对于定容封闭气藏，当气井以稳定产量开井生产后，根据井底流动压力与时间的变化动态，可划分为非稳定阶段、过渡阶段和拟稳定阶段。其中的非稳定阶段称为弹性一相阶段，拟稳定阶段称为弹性二相阶段。后者是评价气井控制原始地质储量（OGIP）的重要方法。陈元千于1991年提出的拟稳定阶段的弹性二相法，在中国得到了广泛的重视和应用，被连续多次列于中华人民共和国石油天然气行业标准。基于在有限封闭边界气藏内，一口以稳定产量生产的气井，考虑天然气、岩石和束缚水的弱弹性膨胀作用的扩散方程，经推导得到了由井底流动压力与累积产量表示的拟稳定阶段弹性二相法的新式。该式可用于评价气井控制的原始地质储量（OGIP），它优越于由井底流动压力与时间关系表示的原式，克服了因产量波动对评价结果造成的影响。通过实测应用表明，弹性二相法的新式是实用有效的。

For volumetric closed gas reservoirs, when the gas well produces with stable production rate, it can be divided into unsteady state stage, transient stage and pseudo steady state stage according to the variation behaviours of flowing bottom hole pressure and producing time. The unsteady state stage is called the elastic one-phase stage, and the pseudo steady state stage is called the elastic two-phase stage. The latter is a dynamic method to evaluate the Original Gas in Place（OGIP） controlled by gas wells. The elastic two-phase method proposed by CHEN Yuanqian in 1991 in the pseudo steady state stage has been repeatedly listed in the National Oil and Gas Industry Standards, and has been widely valued and applied in China. Based on the diffusion equation considering the elastic expansions of natural gas, rock and irreducible water in a gas well with stable production in the finite closed gas reservoir boundary, a new elastic two-phase method for the pseudo steady state stage represented by flowing bottom hole pressure and cumulative production rate is derived, which can be used to evaluate the original gas in place（OGIP）controlled by a gas well. The new formula of elastic two-phase method is superior to the old formula which based on the relationship between flowing bottom hole pressure and production time, and the new can overcome the influence of production fluctuation on the evaluation results. The application shows that the new formula of elastic two-phase method is practical and effective.