Study of Pore Structure and Gas Storage and Transport in a Shale Sample from Sichuan Basin, China
Zhejun Pan1, Weina Yuan1,2, Luke Connell1, and Xiao Li2
1CSIRO Earth Science and Resource Engineering, Clayton South, VIC, Australia
2CAS Institute of Geology and Geophysics, Beijing, China
Understanding gas storage and transport mechanisms in shale is crucial for reservoir evaluation and gas production forecast. Shale matrix has a complex pore structure, with sizes ranging from nanometers to micrometers. The complexity of the pore structure in the shale organic and mineral components leads to the complexity of gas storage and transport behaviors. In particular, due to low porosity and small pore sizes, gas diffusion from the matrix is difficult, making it one of the rate limiting factors for gas production from shales. The aims of this work were to study the pore structure of a shale sample and investigate the gas storage and diffusion behaviors and their relationships with the pore structure.
A sample of Lower Silurian shale obtained from a depth of about 2156 m in the Sichuan Basin of China was prepared for the experimental work. The Sichuan Basin is located within Sichuan Province and Chongqing Municipality in Southwest China. It is tectonically situated in the northwest of the Yangtze metaplatform and surrounded by the Yunnan-Guizhou-Sichuan-Hubei platform fold zone . In Sichuan Basin, shales from six Periods were deposited: Lower Cambrian, Lower Silurian, Lower Permian, Lower Jurassic, Upper Permian and Upper Triassic; Shale of the first four Periods are mainly dark pelitic rock and argillaceous limestone, while the rest two are mainly black pelitic rock and coal rock . The Lower Silurian shale sample studied in this work is considered to be a productive source rock for shale gas [1,3]. The sample was crushed to powder and sieved to three particle sizes for adsorption and diffusion experiments to investigate the particle dependence. The dry and moisturized samples were both used to investigate the impact of moisture on the gas storage and diffusion behavior.
N2 adsorption and SEM experiments were first carried out to study the pore structures. Figure 1 shows the SEM results of the pore sizes, ranging nanometers in the organic content to micrometers in the other components of the shale. The N2 adsorption result shows a peak at pore sizes at about 3 to 4 nm. Then adsorption and diffusion experiments were conducted on a Hy-Energy PCTPro–E&E apparatus, which use a manometric method. The adsorption results show that particle size has no impact on gas adsorption isotherm as expected. However, moisture significantly reduced the adsorption capacity of the shale sample. The diffusion results show that adsorption equilibrium was reached quicker for samples with small particles and for dry samples. All the diffusion data showed a fast early stage followed by a slow stage later.
The adsorption results were able to be accurately described by the Langmuir adsorption isotherms for all the samples. The diffusion data were able to be described adequately by the bidisperse model, with parameters consistent with pore size results obtained from the N2 adsorption and SEM experiments. Molecular diffusion and Knudsen diffusion both play important roles in shale gas diffusion in pores of different sizes as suggested by the bidisperse model results. Moreover, they show different gas pressure dependence with molecular diffusion having stronger pressure difference. Adsorption isotherm and calculated diffusivity showed little particle size dependence as expected. However, gas adsorption and diffusivity were significantly reduced in moist samples, showing that moisture reduces gas storage and transport in shale.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013