不稳定渗流理论在水驱气藏水侵识别中的应用

Application Study of Unsteady Fluid Flow Theory to Identify Water Encroachment in Water Drive Gas Reservoir

提出了利用不稳定渗流理论及应用技术进行水驱气藏水侵识别的新方法,将水驱气藏视为复合地层,建立了渗流数学模型,在获得数学模型的解后,利用Stehfest算法计算出水驱气藏中气井的压力及压力导数特征曲线,压力导数曲线具有4个明显特征:纯井筒储存阶段表现为单位斜率直线;气区的径向流动阶段,表现为斜率是0其值为0.5的水平线;边水向气区推进过程中的流动阶段,其压力导数曲线上翘;复合地层总系统的径向流动阶段,表现为斜率是0的水平线,其值为气水流度比之半.由压力导数曲线看出,随着边水向气区的推进,即随着无因次气区半径的减小,气区的径向流动阶段持续的时间变短,边水向气区推进过程中的流动阶段出现更早,即其压力导数曲线上翘的时间变短.利用压力导数曲线这一特征提出了识别水侵的图版拟合分析方法及常规半对数分析方法,最后进行了实例应用,验证了本方法的可靠性和正确性.

This paper presents a new method to identify water encroachment of gas reservoir, which is based on unsteady fluid flow theory and application technology. The water drive gas reservoir was considered as a composite reservoir, based on which, a mathematic model is established and its solution is achieved by using Stehfest computational method. The pressure cure and corresponding derivative cure of pressure-transient test of gas well in water drive reservoir was also presented. There are four distinct characteristics on pressure derivative curve： on pure wellbore storage stage, a unit slope is developed; on the radial gas flow stage, the slope is zero and its level value is O. 5 ; on the water encroachment stage, derivative cure is upwarding; on the total radial flow stage, the slope is zero and its level value is half of the gas/water mobility ratio. It is observed that the time interval of upwarding is becoming shorter with the radius decreasing on derivative cure. In other words, with the radius of gas area decreasing, the stage of water encroachment is developed earlier. According to the features of derivative cure, two new methods to identify water encroachment are proposed type curve match method and semi-log analysis method. Finally, a field case study is presented to verify the validity of the methods. The work in this paper provided the basic theory to identify water encroachment in water drive gas reservoir.