Method for joint-mining of coalbed gas and coal

Abstract

Provided is a method for joint-mining of coalbed gas and coal. The method comprises: a well-drilling step; a fracturing and penetration step; a coalbed gas extraction step: an ignition step, and an underground gasification step. The method combines an underground coal gasification technology and a coalbed gas extraction technology, not only allows for utilization of the high temperature of underground gasification to heat a coalbed. thus increasing the permeability of a coal seam. and increasing the recovery rate of the coalbed gas, but also allows for utilization of the coalbed gas to perform the drilling and the fracturing and penetration processes, thus increasing the efficiency of underground coal gasification for mining.

Description

FIELD OF THE INVENTION[0001]The present invention relates to a novel process for mining underground carbon-containing organic mineral reservoir, in particular to a method for joint-mining of coalbed gas and coal.BACKGROUND OF THE INVENTION[0002]Coal, as a major source of energy in China, takes up about 70% of primary energy structure and is relatively rich in the total volume. It is predicted that the total volume of coal is about 4.5 trillion tons (within 2000 m), among which the volume of coal accumulated within 600 m is just about 25%. It is estimated from the present coal mining depth (400 m) that there is at least about 80% of the predicted total volume of underground coal in China not yet developed and utilized. The deeper development of coal resource is a long-term task. With the increase of the mining depth, an associated resource, i.e., gas (also referred to as coalbed gas) also increases in content. It is assessed in terms of coalbed gas resource that there is about 36 tri...

AI Technical Summary

Benefits of technology

[0006]The inventor(s) has found that the coalbed gas extraction and the underground coal gasification have some similarities and synergistic effects in drilling, fracturing, fracture expansion and other technological links, so that the collaborative development and utilization of an underground coal resource and a coalbed gas resource may be comprehensively taken into consideration by combining an underground coal technology and a coalbed gas extraction technology. On one hand, a high-temperature medium generated by the underground coal gasification may be used to heat a coal seam. Under the high temperature, the adsorption capacity of CH4 in a coalbed gas is greatly reduced, and the gas is expanded while heated during the process of transferring heat energy to the coal seam, so that it is advantageous for the establishment of a pressure difference, and the seepage rate of the coalbed gas is improved. On the other hand, processes for the coalbed gas in early stage, such as the fracturing stimulation and depressurization by drainage, improve the permeability of a coal seam, so that it is advantageous for the establishment of a fire zone and the implementation of a subsequent gasification process in the underground coal gasification technology; meanwhile, the permeability of the coal seam after heated may be improved by 2-10 times, so it is more advantageous for the desorption of CH4. In addition, from the perspective of products, the cost of the separation of CO2 from methane is lower than that of N2 from methane, and CO and H2 in the gas may be directly synthesized into methane, so the final product, like the coal gas, is methane. It can be seen that the coalbed gas extraction may be combined with the underground gasification, and the both promote and cooperate with each other, so that the recovery rate of the coalbed gas may be improved, and meanwhile, due to the improvement of the permeability of the coal seam, the reaction surface area is increased, so that it is advantageous for the underground coal gasification reaction.

Problems solved by technology

Except for the Qinshui basin and the eastern margin of Ordos basin, it is difficult to realize scale and industrial development in other areas at present.

Furthermore, as there are many high-gas mines or coal & gas outburst mines, with the increase of the mining depth, both the geostress and the gas pressure further increase and the gas drainage becomes more difficult.

Nevertheless, with the increase of the mining depth, the geological environment becomes more complicated that the increased geostress, water influx and ground temperature cause the increase of sudden engineering disasters and serious accidents, for example, rock blast hazards, gas explosion, aggravated strata pressure behaviors, large surrounding rock deformation, rheologic phenomena, increase of ground temperature and the like, which cause great threats to the safe and efficient extraction of deep resources.

However, the existing coal underground technology only concerns the gasification of coal seams.

With the increase of the mining depth of coal, the content of coalbed gas will increase quickly and it is inevitable to have some problems in the extraction of coalbed gas, specifically including: (1) for a coal seam rich in coalbed gas, there may be problems such as safety challenge and waste of resources if gasification is directly implemented to the coal seam; (2) if the conventional coalbed gas mining means is employed without in combination with the underground gasification technology, the subsequent implementation of the underground gasification may cause overlapped functions such as drilling and increased investment cost; (3) injection of CO2 into a coal seam may increase the extraction ratio of CH4 from the coal seam, however, the yield of coalbed gas, which may be increased only in the initial stage of the extraction of coalbed gas, will drop quickly in the later stage as the fracture is likely to close; and (4) as the intermediate of the gasification process and one component of resulting products, CO2, participating in a series of redox reactions, is an important component (about 15-60%) of the output gas and an important factor influencing the calorific value and content of active components of the gas; however, the recycling of CO2 is not taken into consideration in the existing processes.

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