Conception of co-extraction technology of valuable chemicals and natural gas in deep low-rank coal seam

Mild oxidative depolymerization of low-rank coal (lignite and long-flame coal) can generate high-value chemicals and improve the gas transport capacity.In view of the geological background of abundant low-rank coal resources and huge amount of natural gas in coal seams in China,the concept of in-situ oxidation modification-based co-extraction technology of high-value chemicals and gas in deep low-rank coal seams is proposed.The feasibility of this technology is studied from four aspects,including the oxidation depolymerization method to prepare carboxy chemicals,the mechanism of oxidation modification to improve coalbed methane recovery,the co-extraction technology model,and the favorable geological and engineering conditions.It is found that low rank coal has abundant oxygen-containing functional groups (such as carboxyl —COOH and phenolic hydroxyl—OH) and side chains,which provide key precursor structures for the preparation of high-value chemicals (such as carboxylic acid) by oxidative depolymerization.Under the condition of low temperature (<100 ℃) and atmospheric pressure,the mild oxidative depolymerization of low-rank coal with hydrogen peroxide,sodium hypochlorite and other oxidants can prepare carboxyl chemicals with high efficiency and low cost,and realize the non-energy high value utilization of low-rank coal.The oxidative depolymerization reaction has an obvious oxidative modification effect on low-rank coal,mainly manifested by the weakened adsorption capacity of coal surface for methane,enhanced hydrophobicity of water,formation of soluble pores and microfractures,and in-situ generation of CO2,thus increasing the desorption-percolation capacity of coal bed gas.Based on the demand for the high-efficiency production transformation of deep low coalbed methane wells and the secondary utilization of depleted coalbed methane wells,two technical models are designed:a single-well fracturing-dissolution leaching extraction (gas extraction-based) and a group-well fracturing-dissolution leaching extraction (high-value chemical extraction-based).The former is to inject oxidizing fracturing fluid,reasonably control the stewing time after fracturing,give full play to the mechanical-chemical synergistic transformation to increase seepage,separate the high-value chemicals in the fracturing fluid at the rejection stage,and efficiently extract natural gas.The latter is to repeat the transformation of each depleted gas well,form an intercommunicating injection and extraction well network through the fracture network,and continuously generate and extract high-value chemicals.China’s mega-thick (tens of meters to nearly 100 m),deep buried (500-1 000 m) low-rank coal seams are widely distributed,which can effectively reduce the groundwater pollution induced by the leakage of oxidants and reaction products,and provide favorable geological conditions for co-mining.The existing large-scale horizontal coalbed methane wells and hydraulic fracture network can avoid the high well construction cost of in-situ preparation of high-value chemicals and provide favorable engineering conditions for co-production.It is concluded that the co-exploitation of high-value chemicals and natural gas in deep low-rank coal seams can be realized by mild oxidative depolymerization reaction.It enriches the theory and technical system of in-situ modified mining in deep coal seams,is expected to form a new model of coal-bed methane production enhancement and transformation,and increases the comprehensive economic benefits of low-rank coal seam resource exploitation.