一个新的低渗透气藏气井产能预测公式

低渗透气藏普遍具有低孔、低渗、高含水饱和度的特点，因而往往具有较高的启动压力梯度。目前，在进行低渗透气井产能预测时，多采用常规气藏的产能预测公式，启动压力梯度被忽略，导致预测结果与生产实际存在较大误差。通过大量文献的调研，确认低渗透气藏中启动压力梯度确实存在，并且已逐渐成为产能预测中一个不容忽视的重要影响因素。为此，文章基于对低渗透气藏渗流机理的分析和研究，利用Forchheimer由实验提出的描述气体渗流的压降二次方程，同时考虑启动压力梯度的影响，推导出了适合低渗透气藏的气井产能预测公式，并对其进行了实例计算分析，认为低渗透气藏产能预测中必须考虑启动压力梯度的影响，得到了进行产能预测时一些必要参数的获取方法。

A NEW EQUATION FOR ESTIMATING GAS WELL PRODUCTIVITY IN LOW-PERMEABILITY GAS RESERVOIRS

Low-permeability gas reservoirs are generally characterized by low porosity, low permeability and high water saturation, thus usually have relatively higher threshold pressure gradient. The productivity prediction equations of conventional gas reservoirs are commonly used to estimate the gas well productivity in low-permeability gas reservoirs at present, without consideration of the threshold pressure gradient, resulting in large discrepancy between the calculations and production realities. Based on survey and study of large amount of literatures, it is confirmed that the threshold pressure gradient indeed exists in low-permeability gas reservoirs and has progressively become an important influential factor not to be ignored in prediction of productivity. Based on analysis and study of the percolation mechanism in low permeability reservoirs, this paper presents a new equation for estimating gas well productivity in low-permeability reservoirs by using Forchheimer's experiment-based quadratic equation of differential pressure describing gas percolation and concurrently taking into consideration of the threshold pressure gradient. A case calculation with this new equation shows that the threshold pressure gradient must be considered in productivity prediction of low-permeability reservoirs. Methods to obtain some necessary parameters of productivity prediction are also presented.