考虑启动压力梯度的低渗底水气藏见水时间预测

摘要

由于存在启动压力梯度,低渗透底水气藏中的水锥动态不同于常规底水气藏,因而利用常规预测公式计算气井见水时间,其计算结果肯定与真实情况有偏差.建立了底水气藏的水锥过程模型:气井钻开部分气层,射孔段为平面径向流,射孔段以下为平面径向流和半球面向心流的组合.依照该模型,假设储层水平、均质、等厚且具有各向同性,水以活塞方式驱气,气、水的密度和黏度均为常数,气水界面内外的压力梯度相同,忽略重力和毛管力.在此假设条件下,推导出了考虑启动压力梯度的低渗透底水气藏气井见水时间预测公式.将该公式与Sobo-cinski-Cornelius方法进行了实例计算对比,发现该公式计算的见水时间更接近于实测值；且利用推导出的预测公式计算的见水时间随着启动压力梯度的增加不断缩短,这符合定产量生产条件下,启动压力梯度越大则井底压力越小,底水与井底之间的压差越大,从而更容易发生底水锥进的实际情况.%Because of the threshold pressure,the performance of water coning in low permeability gas reservoir with bottom water is quite different from that of conventional gas reservoir with bottom water. Therefore,there is a big gap between the actual performance and the calculated water breakthrough time based on conventional prediction formula. This paper established a water conning model for gas reservoir with bottom water: the gas layer was opened partly in gas wells, gas flows in radial flow form inside the perforation section,and combined with radial flow and semispherical flow below the perforation section. According to this model,it is assumed that the reservoir has horizontal,homogeneous,isotropic layers with equal thickness, water displaces gas in a way of piston-type,the viscosity and density of water and gas are constant,the pressure gradient at the interface of water and gas is the same,and the gravity and the capil-lary pressure can be neglected. Based on the assumption,the prediction formula of the water breakthrough time has been derived by considering threshold pressure gradient in low permeability gas reservoir with bottom water. Comparing the result calculated by this formula with that of Sobocinski-Cornelius formula, the formula calculation results more close to the real value,and we conclude that the water breakthrough time is reduced with the increase of threshold pressure gradient.