Figure 1. The relationship between the adsorption capacity of methane and the TOC for shale at 60 °C. The slope of the fitted line represents the adsorption capacity of organic matter.

Figure 2. Characteristics of the maximum adsorption capacity of organic methane in shales as a function of geological temperature. The adsorption capacity of dry shale is higher than that of aquifers, and methane is mainly adsorbed in organic matter under water-containing conditions.

Shale gas is natural gas that is stored in shale in the free, adsorbed, or dissolved state. For the shale gas that is higher than the mature Lower Paleozoic shale in southern China, the main occurrence of shale gas is free gas and adsorbed gas. For conventional reservoirs, the calculation of free gas is mainly based on parameters such as shale porosity, reservoir temperature and pressure, and gas saturation; however, for shale reservoirs that are mainly nanoscale pores and have a large specific surface area In this regard, the volume of adsorbed gas cannot be ignored, and the presence of adsorbed gas reduces the available storage space for free gas. Therefore, the accurate evaluation of the adsorption capacity of shale not only involves the content of adsorption gas itself, but also the content of free gas, which is of great significance for the evaluation of shale gas resources. The Lower Cambrian black shale is one of the important horizons of shale gas exploration in southern China, but there has been no industrial breakthrough. Compared with the Silurian shale that achieved industrial breakthrough, the Lower Cambrian shale has unique geological and geochemical features: (1) The organic-rich section is more developed, the distribution range is wider, and the organic carbon content is generally higher than Lower Silurian; (2) Higher thermal maturity, greater compaction experienced, and lower overall retained porosity. So far, domestic scholars have carried out a lot of research work on the properties of the Lower Cambrian shale reservoirs, but their understanding of the methane adsorption capacity and mechanism under high temperature and high pressure conditions is not yet clear. It has constrained the evaluation and exploration deployment of shale gas that is higher than mature shale gas in China.

In recent years, the Xiao Xianming Subject Group of the State Key Laboratory of Organic Geochemistry at the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences studied the black shales of the Lower Cambrian Niutitang Formation in Guizhou Province and conducted a series of high-temperature and high-pressure methane supercritical adsorption. The study evaluated the methane adsorption capacity and main control factors of this set of shale and made the following progress:

(1) The absolute maximum methane adsorption capacity of the Lower Cambrian Niutitang shale at 60°C is between 1.8 and 5.85m3/t rock, indicating a good shale gas storage capacity; the maximum absolute adsorption of methane Positive correlation with TOC content indicates that TOC content is still the main factor determining methane adsorption capacity for very mature Lower Cambrian shales (eg, EqVRo≈4.0%) (Figure 1).

(2) Within a certain range of thermal maturation, the methane adsorption capacity of shale is less affected by thermal maturity. When the thermal maturity is too high (eg, EqVRo≈4.0%), the methane adsorption capacity of shale will be reduced. However, the higher TOC content of the Lower Cambrian shale will compensate for the decrease in its adsorption capacity. An increase in thermal maturity will reduce the Langmuir pressure of the shale, making the desorption of the shale more difficult under geological conditions (Figure 1).

(3) Different from the existing understanding, the study found that the maximum capacity of methane and the density of methane adsorption phase in shale are all negatively correlated with temperature, which is related to the increase of the distance between adsorbed molecules at high temperature. Using these relationships, the occurrence of high-temperature deep shale gas and its in-situ gas volume can be more accurately assessed (Figure 2).

The research work was supported by the National Natural Science Foundation of China's Outstanding Young Scientists Fund and the Strategic Leading Science and Technology Project of the Chinese Academy of Sciences. The relevant research results were published on Marine and Petroleum Geology.

Full House Solution

Full House Solution,Hanging Cabinet For Small Bedroom,Cream Bedside Cabinets,Floating Bedside Cabinet

Ningbo Oulin Import&Export Co.,Ltd. , https://www.oulinfurniture.com