Determination of the Aquifer Activity Level and the Recovery of Water Drive Gas Reservoirs

Abstract

The judgment of the aquifer activity level and the recovery of waterdrive gas reservoir are intricate and crucial. The paper is to develop a new criterion to determine the aquifer activity level and a new chart method demarcating the range of recovery variation. In the first place, reasoning and defining the new criterion of the determinate aquifer activity level is obtained by water influx coefficient B, based on the material balance equation of water drive gas reservoirs and the study of previous scholars. Thus a theoretical curve equation and a suppositional curve equation are derived and defined. The former one is pr(1-RgB)=1-Rg, which shows the relationship between the relative pressure pr and the degree of reserve recovery value Rg with different B. The latter one, pra=(1-ER)/(1-aEva)(where a=Sgr/Sgi), relates to the ratio of residual gas saturation and original gas saturation Sgr/Sgi, the ultimate volumetric sweep efficiency Eva and represents the relation between the relative abandonment pressure pra and the recovery ER. Finally, a set of charts is established by the two equations, by which we can set up a table to determine the variation range of the recovery with corresponding Sgr/Sgi value and Eva value when the aquifer is at different activity level.

Water Production Problem in Gas Reservoirs: Concepts, Challenges, and Practical Solutions

“…Most of the time, the type of reservoir aquifer is determined by experience and a quick review of pressure/production data. Some type curves have been presented to estimate the degree of activity of aquifer and the recovery of gas reservoirs by Li et al based on the material balance equation [37]. Knapp et al [31] performed a multidimensional, twophase, compressible fluid flow calculation to simulate the reservoir depletion.…”

“…Li et al [37] presented a method to verify the activity level of aquifer and abandonment pressure of gas using pressure production data. e relation between coefficient of water remaining ω and reserve recovery degree R g was obtained in the following form:…”

“…According to (1), if B is equal to 1, ω and R g are also equally showing a strong aquifer. However, if B goes to infinity, ω becomes approximately zero, denoting the absence of aquifer [37].…”

“…erefore, an aquifer exists for values of B in the range of 1 < B < ∞. In cases that B is greater than 4, the effect of an aquifer can be ignored so that it can range between 1 and 4 for large aquifers [37]. Li et al [37] also classified water-drive gas reservoirs into three categories according to the value of B.…”

“…where P r can be plotted versus R g to define theoretical curves, which are used to estimate the strength of aquifers (Figure 2). Considering the effect of compressibility, the material balance equation is in the following form [37]:…”