Article Cite This: Energy Fuels 2017, 31, 10655-10664
pubs.acs.org/EF
Quantitative Characteristics of Nanoscale Pores in Gas-Bearing Volcanic Rocks of the Yingcheng Formation in the Songnan Gas Field Tiantian Du, Xuanlong Shan,* Jian Yi, and Yue Qu College of Earth Sciences, Jilin University, Changchun 130061, Jilin, People’s Republic of China ABSTRACT: A detailed study is conducted on the nanoscale pores in the volcanic rocks of the Yingcheng Formation in the Songnan gas field in the Songliao Basin of China. Mercury intrusion porosimetry (MIP) and the nitrogen adsorption method are used to analyze the distribution characteristics of nanoscale pores and their capacity to adsorb methane molecules. Volcanic reservoirs in the Songnan gas field consist of rhyolite, tuff, and volcanic breccia. The results of the MIP method indicate that the microscopic pores in the rhyolite, tuff, and volcanic breccia are mainly >1000 nm, 100−1000 nm, and 2 mm, whereas those in tight sandstone range from 0.03 mm to 2 mm and those in shale range from 0.005 mm to 1 mm.7 The pore diameter distribution of shale from North America has been studied using porosity determination, the specific gravity method, specific surface area measurements, and electron microscopy imaging experiments.8 The gas isothermal adsorption curve has been used to study the micropore structure of shale.9 Lowpressure nitrogen adsorption and field-emission scanning electron microscope (FE-SEM) methods have also been used to study the pore characteristics of Silurian black shale in the southwestern region of China; the results of these analyses revealed that the total organic carbon content exhibits a certain correlation with the specific pore surface area and pore volume of micropores.10 The molecular structures and nanoscale pore structures of different deformation series have been studied in different metamorphic and deformational environments using X-ray diffraction (XRD) and the nitrogen adsorption method. © 2017 American Chemical Society
The results of these analyses demonstrated that nanoscale pore structures are the main adsorption spaces of coalbed methane. However, strongly heterogeneous volcanic rocks have seldom been compared to shale and coal in nanoscale research. Volcanic rocks with intergranular micropores have been deemed to be favorable reservoirs for natural gas, based on previous studies of the pores of volcanic rocks in the Songliao Basin.11 In this study, we combine mercury intrusion porosimetry (MIP) and the nitrogen adsorption method to analyze the quantitative characteristics of nanoscale pores with different ranges of diameters. The parameters of pores with different ranges of diameters in the volcanic rocks of the Songnan gas field are clarified. Furthermore, in this study, the relationships between pore diameters, specific surface areas, and pore volumes are discussed, and the lower limit of the adsorption of pore diameters is determined.
2. GEOLOGIC SETTING The Songnan gas field is located in the eastern region of the Changling fault depression, which is located in the southern Songliao Basin in northeastern China (see Figure 1). The bottom of the gas field is composed of Upper Paleozoic, Carboniferous and Permian intrusive and metamorphic rocks, and two reservoirs have developed in this basement. The Huoshiling Formation, Shahezi Formation, and Yingcheng Formation developed during a period of fault activity. The main lithologies of the Huoshiling Formation are andesite, basalt, and trachyte. The Shahezi Formation has developed sedimentary Received: June 22, 2017 Revised: September 5, 2017 Published: September 11, 2017 10655
DOI: 10.1021/acs.energyfuels.7b01787 Energy Fuels 2017, 31, 10655−10664
Article
Energy & Fuels
