Quantifying solute spreading and mixing in reservoir rocks using 3-D PET imaging

Abstract

We report results of an experimental investigation into the effects of small-scale (mmcm) heterogeneities on solute spreading and mixing in a Berea Sandstone core. Pulsetracer tests have been carried out in the regime Pe = 6 − 40 and are supplemented by a unique combination of two imaging techniques. X-ray CT is used to quantify subcore scale heterogeneities in terms of permeability contrasts at a spatial resolution of about 10 mm 3 , while [11C]PET is applied to image the spatial and temporal evolution of the full tracer plume non-invasively. To account for both advective spreading and local (Fickian) mixing as driving mechanisms for solute transport, a streamtube model is applied that is based on the 1D Advection Dispersion Equation. We refer to our modelling approach as semi-deterministic, because the spatial arrangement of the streamtubes and the corresponding solute travel times are known from the measured rock's permeability map, which required only small adjustments to match the measured tracer breakthrough curve. The model reproduces the 3D PET measurements accurately by capturing the larger-scale tracer plume deformation as well as sub-core scale mixing, while confirming negligible transverse dispersion over the scale of the experiment. We suggest that the obtained longitudinal dispersivity (0.10 ± 0.02 cm) is rock-rather than sample-specific, because of the ability of the model to decouple sub-core scale permeability heterogeneity effects from those of local dispersion. As such, the approach presented here proves to be very valuable, if not necessary, in the context of reservoir core analyses, because rock samples can rarely be regarded as "uniformly heterogeneous".

A review of geochemical–mechanical impacts in geological carbon storage reservoirs

“…59,107,119 Thus, the overall or effective rate of reactivity is controlled by the slowest of these three steps, leading to the use of characteristic time scales to provide metrics for the behavior of these systems relative to one another. One such metric is the local Péclet number, defined as Pe = l·u/Do, where Do is the molecular diffusion of the reactants (1.0-7.5 × 10 -9 m 2 s -1 for Ca 2+ at 25°C), l is the characteristic length of the pores that vary from 0.1 × 10 -3 m (for sandstone rocks; Pini et al 120 ) to 1 × 10 -3 m (for limestone rocks; Luquot and Gouze, 119 ), and u is the sample-averaged seepage velocity (u = Q·L/φ). This dimensionless parameter thus sets a relative influence of mixing (e.g., diffusion) versus advective transport on solute generation.…”

Measuring, imaging and modelling solute transport in a microporous limestone

“…Positron Emission Tomography (PET), a nuclear imaging technique, offers the potential to overcome these issues by providing a spatial resolution comparable to clinical Xray CT instruments (O ∼ 1 mm), while requiring minimal doses of radiotracer (O ∼ 10 −12 − 10 −13 mol/mL) and enabling high temporal resolution (O ∼ 10 s) [27,28,29,30]. Recent years have seen an increased use of PET to the study of flow and transport processes in various geomaterials [31,32,33,34,35], including sandstones [36,37,30,38,39]. However, only recently have experimental studies been validated against predictions by numerical models, such as those described above [28,34] and/or used to quantify the degree of mixing in the sample [38,39].…”

“…Recent years have seen an increased use of PET to the study of flow and transport processes in various geomaterials [31,32,33,34,35], including sandstones [36,37,30,38,39]. However, only recently have experimental studies been validated against predictions by numerical models, such as those described above [28,34] and/or used to quantify the degree of mixing in the sample [38,39]. Efforts in this direction are justified by the ultimate goal of improving our understanding of transport processes in heterogeneous porous media.…”

Observations of the impact of rock heterogeneity on solute spreading and mixing

“…However, observations that can distinguish between spreading and mixing and identify larger scale transport processes are restricted to unconsolidated sand or bead pack models. These are limited in their ability to provide spatially resolved observations in three dimensions [Rahman et al, 2005;Rolle et al, 2009;Castro-Alcala et al, 2012;Ye et al, 2015aYe et al, , 2015bYe et al, , 2015cPini et al, 2016].…”