Aquifer water influx is an important mechanism which effects the performance of natural gas reservoirs. The influx of water has serious consequences in the estimation of gas reserves through material balance and in the prediction of future pressures. The effect of water influx from either a finite or infinite aquifer always exists to some extent, however these effects are not always recognized.
Methods of modeling water influx include the Hurst modified steady-state method, and various unsteady state methods such as those of van Everdingen-Hurst, and Carter-Tracy. More recently, the four methods introduced by Leung for finite and infinite aquifers provide relatively simple models to use with substantial improvements in accuracy and efficiency over the previous methods.
Using the methods introduced by Leung, combined with the volumetric's of the gas reservoir, aquifer parameters such as size, total compressibility, influx angle, permeability, porosity and thickness can be determined. With adequate pressure data, the models converge to a unique solution for all aquifer parameters.
The development of the aquifer influx models was the result of the need to better understand the behavior of the McMurray aquifer system in north east Alberta. The 7-5- 81-6W4M well is used as an example, though the application of the models can be extended to any gas reservoir.
The computation of the initial gas-in-place for constant volume reservoir requires the following data:
Initial reservoir pressure
Cumulative gas volume
Stabilized shut-in reservoir pressures
Gas deviation factor
Although this method is currently used, it is not applicable to water-drive gas reservoirs. With pressure reduction, when water enters the space occupied by the gas, the pressures are maintained in part depending on the nature of the water drive.
Pressure support occurs due to a variety of mechanisms - including an infinite acting aquifer, unsteady state finite aquifer, shale water influx, water expansion and rock compressibility/compaction. Through the analysis of DSTs, AOFs, Logs, pressure and production data, the mechanism and future performance of the aquifer can be forecast.
P/Z plots of gas pools attached to a medium to slowly responding aquifers usually follow the straight line of a depletion drive reservoir and then gradually rise above it. This is shown in Figure-1.
Extrapolation of the P/Z will overestimate the gas-inplace. The gas wells water out as the true gas-in-place is approached and while the reservoir pressure remains relatively high (ie. 50% of original reservoir pressure).
The P/Z plot for gas pools influenced by strong aquifers can initially follow a straight line and looks similar to a depletion gas drive reservoir, as long as constant gas production rates are maintained. The extrapolation of the P/Z plot will severely overestimate the gas-in-place in these cases. Again, these gas wells water out as the true gas-in-place is approached and the reservoir pressure as before remains relatively high (ie. ∼75% of original reservoir pressure).
