A Study On Numerical Simulation Method Of Pinnate Horizontal Multilateral Well For Coalbed Gas Recovery

Pinnate horizontal multilateral well is an efficient technology which can increase coalbed gas recovery rate. However, its application is still in test stage in China. As one kind of means for quantitive forecast and analysis, the numerical simulation technique can provide necessary theory basis and guidance scheme for the implement of this technology.The mathematical model and numerical model of vertical well for coalbed gas recovery is developed and completed. The numerical simulation is realized and the factors effecting the gas production rate and the adsorption mechanism are analyzed. Based on them, a mathematical model and numerical model of pinnate horizontal multilateral well for coalbed gas recovery is developed, in which such factors as heterogeneity and anisotropy of stratum, sensitivity of permeability to pressure and friction and acceleration loss in wellbore are considered in this model. The solution process of the numerical model is presented and the numerical simulation of pinnate horizontal multilateral well for coalbed gas recovery is realized.With the coalbed gas pilot test area in Fanzhuang section of Qinshui basin in Shanxi province as an example, the productivity of pinnate horizontal multilateral well of is forecasted, the economic evaluation of recovery effectiveness of pinnate horizontal multilateral well is made and compared with that of vertical well.The increase production mechanism of pinnate horizontal multilateral well for coalbed gas is analyzed based on numerical simulation. It is pointed out that main lateral and side laterals extending widely in stratum makes the stratum pressure drow down rapidly and uniformly in controlled area, which facilitates the desorption and diffusion of adsorbed gas. It is the prime factor that pinnate horizontal multilateralwell can increase gas production rate. The effects of such formation parameters as permeability, Langmuir constant and adsorption time on the gas production rate of pinnate horizontal multilateral well are analyzed and compared with that of vertical well.By computing gas rate of horizontal well with various wellbore structures in the same formation condition, some useful principles on designing wellbore structure are presented. Generally, the recovery effectiveness of multi-main lateral distributing uniformly is superior to single main lateral. There exists the optimal value of space between side laterals, which is similar to the existence of the optimal well pattern and pattern well spacing in vertical well section production mode. The horizontal well lying in the substratum of coal bed can gain better recovery effectiveness in positive rhythm stratum. And the horizontal well lying in the superstratum of coal bed can gain better recovery effectiveness in inverted rhythm stratum.