天然气在致密气藏储层条件下流动呈非达西渗流特征,存在启动压力梯度,压裂是提高致密气藏采收率的重要手段。为提高致密气藏采收率预测精度,利用气藏工程方法,综合考虑启动压力梯度和不同井型渗流形态,推导了常规直井、压裂直井、常规水平井、压裂水平井的采收率计算公式,提出了致密气藏采收率计算新方法，并以X致密气藏为例,进一步研究了启动压力梯度、压差、半缝长、裂缝间距、压裂水平井长度等致密气藏采收率影响因素。结果表明:致密气藏采收率随启动压力梯度的增大而降低,压裂井采收率降幅明显小于常规井采收率降幅。对于压裂直井,随着半缝长增加,采收率提高,但增幅变缓,存在最优值;对于压裂水平井,半缝长对采收率的影响比裂缝间距对采收率的影响更大,且在一定泄气范围内存在水平段最优长度。矿场实例应用表明,致密气藏采收率计算新方法计算精度满足工程要求,能够指导致密气藏开发设计优化。致密气藏采收率计算新方法对致密气藏压裂设计及采收率计算具有重要指导意义。

The seepage of gas in tight reservoir condition is non-Darcy. There exists threshold pressure gradient during the seepage in tight gas reservoir. Fracturing is an important method to improve the recovery of tight gas reservoir. To improve the prediction accuracy of recovery of tight gas reservoir, with the threshold pressure gradient and the seepage patterns of different well types taken into account, the calculation formulas for the recovery factor of conventional vertical well, fracturing vertical well, conventional horizontal well, and fracturing horizontal well were deduced using the gas reservoir engineering method. The effects of different parameters on tight gas recovery were further discussed relying on X tight gas reservoir. The results showed that the tight gas recovery decreased with the increment of threshold pressure gradient, and the decreasing amplitude of gas recovery of fractured well was obviously smaller than that of conventional well. For the vertical fractured well, the gas recovery increased with the increasing of half-length and there existed an optimum length value. For the horizontal fractured well, the effect of half-length on gas recovery is more sensitive than fracture spacing. There is an optimal well length under a certain drainage area. The field application shows that the new method has a high calculation accuracy satisfying the engineering requirements, which can guide the design optimization of tight gas reservoir.