Study On The Dynamic Microscopic Interactions Between Reservoir Fluids During CO₂ Enhance Coalbed Methane(CO₂-ECBM) Recovery

CO₂ sequestration in coal seam is widely concerned with enhancing coalbed methane recovery.In this study,custom built apparatus with low field nuclear magnetic resonance(NMR)were developed to monitor the dynamic microscopic interaction between CO₂,reservoir fluids and coal after injecting CO₂into coal seam.The quantitative evaluation model was established to investigate the dynamic microscopic interaction between CO₂ and water.In addition,this study discussed the inner mechanism of CO₂ enchancing methane from adsorption of CO₂,interfacial tension factor and the CO₂ solubility.The results are as follow.(1)The low-field NMR relaxation was used to monitor the process of CO₂ exchange the adsorbed CH₄ across a variety of time-scales.The T₂ spectra peaks of adsorbed CH₄decreased and those of free CH₄ increased with time after the injection of CO₂.The results show that the replacement capacity of CO₂ decreases with the injection volume of CO₂increasing.Moreover,the injection volume is much higher than adsorbed volume of CO₂.The CO₂ shows high replacement capacity in high rank coals with high vitrinite content.In addition,the high temperature and moisture in coal weaken the exchange between CO₂ and CH₄.(2)The NMR was introduced into a custom built relative permeability measurement apparatus,and a new method to accurately quantify displaced water was developed.During displacement experiment,we measure the change of water in coal cores using the NMR to obtain the displaced water,insteading of measuring water flow from outlet of cores in conventional method.With the new NMR method,the relative permeability curves for Chinese anthracite and bituminous coals were obtained with different confining pressure.The relative permeability curves show characteristics of high irreducible water saturation,low equipotential flow point gas saturation and narrow span of two-phase flow for anthracite and bituminous coals.In addition,additional stress causes a greater shift in curves towards lower relative permeability and higher water saturation.(3)A novel NMR-based approach was developed to evaluate the CO₂ and water wettability of coal.The measured T₂ spectra of coal powder samples with addition of water indicate different phases of water exists and changes with time in coal powder.The results show that the different water spreading among coal powder results from different surface wettability tension for coal samples.The changes of T_2g of P3 shows a positive correlation with surface wettability tension Based on the measured contact angle from the profile of the water adhered to coal powder disc,and T₂ from NMR,of coal powder,we proposed a linearly formulation to evaluate the contact angle using the change of T_2g of P3:?=84.6×T_2g+52.5.(4)We used low-field NMR relaxation to investigate CO₂ and water interactions in coals.After injecting CO₂,the T₂ spectra peaks of micro-capillary water gradually decreased and those of macro-capillary and bulk water increased with time.We assume that the CO₂diffuses through and/or dissolves into the capillary water to access the coal matrix interior,which promotes desorption of water molecules from the surfaces of coal micropores and mesopores.Finally,we built a quantified model to evaluate the efficiency of water replacement by CO₂ injection with respect to temperature and pressure,(?).(5)Base on the NMR relaxation method,we analyzed the CO₂-water wettability characteristics of coal which representing the distribution of water in coals.Results show that CO₂ reduces water wettability of coal leading to the spreads of water on coal particle surface become weak.Low temperature and/or high expose pressure of CO₂ can enhance CO₂wettability of coal.There are three factors affecting the water-coal wetting behavior with CO₂injection:the CO₂ adsorption capacity of coals,the change of interfacial tension with CO₂pressure and CO₂ solution in water.Moreover,with increasing pressure of CO₂,the CO₂wettability of the coal increases.Conversely,the high temperature decreases the effect of CO₂on water wettability of coal.