1. Pore Size Distribution of shale using Advanced Analytical Techniques
Ashutosh Tripathy1*, Vinoth Srinivasan2, T.N. Singh3 EGU General Assembly 2018, Vienna, Austria
Abstract
Methodology
Results
The mercury intrusion method is best suited to study the meso and macro pores.
The MIP results display good conformance with the LPGA results.
The incremental pore volume of shows peaks in the range of 6 to 10nm.
The broad range of volume distribution indicates wide variance in pore sizes.
It can be seen that the Hg curve clearly follows the CO2 curve and is relatively smoother. This can be attributed to the deeper access of the gases to the pore spaces as compared to mercury.
Shale gas is becoming one of the important targets as a future energy source. In addition, they can be utilized as potential sequestration sites for carbon storage to mitigate the global greenhouse effect. However, the pore size distribution of Indian shale gas systems is poorly understood. Knowledge on pore size distribution is one of the fundamental requirements for characterization of shale gas reservoirs and for accurate estimation of gas storage potential. The work presented here is probably the first effort on the characterization of different Indian shale reservoir basins based on the pore size distribution. Mercury Injection Porosimetry (MIP) and low-pressure gas adsorption (CO2) techniques were used to study the nano-scale pore size of the shales.
Shale samples were taken from the Krishna-Godavari basin in India. Geographically, KG basin is located on the southeast coast in the Indian state of Andhra Pradesh. It is a peri-cratonic passive margin basin covering an area of 28,000 square km. The known hydrocarbon-bearing areas are divided into five petroleum systems, the major source rocks being early Permian, cretaceous, paleocene and Eocene. The thermal maturity of the shales ranges from 0.7% to 2% Ro. The cores collected from the site were relatively hard and compact. The stiffness of the sample makes it a good candidate for hydraulic fracturing.
The samples exhibited higher thermal maturity with increasing organic content. The chemical composition of the shale samples was inferred from XRD data, which depicted high enrichment of kaolinite among other clay minerals. The experimental result suggested that the samples exhibited diverse pore size characteristics. The micropores were efficiently accessed using CO2 adsorption which showed a type I isotherm curve indicative of micro-pore infilling. MIP analysis was used to infer the pore throat area. The average pore diameter of the samples was 3.94 nm. The KG shales exhibited a BET surface area of 2.24 m2 /g while the total pore volume amounted to cc/g. The MIP results indicate that the KG shales have a broader range of pore diameters towards the larger pore size (9 nm).
The study shows that KG shales have a broad range of pore diameters. Although the adsorption quantity is not as high as compared to that of other shales, it exhibits enough adsorption to be regarded as a reservoir. With further in-depth study and the application of high-pressure gas adsorption, the storage potential of the shale can be assessed.
A wide range of methods is being applied for assessing the pore size distribution in shale. These methods can be broadly classified into two major categories i) Radiation based and ii) Fluid based (Figure 1). Application of radiation based methods such as scanning electron microscopy (SEM), Transmission electron microscopy (TEM), High resolution X-ray CT scan, Small angle neutron scattering (SANS) and Ultra SANS (USANS) and fluid intrusion methods such as Mercury Intrusion Porosimetry (MIP), Helium pycnometry, N2 and CO2 BET are well established and are most commonly used among shale gas research communities.
The shale sample exhibited a CO2 BET surface area of 2.24 m2/g and average pore diameter of 3.94 nm.
The N2 BET surface area was 4.86 m2/g with an average pore diameter of 20.9 nm.
Table.1. Results of the low pressure CO2 adsorption analysis
Sample Name
BET Surface Area (m2/g)
Total Pore Volume (cc/g)
Average Pore Diameter (nm)
Micro-pore Volume (cc/g)
K-G
2.24
0.002
3.94
0.001
Table.2. Results of the low pressure CO2 adsorption analysis
Discussion
Sample Name
BET Surface Area (m2/g)
Total Pore Volume (cc/g)
Average Pore Diameter (nm)
Micro-pore Volume (cc/g)
K-G
4.86
0.025
20.9
0.003
The CO2 sorption behavior exhibited by the shales can provide significant inferences to the carbon dioxide sequestration potential of the shales. Although the present study is conducted under low pressure, the intrinsic material behavior is independent of pressure.
The H3 type hysteresis observed in case of low-pressure N2 adsorption is suggestive of small slit like pore geometry which can be indicative of clay mineral enrichment in the shale.
