Coalbed methane is a kind of very important unconventional natural gas, it is often referred to as "gas" in the coal field, which can cause coal and gas outburst accidents and gas explosion accidents. According to statistical data from 2002 to 2019, the number of deaths from coal mine gas accidents in China accounts for one-third of all deaths from coal mine accidents. At present, there are 840 high gas and 719 coal and gas outburst mines in China. Some coal mines have evolved from low gas to high outburst mines with the continuous increase of mining depth, and the situation of gas accidents will become increasingly severe.
The main component of gas is CH4, which is considered a relatively clean energy resource compared with coal. However, currently natural gas accounts for only 8.5% of China's energy consumption structure, with coal consumption accounting for 56.2% (2022). In contrast, coal accounts for 11% of energy consumption in the United States and natural gas consumption accounts for as high as 32%.
It can be seen that improving the extraction and utilization rate of gas is of great significance in both gas disaster prevention and gas resource utilization. As is well known, gas extraction is the fundamental measure to achieve disaster prevention and gas resource utilization, However, most coal seams in China is characterized by low permeability, strong adsorption, and difficult desorption. In many mines, the proportion of gas that can be desorbed underground is even less than 10% (while 90% is adsorbed in the nano-pores of coal), resulting in fast attenuation of gas extraction flow (10 to 30 days), low extraction rate, and difficulty in reaching the standard, which will seriously restricting tunnel excavation and production replacement.
In fact, hydraulic punching, deep hole loose blasting, hydraulic cutting, Fracking and other technologies have begun to be explored since the 1950s to improve the extraction rate of gas, which have achieved certain results and are still in use today. These technologies have played a certain role in improving the permeability of coal seams and enhancing the ability of gas migration and seepage, but they are still difficult to promote the desorption of gas in nanoscale pores (especially in coal seams with developed nanoscale pores), so they cannot completely solve the problem of low gas extraction rate in coal seams.
In response to this problem, American Scientist put forward the concept of CO2 enhanced gas extraction in the 1990s and successfully applied it in the San Juan Basin of the United States, becoming the main technical measure to enhance gas extraction. However, after CO2 is injected into the coal seam, it can cause outburst accidents in the outburst coal seam, and it is very easy to cause CO2 in the roadway to exceed the limit during the mining process. Therefore, it is only limited to coal seams that cannot be mined or are not mined for other reasons. Due to the large consumption of coal in China, it is evident that CO2 enhanced gas extraction technology is not suitable for China's national conditions.
The research and development team led by Professor Zhu Chuanjie of China University of Mining and Technology, together with Sichuan Coal Group, has put forward the technology of high-pressure gas water mixed pressure to enhance gas extraction since 2017 after years of scientific and technological breakthroughs, aiming at the low recovery rate of gas due to the low permeability and high adsorption of coal seams. By alternately injecting high-pressure water and displacement gas into the coal seam, the permeability of the coal seam can be improved, and the rapid desorption of high adsorption gas can be promoted. The triple effect of "fracturing permeability enhancement, displacement, and pore unblocking" in the coal seam has been achieved, and a "Chinese solution" has been explored to solve the problem of efficient gas extraction in low permeability and high adsorption coal layers.
At present, this technology has been promoted and applied in major coal producing areas in China (such as Sichuan, Henan, Guizhou, etc.), achieving significant technical results. At the same time, it has solved the problems of low permeability and high adsorption in coal seams, effectively reducing the cost and time of gas control.