Gas-Lift Troubleshooting

Reservoir Drives

The reservoir drive mechanism supplies the energy that moves the hydrocarbon located in a reservoir container toward the wellbore as fluid is removed near the wellbore. There are five common drive mechanisms: Water drive, Gas expansion, Solution gas, Rock or compaction drive, Gravity drainage
One type usually dominates, but drive types can occur in combination. Depending on the drive mechanism, characteristic recovery efficiencies can be expected for a given reservoir.

1. Oil reservoirs.

a.     Solution gas drive.

b.     Gas cap drive.

c.      Water drive.

d.     Gravity drainage drive.

e.     Combination drive.

2. Gas reservoirs.

a.     Volumetric (gas expansion) drive

b.      Water drive

c.       Combination drive

Gas Cap Drive

At initial conditions, the reservoir fluid is saturated. The bubble point pressure can be measured in the reservoir at the depth of the GOC.

As pressure falls, energy is created both by liberation of dissolved gas and  by expansion of the gas cap.
 The driving action of the gas cap pushes oil down structure ahead of the expanding gas cap.
 Fluid contacts must be closely monitored to avoid invasion of the oil column by the gas.
Reservoir pressure drops relatively slowly because of the compressibility of the gas. GOR will only increase with breakthrough of gas in production  wells.

Reservoirs tend to flow longer at more stable pressures. Oil rates are sustained by the slow driving force of the expanding gas.

Solution Gas Drive: Reservoir Energy

At initial conditions, a pure solution gas drive reservoir has no gas cap nor an aquifer. The main source of energy is the liberation of solution gas under the effect of pressure reduction. The expansion of the gas pushes the oil downward and radially into the wells. Other sources of energy (rock, oil and connate water compressibility) are usually minor. Liberated dissolved gas may segregate from the oil phase under gravity forces, and form a secondary gas cap.

Production Characteristics:

The initial GOR is at or below the solution GOR.  The oil rate drops rapidly as gas starts to breakthrough in the reservoir and forms a continuous phase. The pressure decreases rapidly until total virtual depletion.
When pressure becomes very low, Bg approaches 1 and surface gas rates decrease correspondingly. This causes a sharp reduction in the GOR.

Secondary Gas Cap:

Liberated solution gas may be pulled into a producing well at high oil production rates. Liberated solution gas may also move up structure and form  a secondary gas cap in cases of:

·        low oil producing rates.

·         low oil density.

·         high reservoir dip angle.

·         high permeability.

Summary

·        The reservoir pressure declines rapidly and continuously, then flattens out.

·        The surface GOR is initially low, then rises to a maximum and drops from then on.

·        There is no water production unless an aquifer is present.

·        The well requires artificial lift at an early stage.

·        The expected primary oil recovery is in the range of 5% to 30% of the oil.

Water Drive

Reservoir pressure declines slowly. In artesian drives, the pressure remains constant and equal to the hydrostatic value. Gas-oil ratio is limited because the pressure stays high. Oil rate declines slowly. Water-cut increases gradually, and can be as high as 90% or more in the mature life of the reservoir.

Gravity Drainage

Gravitational segregation is the tendency of fluids in reservoir to segregate, under the influence of gravity, to vertically position themselves as dictated by their respective densities (gas above oil, oil above water). Gravity drainage may occur in any type of reservoir. Gravity drainage is particularly important in solution-gas and gas-cap drive oil reservoirs.

 Combination Drive

Most reservoirs do not produce from a single source of energy and by a single mechanism. The ‘combination drive’ is the most frequent occurrence. In a combination drive, both a gas cap and an aquifer are present. Energy available includes influx from the aquifer, gas cap expansion, oil expansion and even evolution of the solution gas. Rock decompression may also play a role.

keywords: gas drive, water drive, types.

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