Research On Coal Permeability Variation Law And The CBM Recovery In CH₄ Displacement With CO₂ Injection Process

This dissertation aims to explore the variation characteristics of coal reservoir’s permeability and coalbed methane（CBM）’s recovery rate after carbon dioxide（CO₂） injection. Based on the coalbed methane geology, surface chemistry, coal geology, adsorption thermodynamics and other disciplines, the experiment of CO₂ and CH₄ adsorption/desorption on pulverized coal and lump coal and physical simulation experiment of CH₄ displacement by CO₂ under different experimental scenarios were carried out to reveal the influences of temperature, pressure and other factors on the gas adsorption and supercritical CO₂ displacement effect, and to discuss the influences of confining pressure and pore pressure on coal strain and permeability in the displacement process. On the basis of the experimental research, the permeability dynamic variation mathematical model of coal reservoir after the CO₂ injection was constructed, the accuracy of model was verified by experimental data, and the influences of rock mechanics, adsorption, slippage coefficient and other parameters on the permeability were analyzed. At the same time, based on the starting pressure gradient experiment, the prediction model of CBM recovery rate under different permeability was established, and the CBM wells’ recovery rate were predicted with the combination of the site parameters, providing a theoretical basis for enhanced CBM recovery rate with CO₂ injection on site.The adsorption/desorption experiments of CO₂ and CH₄ single gas component and their mixture at different temperatures were conducted. The experimental results are as follows: with the increase of temperature, adsorption amount of single gas component decreases while the adsorption equilibrium pressure increases. The adsorption amount of CO₂ is 1.12¹.25 times of CH₄ among different temperature conditions. In the CO₂ and CH₄ mixture adsorption/desorption experiment, desorption curve is not consistent with adsorption curve due to the adsorbed-phase CO₂ is dissolved, making gas mixture adsorption/desorption behaves non-reversible. Meanwhile, the adsorption amount of CO₂ and CH₄ gas mixture on coal increases exponentially with the increases of the concentration of CO₂ component in the mixture. In the process of adsorption, coal is preferentially adsorb CO₂, but its adsorption rate gradually decreases, while the CH₄ adsorption rate relatively increases; the concentration of free-phase CO₂ relatively increases, while the CH₄ free-phase concentration relatively decreases. During the desorption, CH₄ is desorbed preferentially, as pressure decreases, free-phase CO₂ concentration increases while free-phase CH₄ concentration decreases, the desorption rate of CO₂ increases gradually, while the desorption rate of CH₄ changes fast to slow.Experiments of CH₄ displaced by CO₂ on pulverized coal at different temperatures（20℃, 30℃, 40℃）and pressure（2MPa, 3MPa, 4MPa） were conducted to reveal the effects of temperature and pressure on the displacement effect. The results show that in the process of displacement and desorption, the concentration of adsorbed-phase CH₄ decreases with the pressure drops, and increases with the temperature rises; with the change of temperature and pressure, the variation trends of adsorbed-phase CO₂ concentration are opposite to that of adsorbed-phase CH₄ concentration. During the displacement, the separation curves of CO₂ in adsorbed/free-phase under different temperatures are all concave type, while that of CH₄ in adsorbed/free-phase are convex type. And the lower the temperature, the greater the separation degree between the curves is. At the same time, with the temperature decreases from 40℃ to 20℃, the unit pressure desorption rate of CH₄ increases from 10.51%/MPa to 10.83%/MPa. Therefore, the displacement effect is better at low temperature.In the process of different CH₄ adsorption equilibrium pressure displacement experiments, with the increases of CH₄ adsorption equilibrium pressure, adsorbed-phase CH₄ concentration increases and adsorbed-phase CO₂ concentration decreases. The separation curves of CO₂ and CH₄ concentration in adsorbed/free-phase during the displacement under different pressure setting are basically the same, which means they are not significantly affected by CH₄ adsorption equilibrium pressure. When CH₄ adsorption equilibrium pressure increases from 2MPa to 4 MPa, CH₄ desorption rate at unit pressure drop decreases from 11.21%/MPa to 9.59%/MPa, so the better displacement effect appears at low CH₄ adsorption equilibrium pressure.The experiments of supercritical CO₂ adsorption/desorption and CH₄ displacement by supercritical CO₂ at different temperature（35℃, 45℃, 55℃） were carried out to find the effect of temperature on the supercritical CO₂ adsorption/desorption property and its displacement effect. The Gibbs adsorption amount of supercritical CO₂ has a maximum value, the absolute adsorption is in line with the characteristic of type ? isothermal adsorption curve, and the error between absolute adsorption and Gibbs adsorption is positive correlated with pressure. The effect of temperature on the supercritical CO₂ adsorption is consistent with that on gaseous CO₂, with the temperature increases, the adsorption amount decreases.During the displacement experiment under supercritical state, with the increases of temperature, the concentration of adsorbed-phase CO₂ increases, while the adsorbed-CH₄ decreases. The separation curves of CO₂ in adsorbed/free-phase show a concave variation trend, while that of CH₄ in adsorbed/free-phase show a convex type, and the higher the temperature, the