Petrophysical Changes In Cemented-Ash Formations Due To Calcium Rich Fluid-Rock Interactions: An Experimental Study.

Abstract:

Fluid mediated cementation processes, such as hydrothermal mineral alteration and fault healing, represent hydro-thermo-chemo-mechanical processes that result in dynamic petrophysical changes in the subsurface. A wide range of applications, from energy exploration to hazard monitoring, rely on quantifications of subsurface properties, including strength and permeability. There is increasing evidence of the time-dependence of petrophysical properties due to these subsurface processes, which makes the microstructure of subsurface systems dynamic. This requires rock-physics relationships as functions of rock-fluid alteration, which allows for a more accurate remote monitoring of the petrophysical properties in hydrothermal environments. In this study, we have focused on the cementation of volcanic ash with calcium-rich fluids, a process that results in geologically rapid subsurface seal formation through dissolution-precipitation reactions. A set of compressed cemented-ash samples with variable Ca/Si ratio were produced under saturated conditions to represent the stages of cementation as a proxy for reaction time. A complete petrophysical characterization – including mineralogy, acoustic velocities, triaxial strength, porosity and permeability – revealed two stages of cementation with distinct petrophysical relationships, namely calcium under saturation and calcium over saturation. We found that the P-wave velocity, porosity and volume of cement together highlight the corresponding microstructural changes that result in the distinct petrophysical relationships within the two subgroup. For the purposes of remote monitoring, these functional relationships constitute the basis for rock-physics based modeling to generalize lab-based results, which in turn provide the basis for accurate reservoir modeling. These results have both a direct application to volcanic ash cementation and a wider application to any alkali-silica rock-fluid reaction that results in the cementation of the rock, including earthquake faulting within subduction margins and at the concrete-rock interface.