PrintIn Lesson 4, we discussed the drive mechanisms associated with oil reservoirs. For gas reservoirs, there are three drive mechanisms that are associated with conventional gas reservoirs and a fourth drive mechanism associated with unconventional gas reservoirs. These are:
- gas expansion (most significant drive mechanism in conventional gas reservoirs);
- gas desorption (may only be present in certain unconventional gas reservoirs);
- rock and fluid expansion (expansion of the reservoir rock and interstitial water – typically only significant in over-pressured gas reservoirs); and
- natural aquifer drive (or water encroachment).
In this list, I make a distinction between Conventional and Unconventional Gas Reservoirs. Conventional gas reservoirs are reservoirs with sufficiently high permeability to allow for production using conventual well technologies. Unconventional reservoirs are reservoirs with low permeabilities that require special production technologies that allow for economic recoveries of gas. Typically, the threshold to define an unconventional gas reservoir is a reservoir with a permeability less than 0.1 md.
Gas expansion is the primary drive mechanism in most conventional gas reservoirs. Again, the analogy of a gas-filled balloon a very appropriate analogy. If a balloon is filled with high pressure gas and the end is opened to the low pressure atmosphere, then gas will expand and exit the balloon. This mechanism is very efficient and commonly results in recoveries as high as 85 percent of the original-gas-in-place.
A drive mechanism that is associated with certain unconventional gas reservoirs is gas desorption. As we have already discussed, unconventional gas reservoirs are reservoirs with permeabilities less than 0.1 md. These unconventional reservoirs include:
- tight oil and gas sandstones or carbonates;
- shale gas and shale oil reservoirs; and
- coal seam methane reservoirs.
The last two of these unconventional gas reservoir types, shale gas reservoirs and coal seam methane reservoirs, have a high content of organic material in the reservoir rock. This organic rich rock material has the ability to Adsorb gas onto its surface (gas stored by adhesion onto the surface). As pressure is depleted, this adsorbed gas is released to the pore-volume of the reservoir by the Desorption Process. This desorption of gas may dominate production from the unconventional gas reservoirs in which it occurs.
Rock and fluid expansion in gas reservoirs is identical to that in oil reservoirs. It occurs due to the slightly compressible nature of the Interstitial (or Connate) Water and the reservoir rock. This expansion adds energy to the reservoir and acts to keep the reservoir pressure higher than it would be otherwise. This expansion mechanism is always dominated by gas expansion and may only be significant in cases of over-pressured reservoirs.
The final drive mechanism associated with conventional gas reservoirs is aquifer drive, or water encroachment. As with oil reservoirs, this drive mechanism occurs when the reservoir is in communication with a water-bearing aquifer. As the reservoir pressure declines, the rock and water in the aquifer expand, and water is expelled from the aquifer and into the reservoir. This encroachment of water into the reservoir provides pressure support.
These last two drive mechanisms may be slightly deceptive as to whether they aid in gas production or not. Both of these methods tend to keep reservoir pressures high; however, the principle drive mechanisms, gas expansion, and gas desorption, rely on pressure depletion. In addition, water encroachment may also result in trapped gas behind the invading water front.