3.2.1. Mercury Intrusion Porosimetry (MIP). The MIP method is based on the unwettability of the solid surface of mercury. Under the effects of external pressure, mercury can overcome surface tension and enter the pores of rocks. The external pressure is inversely proportional to the amount of mercury that remains outside the pores.13 The mercury intrusion curve established using the MIP method can reflect the characteristics of the connected pores in rocks. Here, the pore diameter distributions detected using the MIP method range from 7.1 nm to 208 μm. Pores that are smaller than 100 nm cannot be accurately measured, because of the size of the Hg atoms, and pores that are larger than 1 μm have reached the micrometer level. Therefore, we use only some of the test results obtained using the MIP method in this study. In addition, we analyze the pore diameters, specific pore volumes, and specific surface areas of pores ranging in size from 100 nm to 1000 nm. 3.2.2. Nitrogen Adsorption Method. Nitrogen is used as an adsorbed gas in the nitrogen adsorption method.14 In this method, the partial pressure gradually increases at a constant temperature, and the adsorbed amount is measured at each corresponding time point. The isothermal adsorption curve is then established based on the adsorbed amount and the partial pressure. Similarly, the isothermal solution curve can be drawn by plotting the desorption amount against the partial pressure.2,9,15 The pore sizes that can be measured using the nitrogen adsorption method range from 0.858 nm to 233.913 nm. In contrast to the MIP method, the results of the nitrogen adsorption method require the use of different data processing methods, which are described below. The BET equation was first proposed by Brunauer, Emmett, and Teller in 1938 to describe the theory of adsorption.16 The distribution of the specific surface area of the micropores can be calculated using the BET equation.17−19 Furthermore, the BJH method is a statistical method that was proposed by Barrett, Joyner, and Halenda to calculate the diameters and volumes of microscopic pores based on the gas pressure.20 The effective characterization range of the BJH method ranges from 2 nm to 100 nm.8,9 Density functional theory (DFT) is a statistical method that is based on the theory of quantum mechanics.21 The adsorption amount of a given pore diameter can be measured based on the integration from the grand potential function of the system to its density distribution. Based on a series of calculated isotherms obtained at a standard pore diameter, the experimental adsorption isotherms can be fitted to calculate the following pore diameter distributions of the samples.9,22,23 The HK method is a semiempirical method used to analyze pore diameter distributions that was established by Horvath and Kawazoe.24 The relationships between filling pressures and effective pore diameters can be analyzed based on the nitrogen adsorption isotherms of the samples.25 The HK method is most applicable to the analysis of pores that are smaller than 2 nm. The application scope of each method is shown in Table 2.
Figure 1. Location of Songnan gas field and positions of wells.
layers, including dark mudstone, sandstone, and muddy siltstone. The Yingchengzu Formation is the main gasproducing reservoir; it contains two groups of rhyolite and tuff.12
3. MATERIALS AND METHODS 3.1. Materials. A total of nine samples from the Yingcheng Formation were collected from 7 wells in the Songnan gas field for further analysis using MIP and the nitrogen adsorption method; these samples include rhyolite, tuff, and volcanic breccia. These samples are located in the Yingcheng Formation and were collected at depths ranging from 3325 m to 4195 m (see Table 1).
Table 1. Wells, Depths, and Lithologies of Samples sample ID
well No.
depth (m)
lithology
1 2 3 4 5 6 7 8 9
YS101 YS5 YS102 YS7 YS102 YS5 YS201 YS6 YS301
3858 4058.88 3772 3325.3 3659.56 3874.5 4195.14 4164.15 3860.62
rhyolite rhyolite rhyolite tuff tuff tuff breccia breccia breccia
4. RESULTS AND ANALYSIS 4.1. MIP Method Results and Analysis. The experimental results of the MIP method are mainly reflected by the pore diameter distribution curve. Based on the purpose of this Table 2. Application Scopes and Parameters of Methods Used in This Study
3.2. Methods. The MIP and nitrogen adsorption experiments were performed at the Key Laboratory of Analytical and Testing Techniques facility in Beijing, China. The instrument used in the MIP experiment is a Pore MasterGT 60 (Quantachrome Instruments, USA). The instrument used in the nitrogen adsorption experiment is an ASAP2460 Version 2.01 (Micromeritics Instrument Corporation, USA). The procedures of these two experiments are based on International Standard ISO15901, and the error is
