Primary Recovery Mechanism ~ The Petro Stuffs

PRIMARY RECOVERY:

It is the recovery mechanism of hydrocarbon from the subsurface reservoir to the surface using the natural energy present in the reservoir as a drive. This natural energy present in the reservoir is mainly responsible for the primary recovery, because due to the pressure difference between the reservoir pressure and the surface pressure (atmospheric pressure) the crude oil present inside the reservoir will flow towards the oil well and reaches the surface. This drive mechanism contributes less than 15% of initial oil-in-place in the reservoir; in case a reservoir is connected to an aquifer then extra pressure support can increase the overall recovery to 40% or even more of the oil-in-place.

There are generally six driving mechanisms that provide natural energy necessary to recover the oil. They are:
· Rock and Liquid Expansion Drive
· Gas Cap Drive
· Solution Gas Drive
·  Water Drive
·  Gravity Drainage Drive
·  Combination Drive

Rock and Liquid Expansion Drive:

When an oil reservoir exists at a pressure greater than its bubble point pressure, then reservoir is called as “under saturated oil reservoir”. At a pressure above the bubble point pressure, crude oil, connate water, and rocks are the only material present in the reservoir. As the reservoir pressure declines, the rocks and fluids expand due to their individual compressibilities. The reservoir rock compressibility is the resultant of two factors they are:

1) Expansion of the individual rock grains.

2) Formation compaction.

This driving mechanism is considered as the least efficient driving force and usually results in the recovery of small percentage of the total oil-in-place.

Gas Cap Drive:

In a Gas cap drive reservoir the main source of energy is the expansion of gas cap which is available at the top of the reservoir (i.e. Just above the oil in a reservoir). Due to the ability of the gas to expand, when the reservoir is breached while drilling because of pressure difference the gas starts to expand this provides a natural energy to produce the crude oil from the reservoir. The degree of pressure maintenance depends upon the volume of gas in the gas cap compared to oil volume. It has considerably larger recovery efficiency than depletion drive reservoir. The expected oil recovery ranges from 20% to 40%.

The ultimate oil recovery from a gas cap will vary depending largely on the following parameters:
· Size of the Original Gas Ca
· Vertical Permeability
· Oil Viscosity
· Degree of Conservation of the Gas
· Dip Angle

Solution Gas Drive:

In this drive mechanism the main source of energy is due to the liberation of gas dissolved from the crude oil and the subsequent expansion of the solution gas as the reservoir pressure is reduced. As the pressure falls below the bubble point the gas bubbles are liberated within the microscopic pore spaces. These bubbles expand and force the crude oil out of the pore space. This drive mechanism requires the reservoir rock to be completely surrounded by impermeable barriers. This drive mechanism is least effective method. Ultimate recovery from the solution gas drive reservoir may vary from less than 5% to about 30%. The low recovery from this type of reservoir suggests that a large quantity of oil remains in the reservoir so these kinds of reservoirs are best one for the secondary recovery application.

Water Drive:

In this the drive energy is provided by an aquifer that interfaces with the oil in the reservoir at oil water contact. As a production continues and the oil is extracted from the reservoir the aquifer expands into the reservoir displacing the oil. There are two types of water drive reservoir:
· Bottom Water Drive
· Edge Water Drive

In this type drive mechanism the water is encroaching into the reservoir in a uniform manner nothing can be done to restrict this encroachment, as the water will probably provide the most efficient displacement mechanism possible. The ultimate recovery from the water drive reservoir is much larger than the recovery under any other producing mechanism. Efficiency depends upon the effective flushing action of the water so that it can displace oil and the degree of activity of the water drive. Ultimate recovery normally ranges from 35% to 75% of the original oil-in-place. The 75% recovery from water drive mechanism occurs rarely in a water driven reservoir.

Gravity Drainage Drive:

The density difference between gas, oil and water results in the natural segregation in the reservoir. The reservoir fluids were subjected to the forces of gravity, as evidenced by the relative position of the fluids i.e. gas on top, oil underlying gas, and the water underlying oil. The reservoir pressure decline rate varies depending on the amount of gas conservation. If the gas conserved, and reservoir pressure is maintained, the reservoir would be operating under combined gas cap drive and gravity drainage mechanism. Therefore, for the reservoir to be operating only as a result of gravity drainage, the reservoir would show rapid pressure decline.

There are three important factors that affect ultimate recovery from gravity drainage reservoirs:
· Permeability in the direction of dip
· Reservoir Producing Rates
· Oil viscosity

Where gravity drainage is good or where producing rates are restricted to take maximum advantage of the gravitational force, recovery will be high. There are reported cases (rarely occurs) where recovery from gravity drainage reservoir has exceeded 80% of the initial oil-in-place.

Combination Drive:

The driving mechanism most commonly encountered is one in which both water and free gas are available in some degree to displace the oil towards the producing wells. There two combinations of driving forces usually present in the combination drive reservoir:
· Depletion drive and a weak water drive.
· Depletion drive with a small gas cap and a weak water drive.

In addition, to this gravity segregations can also play an important role in any of these two drives. These types of reservoirs usually experience a relatively rapid pressure decline, when water encroachments and external gas expansion is insufficient to maintain reservoir pressures.

Ultimate recovery from the combination drive reservoir is generally greater than recovery from depletion drive reservoirs but less than recovery from water drive or gas cap drive reservoir. In most combination drive reservoir it will be economically feasible to institute some type of pressure maintenance operation, either gas injection or water injection or combination of both the fluid injections are performed depending upon the availability of the fluids.