The relatively larger pore sizes can be helpful in designing fracture networks and enhanced permeability.
Pore size Distribution from LPGA results a b
Conclusions
Fig. 3. (a) Physiosorption Isotherm using N2 gas, (b) Pore size distribution of KG shales using N2 LPGA
The low pressure gas adsorption and mercury intrusion porosimetry are effective methods for the assessment of pore size distribution in shales.
The use of MIP alone can give an incomplete picture of the pore size distribution in shales. Thus, it should be used in tandem with low pressure gas adsorption in order to get the compete spectrum of pore sizes.
The results suggest that KG shales have a good potential for natural gas storage and can be a good source of natural gas.
The study can be further strengthened by understanding the fractal behavior of the shale and its implication on adsorption behavior.
High-pressure gas adsorption experiments should be conducted on the shales to estimate the gas storage potential.
Fig. 2. Various methods to access the pores in shales
Low-Pressure gas Adsorption
Background
Low pressure adsorption is an effective method to measure specific pore volume, shape and pore size distribution with specific surface area at pressure <18.4 psia.
Nitrogen and Carbon dioxide having kinetic diameters of nm and nm respectively can easily access the micro-pores present in shale. Thus they are often used as probe gases for the quantification of the otherwise inaccessible micro pores.
As evident from the diameter, CO2 can access the finer micro pore volume as compared to N2.
For gas sorption experiments, the powdered samples are dried for removing the moisture and gas content trapped within prior to the experimentation. Then, injection of N2 is carried out at constant liquid N2 temperature (-197.3°C) under predefined pressure levels.
Stepwise increment in pressure is done up to the saturation pressure point (adsorption) and decreased gradually (desorption). A series of measurements are recorded at each specific pressure level which will be used to quantify the amount of gas adsorbed/desorbed.
Pore size and its distribution has direct impact on the storage and flow capacities of shale gas within the reservoir conditions. Hence, characterizing the shale PSD is an important step for evaluating the reservoir feasibility and in ensuring its economic viability.
Natural gas in shale rock is stored in different forms such as free gas, adsorbed gas within mineral and clay surface or as dissolved gas within shale.
According to the IUPAC, the pores in shale can be classified into three types depending on the pore diameter, namely, macropore (>50nm), mesopore (50nm to 2nm) and micropores (<2 nm).
The economically significant porosity in shale predominantly lies in the micropores, existing in the clay minerals and organic matter.
Researchers have observed that clay minerals play a critical role in sorption properties of shale gas.
Gas adsorption in shales displays a Type IV adsorption isotherm which is often associated with a hysteresis loop.
The hysteresis loops are a result of capillary condensation and this feature is mostly seen in mesoporous materials.
Authors
Fig. 4. (a) Physiosorption Isotherm using CO2 gas, (b) Pore size distribution of KG shales using CO2 LPGA
1 Ph.D. Candidate, Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India,
2 Postdoctoral fellow, Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India,
3 Professor, Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India,
MIP Analysis
The amount of gas adsorbed in the respective relative pressures can be correlated to the volume of micro, meso and macro pores present in the sample.
The hysteresis loops exhibited by the N2 sorption plots can be considered as Type B, which is exhibited by slit type of pores.
The CO2 adsorption plots exhibit a type I curve indicative of micropore infilling.
The pore volume distributions mostly display a bimodal nature with pore sizes lying in two specific ranges , 5-6Å and 10Å.
Mercury Intrusion Porosimetry is an old conventional technique practiced to understand the information of materials such as their pore size distribution, total porosity or pore volume and the specific surface area.
The quantity of mercury that can intrude and the amount of pressure required is directly proportion to the pore size of the material.
The data is then converted into the pore size distribution of the sample using the equation
𝑃= − 2𝜎 𝐶𝑜𝑠 𝜃 𝑟 𝑝𝑜𝑟𝑒
Where, P is the pressure of intrusion, σ is the surface tension of mercury, θ is the contact angle and 𝑟 𝑝𝑜𝑟𝑒 is the pore radius.
References
De Boer, J.H., The structure and properties of porous materials, in: Tenth Symposium of the Colston Research Society. University of Bristol, Butterworths, London, pp. 68–94.
Clarkson, C.R., Bustin, R.M., The effect of pore structure and gas pressure upon the transport properties of coal: a laboratory and modeling study. 1. Isotherms and pore volume distributions. Fuel 78, 1333–1344
Fig. 1. Different types of physiosorption (Sing et al, 1985)