Depleting reservoirs cause water influx, if there is an aquifer adjacent to and in communication with the reservoir. As pressure drops, the inflow of water will increase, and the flow rate will depend on spatial pressure gradients, permeabilities and size of the aquifers. Water influx provides pressure support, but this positive effect on recovery is usually more than counterweighted by the negative effect of “trapped gas”; gas that is left behind the propagating water front and never will be recovered.
Therefore, common practice in gas field reservoir engineering is to try minimizing water influx. This can be done by rapid production, e.g. by producing a series of smaller gas fields sequentially with high flow rates. Another measure is to drill down flank water producers, which will reduce the lateral pressure gradients and thus water influx to the reservoir.
Laurie Dake wrote: “There is more uncertainty attached to the subject of water influx than to any other in reservoir engineering”. Little information is usually available. It has been common practice to estimate water influx from the P/Z plot, as deviations from a linear trend. However, this method is not very sensitive, and can be seriously misleading if the GIIP (Gas Initially In Place) volume has been miscalculated. This can easily happen, as uncertainties in GIIP often are 10-20%.
Take the two scenarios at right:
More GIIP present than originally believed will slow down the pressure decline, just as the case where there is water influx. Both will show the same pressure decline. However, consequences of the two scenarios on total recovery are opposite. This can be dramatic for the field economics.
Gravity measurements can resolve this issue. Surveying is an inexpensive insurance in cases of uncertain water influx and can significantly reduce the uncertainty in GIIP.
in Petroleum Science. 8