Object-oriented approach for the pore-scale simulation of DC electrical conductivity of two-phase saturated porous media

Abstract

Archie’s empirical power laws are strictly valid only for homogeneous, water-wet (WW) rocks deprived of microporosity or substantial clay-exchange cations. When these conditions are not met, non-Archie electrical behavior arises whereby relationships among rock resistivity, porosity, and water saturation no longer exhibit power-law dependence. Currently, such an unreliable behavior of empirical laws can be quantified only through pore-scale modeling of electrical conductivity under specific sets of geometric assumptions and with substantial computation memory requirements. We introduce a new geometric concept to simulate direct-current electrical-conductivity phenomena in arbitrary rock models on the basis of 3D grain and pore objects that include explicit distributions of intragranular porosity, clay-exchange cations, nonwetting fluid blobs, thin films, and pendular rings. These objects are distributed in the pore space following simple heuristic principles of drainage/imbibition that honorcapillary-pressure curves. They provide a simple way to parameterize the 3D pore space and to calculate the electrical conductivity of porous media saturated with two immiscible fluid phases by way of diffusive random walks within the brine-filled pore space. Not only is the random-walk method memory efficient but it also allows the inclusion of clay/brine cation exchange surfaces otherwise not possible with conventional pore-network models. By comparing results stemming from random-walk, pore-network, and percolation simulations, we show the importance of grain surface roughness and thin film thickness, even in water-wet rocks where those factors usually are neglected. For the case of strongly oil-wet rocks, we show that thin films, snap-offs, and pore microgeometry have a primary impact on hysteresis-dominated rock resistivity during imbibition (increasing water saturation). Our simulation method agrees well overall with percolation simulation results and is advantageously unaffected by assumptions concerning site-percolation imbibition.

The Dependence of Electrical Resistivity-Saturation Relationships on Multiphase Flow Instability

“…Multiphase fluid flow in porous media is an important problem for applications including petroleum production [1][2][3], migration of nonaqueous phase liquids (NAPLs) in soils and aquifers [4][5][6], and CO 2 sequestration [7]. Viscous, capillary and gravity forces interact in immiscible two phase flow systems to produce stable or unstable flow regimes [8][9][10][11].…”

“…The saturation exponent is dependent on the presence of nonconductive 2 ISRN Geophysics fluid in the pore space and wettability of the rock [12,19]. Although Archie's Law is widely used to determine fluid saturation from resistivity measurements, it is not always valid as the saturation-resistivity relationship depends on the wettability, saturation history, content of clay minerals, salinity of the brine phase and distribution of water and oil in the rock [3,12,15,16]. Most experimental studies of the dependence between saturation and apparent resistivity were based on the assumption that fluid distribution within the sample was homogeneous, which can be rarely obtained during a transient displacement experiment.…”

Assessing the monitorability of CO2 saturation in subsurface saline aquifers

“…Even though Archie's law has been used widely for describing electrical resistivity of rocks, it is strictly valid only for homogenous and water-wet rocks (e.g., Toumelin and Torres-Verdín, 2008), and introducing clay content into the model is also not straightforward. Pride (1994) derived the macroscopic (dimensions of the grains much smaller than the wavelength of the applied electromagnetic disturbances) governing equations for coupled electromagnetics of porous media.…”

Capillary Pressure Prediction from Rock Models Reconstructed Using Well Log Data

“…This approach will eventually reduce or eliminate the need for cutting cores. Capillary-dominated fluid displacements in the rock pore space are simulated by implementing a quasi-static, pore-scale drainage approach, which is based on concepts from pore morphology and local pore-scale physics (Hilpert and Miller, 2001, Toumelin and Torres-Verdín, 2008.…”