Fluid flow, reaction and permeability evolution in granular media: Experimental results

During burial and diagenesis of granular aggregates, significant permeability reduction may be induced by the formation of low-temperature, authigenic minerals. To quantitatively assess the importance of this process, we have conducted a series of hydrothermal flow-through experiments using de-ionized water and labradorite/quartz sand. Under most of the conditions tested, significant permeability reduction is observed, sometimes exceeding one order of magnitude. Scanning electron microscope observations together with data from additional experiments show that the observed permeability reduction is entirely a result of secondary mineral growth. In all experiments, permeability reduction is fastest early and levels off in the late stages. To explain the permeability behavior as a function of time, a conceptual model is developed in which precipitation of authigenic minerals is rapid at early times, while dissolution of quartz and labradorite is most active, and becomes slower as the system equilibrates. This model is then quantified using well established kinetic theory. Two models are developed; a fully coupled EQ3/6 model and a simplified analytical model, both of which fit the experimental data over the range of temperatures and stresses tested.; In Chapter 3 a new technique is presented that allows easy identification of the secondary minerals formed during experimental diagenesis. Reacting fluids are doped with Ba and Sr; heavy metals which substitute into the alkali-bearing authigenic minerals that precipitate due to alteration of primary labradorite and quartz. When polished sections of post-experiment specimens are viewed using SEM back-scatter imaging, secondary phases appear as bright patches due to their higher density and average atomic number. The more difficult method of X-ray elemental mapping confirms that these zones represent secondary minerals rich in Ba and Sr. This technique allows the spatial distribution of secondary minerals to be mapped out at the specimen scale in a short time frame. Image analysis shows that observed reductions in permeability are caused primarily by mineral precipitation in narrow pore throats and in areas containing an abundance of fine-grained material.