greater the separation degree of the curves is. As the temperature rises from 35℃ to 55℃, the CH₄ desorption rate at unit pressure drop decreases from 6.47%/MPa to 6.38%/MPa, and with the pressure decreases, the adsorbed-phase concentration difference between CO₂ and CH₄ gets greater, that is, more and more CO₂ is absorbed while more and more CH₄ is desorbed. The experimental results indicate that in the range of supercritical state, the displacement effect is better when the temperature and pressure get closer to the critical condition.In order to estimate the effects of confining pressure and pore pressure on gas adsorption, coal strain and permeability during the displacement, a displacement experiment was carried out on the cylinder raw coal sample under the different scenarios of confining pressure（8,12,16,20MPa） and injection pressure（1.25⁶.25MPa）. The results show that adsorbed-phase CO₂ concentration increases with the pore pressure decreases and decreases with the confining pressure increases, and the adsorbed-phase CH₄ shows the opposite variation trend. The variation extent of concentration of adsorbed-phase CO₂ and CH₄ is smaller when the confining pressure gets larger. With the pore pressure decreases, the decreasing rate of adsorption amount shows a variation trend from fast to slow, and the adsorption amount also decreases with the confining pressure increases. We also find that the adsorption ratio of CO₂ and CH₄ increases in a power function with the increases of pore pressure and decreases with the confining pressure decreases. Under the same pressure drop, CH₄ desorption rate decreases linearly with the increases of confining pressure. The CH₄ desorption rate that assisted by CO₂ injection compared with the pure CH₄ desorption rate, improves 106.5%¹19.6%, which means the CO₂ injection promote the CH₄ desorption effectively, and the displacement effect is better at low confining pressure.With the decreases of pore pressure, all of the axial, radial and volumetric strains show a decreasing trend of gently variation-sharp drop-slow down drop with two inflection points at 5.25 MPa and 3.25 MPa, and the strain decreases exponentially with the increases of effective stress. Under the same confining pressure, the radial strain is larger than the axial strain and the difference of axial/radial strain decreases with the pore pressure decreases and deceases with the confining pressure increases, suggesting that the vertical and horizontal strain difference becomes smaller along with the displacement/desorption proceeding and the difference becomes more obvious at the shallow-buried coal reservoirs. The axial, radial and volumetric strains decrease with the confining pressure increases shows that confining pressure will restrain the swelling of coal, thus the axial and radial swelling amount is smaller under high confining pressure.Under the comprehensive influences of effective stress effect, matrix shrinkage effect and slippage effect, the coal reservoir permeability decreases firstly and then increases with the decreases of pore pressure, and the permeability in the late stage of pressure drop increases 1.18%⁹.58% compared with the initial permeability. Meanwhile, confining pressure plays a negative effect on the permeability. During the actual CBM exploration, influences of the three main effects on permeability are not constant, the negative effect of effective stress and the positive effect of matrix shrinkage and slippage are actually changing with the decreases of the pore pressure, and the influences of the three main effects on permeability are gradually weakened with the increases of confining pressure.The coal matrix deformation mathematic model and coal permeability variation model during CBM production after CO₂ injection are established based on the stress-strain constitutive equation of poroelastic medium. The average error between the prediction permeability and experimental permeability is 2.89%, which is of a high accuracy. The analysis of influence factors of permeability model shows that permeability of coal reservoir are correlate negatively with confining pressure, compression coefficient of cleats and Poisson’s ratio, and are positively related to maximum adsorption amount, slippage coefficient.On the basis of the established mathematical model of permeability variation in the well controlled area, the variation law of coal reservoir permeability in horizontal plane is discussed at the same time. During CBM production, in the range of four meters from the wellbore, coal reservoir permeability first increases sharply and then decreases quickly below the initial permeability, and when the propagation radius beyond about 4meters, permeability increases gradually back to the initial state. And the permeability decreases significantly with the increases of confining pressure.Based on the experimental research of permeability and starting pressure gradient, the mathematical prediction model of the theoretical maximum CBM recovery rate before and after CO₂ injection is established. Using the established model to predict the CBM recovery rate before and after CO₂ injection is 12.13%²5.24%, 25.11%⁵8.96%, respectively. The relative enhances 107.02¹60.43%. Moreover, the parameter sensitivity analysis of recovery mathematical model is realized, the analysis results show that CBM recovery rate decreases in a power function with the increases of starting pressure gradient, increases in power function with the increases of critical desorption pressure and permeabilty, increases exponentially with the increases of original gas content of reservoir, and has a logarithmic relationship with gas adsorption constant.