining pressure, following an exponential trend. This trend remains constant across all temperatures investigated. Moreover, at a fixed confining pressure, increasing temperature results in a decrease in permeability. In granite samples that contain both microcracks and a cross-cutting fracture, permeability is more than four orders of magnitude higher than in a microcracked granite without macrofracture. The influence of temperature and pressure on permeability is also significantly reduced in granite samples with both microcracks and a cross-cutting fracture, in contrast to microcracked granite alone. These findings hold significant value in informing numerical models that seek to explore the impact of in-situ conditions on fluid flow within fractured geothermal reservoirs.