考虑天然裂缝影响的低渗透气藏产能分析

Productivity analysis of low permeability gas reservoirs with natural fractures

低渗透气藏可能存在一定的天然裂缝发育区,同时它所体现出的非达西渗流效应也会影响其流入动态,导致其产能特征与常规气藏大相径庭,从而影响气井产能的合理评价,因此亟需建立一套适用性广且能准确描述气藏真实情况的流入动态模型。本文从低渗透气藏非达西渗流机理出发,将地层划分为3个流动区域,并基于稳定渗流及连续介质理论,利用水电相似原理及等值渗流阻力法,通过对天然裂缝系统的简化,建立了考虑天然裂缝、启动压力梯度、应力敏感效应等影响下的低渗透气藏天然裂缝的等效渗流模型。通过模型对各影响因素分析的结果表明:应力敏感效应对产能的影响远远大于启动压力梯度;裂缝距井筒越近,产能的增加幅度越大;开发中后期天然裂缝闭合导致的产能下降不容小觑。

Natural fractures may develop in low permeability gas reservoir and their non-Darcy effects affect the inflow performance,result in a big difference with conventional gas reservoir on production characteristics and in the reasonable evaluation of gas well productivity.Therefore,an inflow dynamic model which can accurately describe and generally applicable to the real situation should be established.According to the mechanism of non-Darcy percolation flow,the flow in strata can be divided into three regions.Based on the theory of steady percolation and continuous medium,the equivalent seepage model of natural fracture is established by introducing principle of hydroelectricity similarity and equivalent flowing resistance method.An equivalent seepage model is proposed through the simplification of natural fracture system and in consideration of the effects of natural fracture,start-up pressure gradient and stress sensitivity.Analysis of influence factors by the model indicates that the effect of stress sensitivity on the productivity is far greater than the threshold pressure gradient;the closer the fracture is to the wellbore,the greater the increase in productivity.The decline in productivity due to the closure of natural fractures in the late development stage can not be underestimated.