Productivity Prediction And Enhanced Gas Recovery Methods For Coalbed Methane Well

Coalbed methane(CBM)is one of the most important unconventional energy.Based on the data of Linlin block,the CBM reservoir geological model and numerical model was build to study the factors which affect the CBM productivity.The productivity of different well types(including vertical well,horizontal well and multi-branch horizontal well)was predicted and methods to enhance coalbed methane recovery(ECBM)were researched.The Liulin block,located in the Shanxi Province,central North China,is on the east edge of Ordos Basi,which be-longs to the west flexure belt of the Hedong coalfield.The coal seams of the Liulin block are part of a mono-clinic structure inclined to the west and southwest with a dip angle of 5°.The drilling depth of the coalbeds varies from 300 to 1300 m.The main targeted coal beds include the No.3 & 4 of the Lower PermianShanxi Formation and the No.8 & 9 of the Carboniferous Taiyuan Formation.The reservoir is typical of very low porosity and permeabilityIt is difficult to measure the permeability using the conventional method.The improved pressure depletion method was used to measure the porosity and permeability of tight rock.The principle is that using a smaller volume gas container with a high pressure applied to one end of the rock sample and recording the process of pressure depletion.According to the variation of pressure with time,calculate the permeability and porosity of core.The computational model was show.Different factors(gas composition,temperature,confining pressure and displacement pressure)were studied,which affected the rule of pressure depletion and permeability.Based on the gas-water two-phase flow equation,coalbed methane mathematical model was set up,the model including the adsorption,desorption,diffusion and the contraction/expansion effect of coal matrix.Based on characteristics of coal seams,the component model was adopted to analyze the dynamic change of pressure drawdown distribution during the CBM production and the effect of coal matrix contraction/expansion on permeability and porosity,results show that with the exploitation of CBM,the porosity and permeability of microfracture increased gradually and the initial increase amplitude is bigger,then gradually slow.The effect of micro fracture permeability,hydraulic fracture(fracture permeability and fracture length),micro crack spacing,Young's modulus,Poisson's ratio,Langmuir volume and Langmuir pressure on the productivity of vertical well were analysed.According to the basic parameters,the geological model of a multi-branch horizontal well was set up,the fitting analysis of production history and productivity prediction were analyzed.The impact of coal seam thickness,micro fracture permeability,micro fracture porosity,water saturation and minimum bottom hole flowing pressure on the productivity were analysed.Using stochastic modeling method,the 3D geological model of Liulin block was established for fitting analysis and productivity prediction of the whole block.The total gas in place is 363.4×108 m3,which is basically equal to the total gas calculated by volume model method.Production data of fractured vertical wells and multi-branch wells of Liulin block were analyzed.Production history show that vertical wells' performance was significantly poorer than expected,especially after 1-2 years of production,only about 30% of expected,while multi-branch horizontal wells' high gas production rate was basically identical with the results of numerical simulation.According to the geological characteristics of Liulin blocks,production history and numerical simulation model,productivity of different types well were analyzed,and compared with the field production data.The mechanism of different well types,productivity and its influencing factors were studied.Results show that the damage of fracturing fluid to reservoir and the closure of induced fracture were the main reasons for the poor production of fractured vertical wells.While multi-branch horizontal well was more appropriate for exploitation of CBM in the Liulin block.Through numerical simulation,the control area of multi-branch horizontal well and the influence of coal thickness,reservoir coefficient and the structure parameters on yield were analyzed.The mechanism of ECBM by injection CO2 and N2 was analysed.The numerical simulation results show that the production of ECBM(injection CO2 and N2)is about 1.36~1.5 times of the production with no gas injection.At the same time,the influence of gas injection rate and injected gas composition on recovery and breakthrough time were studied.