The Study Of Main Influence Factors On Productivity Of Coalbed Methane Well In JL Coalfield

The maim task of coalbed methane exploitation is to improve the production of coalbed methane well, and the main factors affecting the productivity of coalbed methane well are gas content, permeability, the thickness of coal seam, the critical desorption pressure and et al. Coalbed methane is mainly adsorbed on pore inner surface of the coal matrix, so the gas content of coal seam mainly refers to the amount of adsorbed gas. Because the gas content, the thickness of coal seam, and the critical desorption pressure are unchangeable, increasing the desorption volume and enhancing permeability are considered as the main methods to improve productivity of coalbed methane well. Clarifying the mechanism of coalbed methane desorption and the permeability dynamic variation rules in the mining process of coalbed methane, will provide theoretical basis for the accuracy prediction and improvement productivity of coalbed methane well, formulation and adjustment of drainage system.Based on geological and production data of the JL coalfield, theoretical analysis and field production practice have been combined to investigate the main factors of productivity of coalbed methane well in JL coalfield, to protect reservoir and avoid reservoir damage in the production process, and further put forward the reasonable drainage system.First, the environmental and climatic conditions, the structure characteristics of coal seam, the characteristics of coal rock and coal quality, the physical characteristics of coal reservoir, gas bearing characteristics, the pressure and stress characteristics of coal reservoir, the top and bottom of the mechanical properties of coal reservoirs, and so on, have been systematically introduced.Three stages, namely water phase, gas-water two phase, and gas phase have been derived from the characteristics of production process of coalbed methane with time in JL coalfield, and the coalbed methane productivity equation has been established based on phase differentiation. The effect of geological factors on well production is performed numerically, and the relationships between the productivity of20evoluation wells and geological, engineering, drainage in JL coalfield are analyzed. The main factors that affect the productivity of coalbed methane well in JL coalfield have been obtained by the grey correlated analysis. Correlative degrees of factors are as follows:permeability>thickness> fracturing fluid volume>critical desorption pressure>sand concentration>gas content. These factors can be divided into two groups:one is material base that refer to abundance and desorption ability; the other is permeability that refer to initial permeability and hydraulic fracturing.Next, based on the Langmuir isothermal adsorption equation, coalbed methane desorption efficiency is defined and the desorption efficiency curve is further employed to characterize the staged desorption of coalbed methane. By using the curvature of desorption efficiency curve,3key pressure points are proposed, namely, turning pressure, transition pressure, and sensitive pressure. With the combination of these3key pressure points and the critical desorption pressure point, the isothermal depressurization desorption process of coalbed methane can be divided into5desorption stages, i.e. zero desorption stage, slow desorption stage, transitional desorption stage, quick desorption stage, and sensitive desorption stage. The desorption stages have different contribution to productivity:zero desorption stage almost shows no contribution; the stages of slow desorption and transitional desorption make small contribution to the production of gas; while quick desorption stage, especially sentitive desorption stage, is the main contribution stage of coalbed methan well high and stable productivity.Then, the permeability dynamic variation rules in the production process of coalbed methane were studied as the focus. As the coal adsorption capacity is affected by coal rock contains moisture content, which indirect impacts the permeability of coal reservoir. It established "the permeability model considering moisture content combined with the effect on permeability of the consideration of effective stress and coal matrix shrinkage two mechanisms". Based on the model, the permeability dynamic variation rules are analyzed with decline of the formation pressure in the drainage process of coalbed methane. Conclusions are as follows:(1)0<prc<prb<pi: in this case, the pore pressure begins to decrease with coalbed methane production, and the negative effect of effective stress is the dominant firstly, which leads to the permeability reduction. When the formation pressure is reduced to the rebound pressure prb, the positive effect of coal matrix shrinkage offset the negative effect of effective stress, resulting in the lowest permeability. When the formation pressure is reduced below the rebound pressure prb, the positive effect of coal matrix shrinkage start in a dominant position, and the permeability begins to rise. When the formation pressure is equal to the recovery pressure prc, the permeability returns to the initial value. When the formation pressure continues to decrease, less than the recovery pressure prc, the permeability will be further increased, greater than the initial permeability.(2)prc> Prb> Pi: for this condition, since the initial formation pressure is less than the rebound pressure prb, the positive effect of coal matrix shrinkage dominates as soon as coalbed methane starts to produce. The permeability has been on the rise.(3) prc=Prb=Pi:under this situation, since the initial formation pressure is equal to the rebound pressure prb, the positive effect of coal matrix shrinkage is equal to the negative effect of effective stress when coalbed methane starts to produce. As the formation pressure further decreases, the positive effect of coal matrix shrinkage further increases and the permeability begins to gradually increase.(4)When the recovery pressure prc is less than0, the permeability decreased in the mining process and cannot return to the initial permeability.Due to the special properties of coal seam reservoirs, pulverized coal is inevitable in the production process. Therefore, the generating mechanism of pulverized coal, the output mechanism of pulverized coal, and the impact of pulverized coal migration on permeability have been systematically analyzed. Finally, drainage system has been optimized for each production stage.