| As the gas resource, coal bed methane which is associated with coal has become the focus of unconventional gas exploration and development for its clean, calorific value and broad development prospects. In China, the geological conditions and the occurrence conditions of coal bed methane reservoir are complex, the development of coal bed methane is controlled by the coupling effects of stress field, groundwater pressure field and temperature field. In this paper, based on the coal seam gas heat injection exploitation, the experimental and simulation method is used to study the gas liquid two phase flow under the coupling effects of stress field and temperature field of coal. The main research contents and results are as follows:1) According to the mechanism of coal bed methane production, the exploitation of coal bed methane is the movement process of gas desorption, diffusion and seepage. Based on the vaporization and condensation dynamic equilibrium principle, Fick law and Darcy law, a numerical model for the gas-liquid two phase flow simulation in the strata is obtained. In the ordinary solving process, analysis of capillary force and gravity of different conditions for simplified calculation. The simplified flow differential equation is obtained.2) A large number of unsteady state two phase flow experiments were carried out by using self development gas-liquid two phase high precision osmotic system. The results show that the whole process contains three stages: water flow stage, gas-liquid mixing flow stage and gas flow stage, and the seepage characteristics of each stage are obtained. The water saturation rate of coal is reduced to 0.5, and then the water saturation decreases sharply, but this stage only has the gas flow. The change of the degree of saturation can be predicted for gas saturation rate.3) Under different stress conditions,the two groups of coal samples were carried out under different conditions of single-phase flow and two-phase displacement seepage experiment. Experimental results show that: in the same axis pressure σ1, confining pressure σ1(σ1>σ2) conditions, increases the pore pressure P1, water permeability increases nonlinear. When the pore pressure reaches a certain value, the permeability will increase suddenly. And under relatively low stress, and pore pressure affects on the permeability more obvious. In comparison to the water phase permeability and water effective permeability: the gas increases the flow resistance of the water. With the higher the gas displacement pressure is, the greater the resistance will be. And this will induce the greater difference of the effective permeability.The characteristic of relative permeability curve of coal: with the increase of gas saturation, gas relative permeability curve will increase sharply, the relative permeability of liquid phase decreases slowly in the lower range. After the gas breakthrough, it quickly dropped to zero, and the coal sample had a higher bound water saturation.4) In the process of 17 kinds of pressure combination experiment, the water permeability and gas permeability decrease with the effective stress increases in the relatively low effective stress range(less than 4~ 5MPa). With the increase of effective stress, the permeability in the late stage decreases slightly. Compared with the water permeability and gas permeability of same coal sample, the trend of the change is same. At the same time, with the normalized treatment of the permeability data, the results trend to be consistent, whether it is the choice of the water phase as the fluid or gas phase as the fluid. Mathematical fitting of experimental data shows that the normalized permeability of coal and the effective stress have good power function relationship. After thermal effect, the stress sensitivity of the coal decreases. In the coal bed gas thermal recovery process, the coal stress sensitivity decreased will lead to the drainage and gas production process more smooth, and this process avoid the sudden large output of gas encountered in the traditional process of pressure reduction.5) By using gas liquid two phase flow device and temperature control test bed, the single phase permeability experiment and gas-flooding-water experiment of the coal with the change of the temperature in the range of 30℃ to 180℃ was carried out. Under the condition of temperature, the law of two-phase fluid flow and the flow changes process were studied. With the increase of temperature, the liquid production increased under each pressure. The liquid yield increase was more obvious and mainly in the high temperature phase(above 120℃). It is indicate that: the temperature in the range of 120℃ to 180℃, the higher temperature is, the larger thermal expansion deformation of coal will be. Also, in the higher temperature, the porosity and fracture connectivity is better. At the same time, the water molecules at high temperature get greater heat energy; this phenomenon is good for the liquid output of coal. Instead, under the same temperature conditions, the pore and fracture inside the coal is compressed when increases under axial pressure and confining pressure enable effective stress changes. Thus, part of the liquid cannot be discharged, the bound water increased and water yield decreases.6) The three stages of the whole process of gas flooding water two-phase flow under the condition of temperature: the first stage is the liquid linear seepage stage, with the increase of temperature, the percentage of liquid flow decreases. When the temperature is higher than 120℃, this phenomenon will be more obvious. And the second stage is gas-liquid mixed stage, liquid flow velocity increases rapidly and gas flow rate is stable. With the increase of temperature, the percentage of liquid flow in this stage decreases gradually. But when the temperature is higher than 90℃,the percentage of liquid decreases more obviously. In the third phase, temperature in this stage affects the gas flow under the bound water more obvious and increases in the percentage of liquid flow. When the temperature less than the 60℃, the liquid flow rate is about 10% and the yield of liquid is mainly happened in first and the two stage. When the temperature reached 150℃, the liquid flow rate reached 50%, and the proportion of liquid flow rate was more than one or two stages.7) The temperature in the range of 30 ℃ to 180 ℃, variation of permeability was divided into two stages: At low temperature range(30℃ to 120℃), the influences on fluid is dominated by the temperature in this stage. The fluid viscosity changed significantly during the temperature change. Gas viscosity increased slightly, and the change trend of permeability measurement was consistent with the change trend of viscosity. In the high temperature range(120℃ to 180℃), the changes of the porous media structure become the main factors, the viscosity of liquid and gas are relatively slight, but the change of permeability is significantly. It is indicated that: the pore fracture of coal sample has a great change affected by the temperature, the flow passage is better than that of the low temperature section, which is good for the fluid flow. At the same time, with the increase of temperature, gas water two phase flow area becomes wider. And the bound water saturation decreases, the relative permeability decreases with the decrease of water saturation. At the higher temperature, the influence of the relative permeability curve of gas water two phases was reduced gradually. In order to explain the pressure drop caused by temperature change, the temperature sensitivity coefficient is proposed, the relationship between the sensitivity coefficient of pressure drop and temperature are presented.8) According to the Euler and Lagrange’s viewpoint and the flow interface, and the pseudo pressure functionp=2∫PP0p/μdgZdp is introduced into the two-phase flow equation. The pressure function is divided into three kinds of situations: ①p/μgZ as a constant, pg =pg; ②μgZ as a constant, pg =pg2; ③μgZ is a linear function of the pressure p,p=2∫PP0p/ap+bdp. Thus, the model of interface of gas liquid two phase flow is obtained without considering the shrinkage displacement, compressibility displacement model and displacement model of pseudo pressure linear function equation. The flow equation and boundary conditions of the three models are derived in detail, and the relationship between the flow rate and the position function is obtained. By using MATLAB, the three models were calculated, and the experimental parameters were selected under the same stress condition. The curves of cumulative output water flow with various time and the gas-liquid flow interface positions were obtained. Compared with the experimental results and errors of the three models, the applicability of the three models is also obtained. It provides a more reliable model for the drainage prediction of the actual project. |
