Study On The Influence Of Coal Reservoir Characteristics And Production Dynamics On The Effect Of CO₂-ECBM

CO₂-ECBM technology is an important method to realize the efficient exploitation of coalbed methane.The optimization of CO₂ injection and production well group and the determination of the best time of CO₂ injection are the core key technologies to ensure the stimulation effect of coalbed methane production Wells.This article on the southern qinshui basin,Shan Xi persimmon Shizhuangnan blocks of coalbed methane geology and development engineering background,with the main purpose of the coalbed methane exploration and development in the area of coal seam 3#coal as the research object,based on the coal bed gas geology,coal bed methane exploration and development,such as CO₂-ECBM theory,uses the related basic theory analysis the DT and SH coal mine,sampling and basic experiment,columnar sample CO₂ flooding experiment of coal bed methane simulation,reservoir geological modeling and CO₂-ECBM process numerical simulation method,etc.The effects of porosity,permeability,gas content,water content and their dynamic changes on the process of CO₂replacement and CH₄ displacement are studied.Suggestions on the site selection of CO₂-ECBM project and the optimal timing of CO₂ injection are put forward.The research results have theoretical guiding significance for the optimization of injection and production well group and process optimization of CO₂-ECBM project in coalbed methane development area.The main research achievements of this paper are as follows:(1)The 3#coal samples of DT and SH coal mines belong to semi-bright coal with low moisture content and ultra-low ash content.Organic microcoalite is dominated by vitrinite with deep gelation.The pores in the coal tested by mercury injection and low temperature liquid nitrogen are mainly medium and large pores.The mercury retracting and retracting curve presents an open-mouth curve with a certain volume difference and hysteresis loop,but there are significant differences in the adsorption and desorption curves.It has obvious fractal characteristics that the pore diameter of DT coal sample is larger than 226.8nm and SH coal sample is larger than 120.8nm.Due to the influence of coal metamorphism,the V_L and P_L of CO₂ and N₂ adsorption from the coal samples in SH Mine are higher than those in DT Mine,but the reservoir permeability is obviously lower than that in DT Mine.(2)the injection of CO₂-ECBM experimental simulation show that the"higher confining pressure,effective stress increase,coal matrix pore fissure under compression",The displacement time of DT and SH samples has a positive linear correlation with the confining pressure,while the displacement efficiency has a negative linear correlation with the confining pressure.The displacement efficiency increases slowly with the increase of gas injection time and eventually tends to be stable.It is found that equal percentage displacement time is negatively correlated with sample permeability in a logarithmic function type,while displacement efficiency is positively correlated with permeability in a logarithmic function type when compared with different samples.Because the development of pore and fissure in coal is more conducive to the displacement and displacement of N₂ by CO₂,the CO₂ displacement efficiency of DT samples with high permeability is more than 85%,and the CO₂flooding time is only 1/6 of that of the samples with medium and low permeability.(3)The numerical simulation results of CO₂-ECBM process show that with the increase of coal reservoir permeability,the cumulative injection of CO₂ injection well,the cumulative gas production of CBM producing well,and the CBM recovery in the well control range all increase.When the gas content of the coal reservoir increases,the cumulative gas production and CBM recovery of the producing well also increase correspondingly,but the cumulative CO₂ injection decreases.The lower the porosity and water saturation of the coal reservoir,the higher the cumulative injection in the CO₂injection well,the cumulative gas production in the producer well,and the CBM recovery in the well control range.(4)The site selection of CO₂-ECBM project should meet the requirements of low porosity,high permeability,high gas content and weak water cut of the reservoir to the greatest extent.It can be the best time for CO₂ injection when the daily production of CBM well is low and the water saturation of the coal reservoir around the wellbore is low.