A reservoir rock will produce fluid into the wellbore as a consequence of the fluid in the pore space which exists at high pressure and the rock being in a state of compaction. Thus the reservoir as such contains an enormous amount of compressive energy stored within the compressible hydrocarbon fluid which can be utilised to allow fluid to be produced from the reservoir into awell. Under natural flowing conditions the pressure is also significant enough to allow fluid to be flowed to surface and finally into treatment facilities.
The response of the reservoir to the pressure depletion process which occurs on production, will be dynamic and the fluid remaining in the reservoir will change both in terms of its volume, flow properties and in some cases its composition. Themanner in which the reservoir system responds to the depletion process will be naturally governed by the drive mechanism. The long term production capacity of the reservoir will be defined by the extent and rate of pressure depletion. The depletion effects can be offset to some extent by the injection of fluid back into the reservoir.
Once the reservoir delivers fluid to the wellbore, sufficientpressure energy needs to exist to lift the fluid to surface if the well is to operate under natural flow. In the event that insufficient energy exists to allow production to occur or to occur at an economic rate, the well may require assistance by the application of artificial lift to provide all or a portion of the vertical lift pressure losses.
1. RESERVOIR DEPLETION CONCEPTS
The basicconcept regarding the production of fluid from a reservoir is that for fluid to be produced as a result of its high pressure, then the reservoir system will deplete and must therefore compensate for the loss of the produced fluid by one or more of the following mechanisms:
• Compaction of the reservoir rock matrix
• Expansion of the connate water
• Expansion of hydrocarbon phasespresent in the reservoir:
o If the reservoir is above the bubble point, then expansion of the oil in place.
o If the reservoir is below the bubble point then expansion of the co-existing oil and gas phases
o Expansion of any overlying gas cap.
• Expansion of an underlying aquifer.
In most cases, as oil is produced, the system cannot maintain its pressure and theoverall pressure in the reservoir will decline.
The pressure stored in the reservoir in the form of compressed fluids and rock represents the significant natural energy available for the production of fluids and requires to be optimised to ensure maximum economic recovery.
The mechanism by which a reservoir produces fluid and compensates for the production is termed the reservoir drive mechanism.1.1 Reservoir Drive Mechanisms
The reservoir drive mechanism refers to the method by which the reservoir provides the energy for fluid production. There are a number of drive mechanisms and a reservoir may be under the influence of one or more of these mechanisms simultaneously.
1.1.1. Solution Gas Drive
If a reservoir contains oil initially above its bubble point then, as productioncontinues, the removal from the reservoir of the produced oil will be compensated for by an expansion of the oil left in place within the reservoir. This will by necessity lead to a reduction in pressure and eventually the pressure within the reservoir will drop below the bubble point. Gas will then come out of solution and any subsequent production of fluids will lead to an expansion of both the oiland gas phases within the reservoir (Figure 1).
The gas will come out of solution as dispersed bubbles throughout the reservoir wherever the pressure is below the bubble point but will be concentrated in areas of low pressure such as the near wellbore area around production wells. However, as discussed previously, the relative permeability to the gas will not be significant until the gas...