pied by gases. This volume is then reduced by water saturation—the percentage of the void filled by water.
The quantity of gas that can be produced depends on the permeability, or interconnectedness, of the reservoir rock, which determines how easily fluids can move through the reservoir, as well as on the initial reservoir pressure. Reservoir pressure generally increases with depth and is the pressure on the fluids within the pores of the reservoir. Initial pressure influences how quickly gas can initially be produced (all else equal).
The porosity, permeability, water saturation, and reservoir pressure will determine, in part, the amount of gas that can be produced from a single well drilled into a reservoir. However, the diameter of the pipe, the completion job chosen for the well, and the drilling path—the physical path along which the well is drilled—will also impact the primary production capability of the well. A complete assessment would require knowing the size of pipe used in the well, the number of perforations in the pipe, and any fracturing of the reservoir rock to enhance the ability of gas to flow to the pipe for production.
Total production from the well over time can also be impacted by the production path. Consider a well ready for initial production. The operator of the well could choose to produce the well at “absolute open flow.” That is, it could allow the well to produce against the full force of the reservoir pressure. While this would generally result in high production at first, pressure might decline rapidly, producing too low a flow to maintain economic viability in later years. In other words, because there are minimum costs associated with operating a well, operating it at high rates initially will cause it to be very profitable at first, but fairly unprofitable soon after, even though significant helium remains.
An analogy can be made to the blowing up of a balloon. Assuming no reservoir damage from the aggressive production choice, releasing the gas from the reservoir would reduce the pressure in the reservoir, as would letting the air out of the balloon. If you blow the balloon up and then let the air escape unimpeded, the air will rush out but then decline over time as the pressure in the balloon declines. When there is no more air escaping there will still be some air inside the deflated balloon.
This well could also be produced more conservatively by letting the gas flow against a percentage of the available pressure, reducing initial production and also changing the pressure regime within the reservoir over time. For instance, in a second identical balloon, the air would be allowed to escape but with the throat of the balloon opened only partially. There would be different production and pressure paths. Production would be lower initially than from the first balloon, pressure would decline more gradually, and air would escape for a longer period of time.
The balloon example does not, however, reflect the complicated pressure gradients and barriers to flow that exist in actual reservoirs. In the balloon, the quantity of air left when the throat is wide open or only partially open might vary somewhat,