Reservoir drives are sometimes referred to as the reservoir's natural energy. As we search for hydrocarbons, we need to understand the role of pressure and compressibility, which powers the reservoir drive. An effective oil and gas recovery strategy is one that takes into account the effect of the natural reservoir energy.
Engineers are constantly trying to improve the reservoir's recovery factor, which is the ratio of the volume of oil or gas brought to the surface to the volume originally in place in the reservoir. The higher this ratio, the more effective the recovery strategy will be. One way to do this is to understand the reservoir drive mechanisms that can be manipulated when preparing reservoir management and production strategies.
What Drives the Flow in a Well?
At this point it is important to note that fluids trapped in a reservoir are present in equilibrium and have no communication with the surface, otherwise if they did, then the oil or gas would naturally flow upwards to the surface and in that case there is really no need to drill a well to tap the reservoir. (Learn about the Three Types of Oil Reservoirs.)
Wells are drilled to produce from reservoirs that have no communication with the surface. These reservoirs are in pressure equilibrium. The principle behind wells flowing to the surface is that a well drilled into the reservoir disrupts the natural pressure equilibrium and hence we have fluids flowing from the zone of high pressure to the zone of low pressure. Remember that in the formation of oil or natural gas, fossils are buried under great heat and pressure with layers upon layers of rocks over time lying on top of the reservoir rock and exerting an overburden pressure on the rocks and fluid. Hence, the reservoir is always operating at a higher pressure than the pressure found at the surface. It is therefore natural for the fluids in the reservoir to tend to flow to the drilled well because the drilled well has a connection with the surface that operates at a lower pressure. This understanding will guide us as we delve into looking at the various types of reservoir drive mechanisms.
Types of Reservoir Drive Mechanisms
The various types of reservoir drives include:
- Water drive
- Solution gas drive
- Gas cap drive
- Gravity drive
- Rock compaction drive
- Combination drive
All of these options are possible for any given reservoir, but not all of them are always available to be manipulated. Some reservoirs combine two or more of these drive mechanisms, which is the case in a combination drive. Also, all of these drives in one way or another function by a form of rock or fluid expansion.
With the water drive mechanism, there is always water present in the oil or gas reservoir at all times. This water is called connate water, but is not the water behind water drive. Connate water occupies some of the pore spaces with the oil or gas as a function of wettability and capillary pressure and is naturally produced with the oil at a given saturation. (Learn more about pore spaces in Grasping the Concept of Rock Permeability.) However, water drive derives its energy from a water reservoir called an aquifer.
As the well is opened up for production, oil flows through the reservoir pores and into the wellbore. This creates a void, and since nature is always seeking a balance or equilibrium, water from the aquifer moves to fill the voids left by the produced oil. For water drive to be effective certain conditions must be met:
- The volume of the aquifer must be very large to ensure the continuous availability of water to recharge the voids left by the produced oil. Another way this condition can be met is by recharging the aquifer by water injection as a means of secondary pressure maintenance. This is why some recovery strategies incorporate water injection from the onset instead of waiting until the pressure declines.
- The aquifer must have good rock properties. Do not forget that the aquifer is a reservoir too — just that it is a water reservoir. This is why properties like permeability must at least be equal or even better than that of the oil reservoir. This will ensure that the water moves as fast as possible to replace the voids left by the produced oil.
- There must be a connection between the aquifer and the oil reservoir. This condition is saying there must not be any seal between the aquifer and the oil reservoir because any seal will prevent the flow of water from the aquifer into the oil reservoir. The aquifer should actually be an extension of the oil reservoir such that any change in pressure in the oil reservoir is felt in the aquifer. That way, the aquifer quickly reacts to fill any voids left by oil production.
Solution Gas Drive
A solution gas drive refers to the gas present in solution or dissolved in the oil. Light oil, for instance, has a high concentration of dissolved gas compared to heavy oil. As the oil is produced, the oil expands as a result of this dissolved gas. Remember that this gas is dissolved in the oil under great pressure. As we produce the oil, the pressure drops and the gas expands. Since the gas is present in the oil, the gas expansion causes oil expansion and thus the remaining oil fills the voids left by produced oil. This will continue until a pressure called the bubble point pressure is reached — this is the pressure at which the first gas bubble leaves the oil. Once this happens, the oil will shrink instead of expanding. Don’t forget that it was the presence of the gas in the oil that caused the oil to expand in the first place. This oil shrinkage will result in the lowering of oil viscosity and negatively affect oil recovery.
Gas Cap Drive
Here, free gas is already present in the reservoir. Free gas is different from solution gas. True to its name, the free gas is free, and is not present in solution in the oil but exists freely at the top of the oil. Reservoir fluids settle according to their density, with gas occupying the topmost position, followed by oil then followed by water. This gas at the top of the oil leg is the gas cap. As we produce the oil, the free gas expands to fill the voids left by the produced oil. This expansion drives even more oil to the wellbore.
This kind of reservoir drive mechanism is most effective in reservoirs with high vertical permeability. The reservoir fluids are driven as a function of gravity and thus it is most common in fractured reservoirs. It helps if the reservoir has a free gas cap to further aid in pushing the oil from the pores to the wellbore. Another important factor is for the reservoir to have a steep geometry. Gravity aids oil production from every reservoir in one way or the other. It may not be dominant in some reservoirs but it is still present.
Rock Compaction Drive
This kind of drive mechanism relies on the compaction of the reservoir rock matrix as voids are created in the rock pores. Remember that an oil reservoir is just like a sponge. The principle behind this drive is that as oil is produced, voids are created in the pores that were originally occupied by the oil. As this happens, the overburden pressure from overlying rocks squeeze the reservoir rock compressing the rock matrix. As the matrix is squeezed more and more, oil is expelled from the reservoir rock. This is the principle behind compaction drive. It is most prevalent in shallow reservoirs or reservoirs in unconsolidated formations because formation compressibility is higher in these conditions.
This is the most common drive mechanism that results in a situation where two or more drives are contributing to oil production. Most reservoirs are producing from a combination of two or more drive mechanisms, although one of the drives may be the most active. For example, a reservoir can be driven by water and solution gas at the same time, or with gas cap, rock compaction and water at the same time with one of them being the most dominant.
A Note about Artificial Drive Mechanisms
Some recovery strategies do not rely on the natural reservoir energy alone to push the hydrocarbons to the surface because sometimes the reservoir drive may be too weak to push the oil from the pores to the wellbore and all the way to the surface. In this case, an external source of energy is required. This is why artificial lifts are used or a secondary source of energy may be recommended. (Discover a common use of artificial lifts in the article The Unique Challenges of Coal Bed Methane Production.)
Any method of recovery must recognize the rock and fluid properties and available drive mechanisms present in the reservoir. For example, a reservoir that has a weak aquifer, whose energy is not enough to lift the oil all the way from the pore to the surface, can be assisted by injecting water from the surface into the aquifer. This is called water injection and it is a secondary means of recovery. This recovery strategy where secondary recovery is combined with a primary reservoir drive right from the beginning of production is becoming more common.