Figure 1. The relationship between TOC and methane adsorption capacity of shale at 60 ℃. The fitted slope shows the adsorption capacity of organic matter.
Figure 2. Characteristics of maximum adsorption capacity of organic matter in shales with geologic temperature. The adsorption capacity of dry shale is higher than that of water-containing shale. Under the condition of water, methane is mainly adsorbed in organic matter.
Shale gas is natural gas that is stored in shale in free state, adsorbed state or dissolved state.For the Paleogene shale under high overripe condition in southern China, the main occurrence state of shale gas is free gas and adsorbed gas For conventional reservoirs, the calculation of free gas is based on parameters such as shale porosity, reservoir temperature and gas saturation, etc. However, for shale reservoirs with predominantly nano-scale pores and large specific surface area Layer, the volume of adsorbed gas can not be ignored, and the presence of adsorbed gas will reduce the available storage space for free gas. Therefore, accurately assessing the adsorption capacity of shale not only involves the content of adsorbed gas itself but also the content of free gas, The resource evaluation of shale gas is of great significance.The Lower Cambrian black shale is one of the important layers of shale gas exploration in the south of China, but there has been no industrial breakthrough.Compared with the Lower Silurian shale with industrial breakthrough , The Lower Cambrian shale has unique geological and geochemical characteristics: (1) the organic-rich layer is more developed and has a broader distribution and higher organic carbon content than the lower Silurian; (2) higher thermal maturity , The experience of compaction stronger, Paul So far the porosity is generally low.So far, domestic scholars have carried out a great deal of research on the physical properties of the Lower Cambrian shales, but their understanding of methane adsorption capacity and mechanism under high temperature and high pressure It is not yet clear, which to a certain extent restricted the resource evaluation and exploration deployment of the over-mature deep shale gas in our country.
In recent years, the Xiao Xianming group of the State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, carried out a series of high-temperature and high-pressure methane supercritical fluid adsorption Studied and evaluated methane adsorption capacity of the set shale and its main controlling factors, and made the following progress:
(1) The maximum absolute methane adsorption capacity of Niutitang shale in Lower Cambrian range from 1.8 to 5.85 m 3 / t rock at 60 ℃, showing good shale gas storage capacity. The maximum absolute methane adsorption capacity It shows a positive correlation with the TOC content, indicating that the TOC content is still the main factor determining the methane adsorption capacity (Figure 1) for very mature Lower Cambrian shales (eg EqVRo≈4.0%).
(2) The methane adsorption capacity of shale is less affected by thermal maturity within a certain range of thermal maturity. When the thermal maturity is too high (such as EqVRo≈4.0%), the methane adsorption capacity of shale will be reduced , But the higher TOC content of the Lower Cambrian shale compensates for the decrease of its adsorption capacity.The increase of thermal maturation will decrease the Langmuir pressure of shale so that shale desorption becomes more difficult under geological conditions ).
(3) Different from previous understanding, the study found that the maximum capacity of methane and the phase density of methane adsorption in shale are negatively correlated with temperature, which is related to the increase of spacing of adsorbed molecules at high temperature.Using these relations, Can more accurately assess the occurrence of deep shale gas in high temperature and its in situ gas (Figure 2).
The research work has been supported by the National Natural Science Foundation of China Outstanding Young Scientists Fund and the Strategic Pilot Science and Technology Project of the Chinese Academy of Sciences. Relevant research results were published on Marine and Petroleum Geology.
