Simulation Of Methane And Water Dynamic Change Law Based On NMR During CBM Output Process

Taken Anthracite coal in Qinshui Basin as the research object, this thesis discusseed the feasibility of using 2D NMR T1-T2 technique to distinguish the signal of methane and water, and the feasibility of using briquette coal instead of raw coal to study methane and water state during coalbed methane output process. The effect of CO2 on the state of the water in coal reservoirs was analyzed by observing the changes of T2 spectra in the process of CO2 displacing water. The effect of water and displacement pressure on the output of methane was analyzed by observing the changes of T2 spectra in the process of heavy water displacing methane. Through the whole experiment process, the following conclusions are drawn.2D NMR and briquette coal displacement experiments indicated that T1 and T2 ratio of methane and water is very close, which is difficult to distinguish methane and water signal in coal reservoir by two-dimensional spectrum T1-T2. During the whole experiment process, the binder is required to no nuclear magnetic resonance signal when using briquette coal instead of raw coal.CO2 displacement experiments of coal samples with different permeability showed that it is difficult for coalbed methane migration in the low permeability reservoir. Reservoir stimulation plays a major role when mining the deep coalbed methane. The wettability of coal could be changed by CO2, which will make the adsorption water gradually changes to free water in coal reservoir.The adsorption and desorption results of dry and moisture coal sample demonstrated that the adsorption of methane is gradually absorbed by the large pores to the small pores. With time changing, the adsorption and desorption of methane on the coal surface are in accordance with the logarithmic relationship. The whole adsorption process can be divided into three stages, among which the first two stages are linear change, and the third stage is logarithmic change. The rate of adsorption is very large in the first linear stage, which indicated that rate of desorption is very high in the initial stage of desorption. At the same time, it was founded that there was no correlation between the adsorption rate and the pressure when the amount of methane adsorption in a certain range.The water in the coal pillar not only hinders methane migration, but also reduces the adsorption rate of methane on the coal surface. On the one hand, the reason is that the adsorption ability of water on the coal surface is stronger than methane; on the other hand, a part of the pore may be filled with water. Meanwhile, due to the irregular pore, the water molecules clog pores, which lead to methane molecules can not enter these pores. Effect of coal pillar water on the migration velocity of methane is due to the interaction force among coal surface, water molecules and methane molecules. The migration channel is blocked by water can also lead to the migration velocity reduce. The water of coal pillar can also reduce the desorption rate of methane, but it has little effect on the overall rule, which shows that the effect of water on methane desorption is due to the effect of water on methane adsorption in coal reservoir.The experiment of heavy water displacing methane showed that the effect of displacement pressure on methane output plays a dual role. The displacement pressure can promote methane migration, and hinder methane desorption as well as. During the continuous output process of coalbed methane, the effect of water content of coal reservoir on coalbed methane desorption is not the main factor, the key is the control of bottom hole differential pressure. The experimental simulation showed that the bottom hole differential pressure of coalbed methane wells in Daning area should be controlled between 0.5-1.5 Mpa. At the same time, it is very significant to control the displacement pressure in the process of coalbed methane exploitation by gas injection.