Optimization Of High-efficiency Development Technology And Numerical Analysis Of The High-rank Coalbed Methane Reservoir

The efficient exploitation of coalbed methane(CBM)is an important carrier to promote the transformation and the upgrading of the energy structure of China for achieving the green development.For a long time,CBM has been regarded as a main culprit endangering the safety production of mining of coal.Normally,the CBM existing in the laneway of the coal mine is usually pumped through the mine ventilation system into the atmosphere,without any treatment of pollution control measures.Nevertheless,with the deepening of domestic and foreign scholars' research on atmospheric protection issues,the CBM is a greenhouse gas with strong damage to the atmospheric ozone layer.Therefore,CBM extraction from the ground could not only provide China's increasingly scarce natural gas resources with an important supplement but also protect the ozone layer from the source of pollution.The condition of high-rank CBM reservoir in China is so complicated that the extraction efficiency of its is low.With the deepening of CBM extraction practice in the southern Qinshui Basin,research institutes cooperating with the owner of the mineral rights have conducted numerous pilot experiments on the applicability of various special types of well and tested new stimulation technologies in the target area.However,the cost of on-site testing is high and there are many uncontrollable interference factors.It is difficult to carry out a long period of the test cycle with a large number of parameters to be measured.Because the numerical experiment can overcome those disadvantages,in numerical experiments,the CBM production process can be simplified into solving a multiphysics coupled problem.By summarizing and analyzing the geological data and drainage data of low-yield and high-yield wells,it was confirmed that the main contradiction that restricting the full release of the production potential in this area is that the development technology is mismatched with the conditions of CBM reservoirs.Therefore,this paper focuses on the deposit characteristics of CBM in the target area.By comprehensive application of field data investigation,theoretical analysis and numerical calculations,the multi physical processes of reforming CBM reservoirs with fishbone horizontal wells and high-pressure hot nitrogen are studied.The following progress and conclusions have been made:(1)This paper combines the effective stress principle of double-porosity medium with the permeability evolution equation.Based on the hypothesis of dual porosity and single permeability,the stress-strain field and gas diffusion seepage field of coal are cross-coupled successfully.With the help of COMSOL Multiphysics,the CBM recovery by pressure drop in the fishbone horizontal wells was simulated,on the premise of ensuring the accuracy and efficiency of the solution.The gas production period of the fishbone wells and recovery rate under different working scenarios are compared and analyzed.In addition,by combining the failure criterion of three-dimensional surrounding rock with the fluid-solid(HM)coupling theory,the borehole stability at the intersection of principal branch and sub-branch of the fishbone well was studied,which provided a reliable theoretical support for the optimization of well parameters.(2)The main controlling productivity factors of the target area were screened out from the determination of the resource conditions and mining conditions of the CBM well productivity of Zaoyuan block in the Qinshui Basin.By analyzing the distribution characteristics of the main control parameters in the target area and comparing it with the output data of the typical wells,the optimization principle of the high-efficiency development technology of the CBM in the target area is established.(3)The gas pressure in CBM reservoirs in the target area is low,and the coal and rock have strong adsorption and low permeability,which restricts the release of production capacity in the reservoir area controlled by the well pattern.Analyze the results of numerical experiments based on the HM coupling theory for the process of evacuating CBM from fishbone wells,and then combine the data of the maintenance operations of CBM wells to select the optimal wellbore parameters for the fishbone wells.Then,a new process integrating CBM extraction,nitrogen reaming and nitrogen foam deplugging technologies was proposed for the sugar point.By adopting the glass fiber reinforced plastic(GFRP)screen tube to strengthen the main branch of the horizontal fishbone well,the stability of the wellbore is enhanced.GFRP screen tube can make the main branch withstand the cyclic impact pressure caused by nitrogen reaming.When the production is decreasing,nitrogen foam deplugging technology could be used to remove the blockage and recover the capacity quickly.(4)The thermo-hydro-mechanical(THM)coupled equations are established on the basis of the single-porosity and single-permeability hypothesis.By solving these equations to study the evolution law of physical properties of rocks of reservoirs with the injection of high pressure hot nitrogen into the wellbore of vertical wells.The results of the coupled simulation show that the temperature of the surrounding coal of the wellbore is greatly increased,after continuously injecting high-temperature and high-pressure nitrogen into the vertical well for 14 days.On the one hand,the coal seam in the borehole is pyrolyzed and gasified in situ under high-temperature nitrogen atmosphere,and the wellbore expansion rate has reached 30%.On the other hand,high-pressure nitrogen passes through the shaft wall,resulting in a significantly drop of effective stress in the near well zone,the shrinkage of coal matrix and the opening of cleat,which makes the permeability of coal seams in near-wellbore increase by an order of magnitude.The seepage channel of CBM is thoroughly dredged in this project.The process can relieve the damage caused by the Jamin effect and the water blocking effect on the seepage pathways in the reservoir rocks.It is suitable for applying the deblocking and stimulation operation in the disintegrated structural coal enrichment area.