Based on Gas liquid porometry – assessment of an alternative method for the determination of flow relevant parameters of porous rocks, presented at the International Symposium of the Society of Core Analysts held in Napa Valley, California, USA, 16-19 September, 2013.
Customarily, mercury injection porosimetry has been used to determine the flow of different parameters in porous rocks, such as permeability and pore throat size distribution. But, mercury porosimetry is burdened by several drawbacks. As a process, it is time consuming and destructive, and it is not environmentally friendly, as it requires fairly large quantities of mercury.
Alternative techniques have been explored to reduce the waste, time, and costs associated with mercury injection porosimetry, along with making the process more environmentally friendly.
Gas Liquid Porometry (GLP) is a recognized method for membranes and filters, but has also proven effective in petrophysics applications, making it a viable alternative to mercury injection porosimetry. It allows users to gather information and a number of parameters with good levels of accuracy and reproducibility in a single, fast measurement, effectively reducing the measuring time from several hours to just minutes.
A standard GLP test requires a porous sample to be injected with an inert and nontoxic wetting liquid, as well as the use of an inert gas like nitrogen to displace the liquid out of the porous network (called a “wet run”). The “wet curve” shows the measured gas flow against the applied pressure (inverse proportional to the pore throat size).
The gas flow against the applied pressure on the dry sample (called a “dry run”) is also measured. Information on the porous network can be gathered (Figure 1) from the wet curve, the dry curve and the “half-dry curve” data by dividing the flow values of the dry curve by two.
Figure 1: Measuring curves and resulting parameters in GLP (d = dry curve, w = wet curve, d/2 = half-dry curve, FBP = largest pore, MFP = mean flow pore: pore size at which 50% of the total gas flow can be accounted; minimum pore size: calculated at the pressure where the wet and the half-dry curves meet).
Additionally, users can calculate the cumulative filter flow distribution against the pore size and the corrected differential filter flow, which illustrates the flow distribution per unit of change in size, which is often defined as pore size distribution.
Application to Cretaceous Sandstones
“Bentheimer sandstone”, which is made up of quartz sandstone with 10-14% feldspar and up to 6% clay, has been explored in great detail by core analysis and special core analysis (CAL / SCAL). Scanning electron microscopy and µ-CT imaging show that throat sizes smaller than 5 µm correlate with pore networks, shaped by dissolved mineral phases. Throats smaller than 500 nm can be linked to the local pore networks between the clay minerals (throats < 0.04 µm).
GLP measurements of 4-5 mm slices from the end of the plug were carried out through the use of a POROLUX™ 1000 porometer (POROMETER nv). Further CAL / SCAL analysis were carried out, including permeability, BET surface, porosity, NMR, and µ-CT, to characterize the equivalent porous networks and to correlate results of Hg-injection and GLP.
Every sample measurement was repeated six times to assess the reproducibility of GLP. Each cycle took approximately one hour, meaning that this experiment was completed in less than seven hours, which is the same as the time needed for a single, conventional Hg-injection test using the same measuring range.
Figure 2 illustrates the average of the GLP curves and binned (left hand side). In order to best compare the results directly with the conventional pore radii distribution – in terms of pore throat bin – a polynomial fitting (best fit: sixth order polynomial; residual sum of squares: 7.45; R-squared: 0.962) is required. This type of fitting also enables users to recalculate the area underneath the fitting curve to the same bin sizes used for the mercury experiment (right hand side).
Figure 2: Averaged and re-calculated pore throat distribution (left hand side) and polynomial fitting for better comparison with results of the conventional mercury injection experiment (right hand side).
To compare, the resulting pore throat bins have been plotted within the pore throat radii distribution gathered with mercury injection porosimetry. Figure 3 illustrates the results stemming from both techniques, demonstrating strong unity between each other. GLP measurements are more prone to “scatter” on the flanks of the main pore throat size, due to the effect of the recalculation for the comparison of both measurements.
Figure 3: Pore throat radii distribution derived by Hg-injection (faded out) and by GLP (light green).
CFP has been found to be an adaptable and viable alternative for the measurement of the effective pore throat radii distribution of porous and permeable sandstones, for example the Bentheimer sandstone. GLP demonstrates good reproducibility and good comparisons with mercury injection experiments. This technique allows users to reuse valuable specimens for future investigations as a result of the combination of a nontoxic wetting fluid and the application of substantially smaller pressures.
This information has been sourced, reviewed and adapted from materials provided by Particulate Systems.
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