Microscopic pore structure of rock salt plays a dominant role in its permeability. In this paper, microscopic pore structure of a set of rock salt samples collected from Yunying salt mine of Hubei province in China is investigated by high pressure mercury injection, rate-controlled mercury penetration, and nitrogen absorption tests. The pore size distribution is further evaluated based on fractal analysis. The results show that pore size of rock salt varies from 0.01 to 300
Rock salt is an ideal underground medium to store energy (oil and natural gas) with 90% of the world’s energy repository constructed in rock salt medium or deserted salt mines [
As a polycrystalline polymer, rock salt features great leakproofness with a porosity rate of less than 0.5% for undisturbed rock salt and a permeability rate of 10−21–10−20 m2 [
Gueguen and Dienes [
However, Stormont and Daemen [
A sketch diagram of pore and pore throat in rock salt. (a) demonstrates two pores connecting with a pore throat; (b) displays a pore throat within a single pore.
In order to better understand the influences of pore structure on permeability of rock salt, the present research implements mercury intrusion and gas adsorption testing methods to test the pore structure of rock salt in Yunying mine of China and analyze the porosity rate of rock salt, average pore throat ratio, average pore radius, size of pore, and the influence distribution geometry has on permeability rate so as to provide physical basis for the research on permeability rate.
The rock salt samples are collected from Yunying salt mine of Hubei province in China. Surface of samples are first cleaned and cut to be cubically shaped with size of 8 × 8 × 8 mm3. Then, the samples are covered perfectly using ethyl alcohol. Ten samples are grouped into two categories: group one contains Samples No. 1–No. 4 being tested by deploying high pressure injection and group two includes Samples No. 5–No. 10 being examined by rate-controlled penetration. The principle and testing setups mercury injection method is showed in Cuevas [
The setup of mercury injection test is organized as follows: Dry the samples by putting samples in vacuum oven with the temperature 80°C lasting for 4 hours; weigh the sample when this process is completed. Open nitrogen pressure relief valve and set the outlet pressure of 0.28 MPa and then open vacuum pump to start test, and simultaneously switch on the data logging system. Put samples into NOVA to start first with low pressure test; then test the samples with deploying high pressure; the parameters including the accumulative volume of the mercury, mercury rate, and porosity are automatically recorded. Shut down testing facilities and take the samples out of the devices.
The nitrogen absorption tests are set as follows: put samples into vacuum drying oven at 150°C and they are vacuumed for 10 hours and then put the samples into device NOVA to determine the adsorption-desorption curve.
The pore size distribution of the rock salt is determined based on high pressure mercury injection tests. Figure
Accumulative mercury quantity versus pore size curve of Sample No. 3.
The testing results show that the porosity of the rock salt samples collected from Yuying mine varies between 0.26% and 3.0%, as shown in Table
Classification of the pore size of rock salt.
Sample number | Total porosity | Macroporosity | microporosity | Subporosity |
---|---|---|---|---|
No. 1 | 3.0660 | 18.90% | 22.56% | 58.54% |
No. 2 | 0.9900 | 15.98% | 26.32% | 57.64% |
No. 3 | 2.4099 | 16.70% | 25.10% | 58.20% |
No. 4 | 2.9468 | 10.81% | 37.06% | 53.04% |
No. 5 | 0.9343 | 51.74% | 12.17% | 86.09% |
No. 6 | 0.9526 | 0% | 30.52% | 63.48% |
No. 7 | 3.743 | 10.56% | 29.18% | 60.26% |
No. 8 | 0.4742 | 52.73% | 18.18% | 29.09% |
No. 9 | 0.5526 | 10.4% | 28.64% | 61.22% |
No. 10 | 0.6339 | 8.46% | 21.18% | 70.36% |
Both pore volume and pore throat volume can be precisely determined by rate-controlled penetration experiments. Thus, the pore structure is investigated very specifically. In this study, six samples including Nos. 5–10 are tested by rate-controlled penetration experiment. Table
Porosity and the average throat radius.
Sample number | No. 5 | No. 6 | No. 7 | No. 8 | No. 9 | No. 10 |
---|---|---|---|---|---|---|
Porosity | 0.9343 | 0.9526 | 3.743 | 0.4742 | 0.5526 | 0.6339 |
Average throat radius | 0.0155 | 0.0143 | 0.0141 | 0.0169 | 0.0168 | 0.0185 |
Considering that nitrogen absorption experiment can test the pore size from 0.35 nm to 100 nm and the rate-controlled penetration test can test pore size ranging from 7 nm to 200
The determined specific surface area and pore size using nitrogen absorption method.
