Natural Gas Industry B (Feb 2021)
Change laws of water invasion performance in fractured–porous water-bearing gas reservoirs and key parameter calculation methods
Abstract
In order to develop fractured–porous water-bearing gas reservoirs efficiently, it is necessary to understand the change laws of water invasion performance. Based on the π theorem, this paper adopted the dimensionless parameter that can reflect the change characteristics of water invasion performance to design the physical simulation experimental scheme of water invasion performance. The full-diameter cores taken from the reservoir of one certain fractured–porous water-bearing gas reservoir in the central Sichuan Basin were selected to carry out physical simulation experiment of water invasion performance. Then, by virtue of numerical inversion, the experimental results were converted into evaluation parameters of water invasion performance in water-bearing gas reservoirs. Afterwards, the change laws of water invasion performance in fractured–porous water bearing gas reservoirs were investigated, and the method for calculating key water invasion performance evaluation parameters and water volume was established. Finally, water invasion performance analysis and aquifer size evaluation were conducted with the Xu 2 Member gas reservoir, Upper Triassic in the Zhongba gas field of the Sichuan Basin as an example. And the following research results are obtained. First, in fractured–porous gas reservoirs with edge and bottom water, water invasion and gas production occur simultaneously. Before water production, there is a linear relationship between bottom hole pressure and reserve degree of reserve recovery. After water production, water invasion speeds up and the water–gas ratio (WGR) rises rapidly and finally approaches to a stable WGR of higher level. Second, the bottom hole pressure drop rate before water production and the stable WGR are mainly affected by the water volume, and the degree of reserve recovery per unit pressure drop and the stable WGR can be used to evaluate the water volume. Third, a method for mutual interpretation and evaluation of the aquifer size and the reserve degree of reserve recovery per unit pressure drop before water production and the stable WGR of fractured–porous gas reservoirs with edge and bottom water is established. In conclusion, the water invasion performance curve of the gas reservoir agrees with the core simulation experimental results. Based on the known water volume of a gas reservoir, the stable reserve degree of reserve recovery per unit pressure drop before water production and the stable WGR can be predicted; and on the basis of the stable WGR, the water volume of a gas reservoir can be predicted. The prediction results are in line with the production performance, providing a theoretical support for water invasion performance prediction and overall water control of fractured–porous water-bearing gas reservoirs.
