Acid And Nitrogen Foam Compound Transformation Process And Its Yield Increasing Mechanism In Low-Pressure And Low-Permeability Coal Seam

The overall coal seams in China are dominated by low-pressure and low-permeability coal reservoirs.The main controlling factors of low pressure and low permeability in coal seams in different regions are different.Scientific and suitable stimulation technology is the key to the efficient development of coalbed methane.Taking the No.3 coal seam of the Lu’an Mining area as the research object,this thesis reveals the main controlling factors of low pressure and low permeability and determines the key controlling factors of the transformation and stimulation.Combined with physical and numerical simulations,a new fracturing fluid system of acid and nitrogen foam and a composite fracturing transformation technology were formed.The stimulation mechanism of the process was expounded.Based on the above research results,a coalbed methane well fractured by acid and nitrogen foam was successfully carried out in the No.3 coal seam of the Yuwu mine field,Lu’an Mining area,which verified the suitability and effectiveness of the acid and nitrogen foam composite fracturing process for low-pressure and low-permeability coal seams.The main achievements and understanding of this thesis are as follows:The No.3 coal seam in the Lu’an Mining area is mainly meagre lean coal,with a thickness of 0.80-10.25 m,a burial depth of 57.34-931.41 m,and a gas content of 4-25.9 m~3/t.The coal seams are mostly primary structural coal and fragmented structural coal,and the endogenous fissures are relatively developed.The pore types are mainly micropores and small pores,and mesopores and macropores are not developed.The coal reservoir pressure gradient is lower than 0.75 MPa/hm,and the permeability is generally less than 0.1 m D.It is a typical coal reservoir with low pressure and low permeability.The analysis shows that the dew point elevation of Xin’an Spring is the main controlling factor of low pressure in the No.3 coal seam,and the minimum horizontal principal stress and mineral filling are the main controlling factors of low permeability.Therefore,the research and development of fracturing technology with pressurization,permeability increase and coal reservoir protection is the key to the improvement and production increase of coalbed methane in the No.3 coal seam in the study area.Aiming at the characteristics of low pressure and low permeability of No.3 coal seam,laboratory experiments of optimizing the concentration of acidification and increasing permeability and contrast experiments of physical parameters before and after acidification were carried out.The evolution law of mineral content,pore and fracture structure and adsorption and desorption characteristics of coal samples before and after acidification were analyzed.A new type of acid and nitrogen foam composite fracturing fluid system was proposed,and its main performance was tested.Combined with physical simulation and numerical simulation,the acid and nitrogen foam composite fracturing process was formed.The research shows that the process has a remarkable effect of pressurization and permeability increase,a good effect of rubber breaking,and no filter cake is formed.The combination of sand carrying rubber fluid and rubber breaking agent is helpful to carry sand and increase the stable flow conductivity of fracturing fractures.The coal seam has a small filtration coefficient and small filtration amount,which can be used as the technical basis for actual fracturing design and operation.Based on the above research results,according to Langmuir theory and isothermal adsorption experiments,the pressurization effect of the acidification and nitrogen foam transformation process on low-pressure coal seams is quantitatively evaluated and further reveals that the acid and nitrogen foam fracturing process of the low-pressure low-permeability coal seam of the whole turbo increases the permeability and energized mechanism,illustrating its modification effect to increase production.Compared with conventional fracturing process,it is proposed that the acid and nitrogen foam composite fracturing process has advantages in many aspects,including three-stage fracture making,double pressurization double permeability enhancement and reservoir protection.The field test results are remarkable.The maximum gas production is 2618m~3/d,the average gas production is 1585 m~3/d,and the cumulative gas production is4.96 million m~3.The test well has the characteristics of high daily production,long-term stability and large total gas production,which verifies the validity and feasibility of acid and nitrogen foam composite fracturing technology in No.3 coal seam with low pressure and low permeability in the Lu’an mining area.There are 87 pictures,63 tables and 187 references in this thesis.