Sample number | XF-01 | XF-02 | XF-03 |
---|---|---|---|
Average pore throat radius (nm) | 6.04 | 13.54 | 38.07 |
Specific surface area (m2 g−1) | 13.3 × 103 | 7.16 × 103 | 0.3 × 103 |
Micropore volume (cm3 g−1) | 80 | 100 | 1400 |
By analysis, the samples XF-02, XF-03, and XF-04 exhibit obviously peaks in their BJH (Barrett Joyner and Halenda) distribution curve, as shown in Figure
The BJH distribution of pore size of the rock salt samples.
Figure
The pore size distribution of Sample No. 3.
The study above revealed that the pore throats have a size generally smaller than 2.0
Taking Samples No. 5–No. 10, for example, the pore channels are analyzed by following fractal theory for the size both smaller and bigger than 2.0
Major parameters for the distribution of pore throat.
Sample number | No. 5 | No. 6 | No. 7 | No. 8 | No. 9 | No. 10 |
---|---|---|---|---|---|---|
Average throat radius ( | 0.0155 | 0.0143 | 0.0141 | 0.0169 | 0.0168 | 0.0185 |
Sorting coefficient | 0.5802 | 0.6187 | 0.7113 | 0.5537 | 0.9104 | 0.6979 |
Fractal dimension | 2.47 | 2.38 | 2.53 | 2.23 | 2.19 | 2.62 |
Fractal coefficient | 4.086 | 22.420 | 11.821 | 16.900 | 11.900 | 9.071 |
Relation coefficient ( | 0.997 | 0.955 | 0.995 | 0.874 | 0.932 | 0.985 |
On the other hand, pore channel is also analyzed using the fractal dimensions, as shown in Table
The pore channel distribution and fractal dimensions.
Sample number | No. 5 | No. 6 | No. 7 | No. 8 | No. 9 | No. 10 |
---|---|---|---|---|---|---|
Fractal dimension | 6.29 | 3.75 | 6.04 | 4.54 | 4.87 | 5.95 |
Fractal coefficient | 19.00 | 8.50 | 6.90 | 6.84 | 8.40 | 12.30 |
Relation coefficient | 0.451 | 0.684 | 0.705 | 0.948 | 0.922 | 0.811 |
Pore throat reveals the connection among the pores which is considered to be crucially important to the permeability of rock mass. The parameters including average capillary pressure,
Ten samples are tested by the capillary tests and the obtained parameters are displayed in Table
Relationship of throat microgeometrical parameters with porosity and permeability.
Microgeometrical parameters | Maximum | Minimum | Correlation with porosity | Correlation with permeability |
---|---|---|---|---|
Skewness | 1.08 | 0.39 | | |
Sorting coefficient | 1.3449 | 0.3023 | | |
Variation coefficient | 0.797 | 0.5006 | | |
Median pore throat radius | 1.115 | 0.0086 | | |
Median pressure | 36.293 | 0.3871 | | |
Displacement pressure (MPa) | 0.07176 | 0.0471 | | |
Maximum mercury saturation | 96.04 | 71.43 | | |
Table
Relationship between porosity and permeability of rock salt.
The low permeable medium has generally small throat radius, but with big pore size. The throat has larger number than pore and thus the throat plays determining role in the permeability of rock salt [
Figure
Pore throat radius and its contribution rate to permeability.
In this paper, the microscopic pore structure and its influences on permeability of rock salt samples collected from Yunying salt mine in China are studied. High pressure mercury injection, rate-controlled mercury penetration, and nitrogen absorption experiments are carried out to determine the pore size and its distribution. The impact of pore size and structure on permeability is then analyzed. Major findings of this study include the following: The porosity of rock salt is determined to be varying between 0.26% and 3.00%. The pore size covers a range from 0.01 to 300 Pore channel size of the rock salt is distributed randomly, but the pore throat radius conforms with the law of fractal theory and the fractal dimension is determined to vary between 2.19 and 2.62. The displacement pressure, The porosity of the rock salt is observed to be not correlated obviously with its permeability. On the other hand, maximum pore and median pore throat radius are both related to permeability of the rock salt. The variation coefficient of pore throat distribution is estimated to be 0.663, indicating they are strongly correlated. Thus, pore throat radius plays dominated role in permeability of rock salt. The further research shows that the influences of big throat radius on permeability of rock salt become lower and the contribution of small throat radius on permeability increases drastically with increasing the fractal dimension.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The project was also supported by the National Natural Science Foundation of China (no. 51474259) and Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) no. CUGL150818 and no. CUGL 150610. The funding provided by China Scholarship Council (CSC) during the visit at University of California, Berkeley, is deeply appreciated.