At the depth of the Earth’ crust with high pressure the rock and fluid contained in it being in a state of compaction. The reservoir contains such an enormous amount of compressive energy, which can be utilized to allow fluid to be flowed through pore space and produced from the reservoir into a well. The energy might be significant enough to allow fluid to be flowed to the surface and finally into treatment facilities on the ground.
The reservoir will response to the pressure depletion process, which occurs on production, in the change of its volume, fluid flow properties and in some cases its composition. The way 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 basic concept 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 phases present in the reservoir: a) If the reservoir is above the bubble point, then expansion of the oil in place; b) If the reservoir is below the bubble point then expansion of the co-existing oil and gas phases; c) Expansion of any overlying gas cap.
- Expansion of an underlying aquifer.
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 optimized to ensure maximum economic recovery.
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.
Solution Gas Drive
The reservoir contains oil initially above its bubble point pressure, as the production continues the removal from the reservoir of the produced oil will be compensated by an expansion of the oil left in place. This will lead to a depletion in pressure and eventually it 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 oil and gas phases within the reservoir. As gas come out more and more from solution its build up in the reservoir until its saturation high enough to allow it to be produced more easily and free gas will be produced in increasing quantities associated with the produced oil. We can observe the evident by the gas oil ratio (GOR) changing over time in this drive mechanism.
Gas Cap Expansion Drive
Very often we see the reservoir initially has a gas cap on the top of the oil. It occurs when reservoir initial pressure equal to or at some later stage falls to the bubble point pressure, the gas released from solution over time migrate to above the top of the oil and form the gas cap. In this case of drive mechanism, the production of fluids occurred primarily with gas expansion as it moved towards the lower pressure zone nearby the wellbore. To maximize the capacity of gas cap expansion, gas must be controlled from break through into the wellbore and retain its volume in the gas cap.
In a reservoir with a water drive mechanism, reservoir pressure can be maintained by either aquifer expansion or via injection of water into the reservoir. The water normally contained within an aquifer system can be defined as edge or bottom water drive depending upon the structural shape, dip angle and oil water contact (OWC) within the reservoir.
Produced oil from reservoir is compensated by expansion of the aquifer leads to a steady elevation in the oil water contact towards the wellbore, and this will affect the production zone within the reservoir, which is the perforated section of the wellbore. The rise of the OWC is not uniform due to heterogeneities of formation and the significant pressure drop around the wellbore. The phenomenon is referred to as coning and fingering, this could lead to premature arrival of water to the wellbore.
Although water drive is encountered as a naturally occurring drive mechanism in many fields that contain reservoir with aquifer, water drive supported by injection can also appear in an early stage of a field development. This extends the period of production above the bubble point, maximize rates and improves recovery by immiscible displacement (water and oil).
In gravity drive, the hydrostatic pressure of the oil column and gas cap can provide the energy for oil to flow into the wellbore. For such a system to be effective drive mechanism requires great structure dip, low oil viscosity, good vertical and horizontal permeability, preferably an active gas cap and negligible aquifer activity.
In general, the compaction drive reservoirs are rare. The oil within the reservoir pore space is compressed by the weight of overlying sediments and the pressure of the fluids they contain. If fluid is withdrawn from the reservoir, then it is possible that the pressure depletion in the pore space attributable to the production of fluid can be compensated by the overlying sediments compacting lower sediments. The impact of this is to create a reduction in porosity and thus a potential compression effect.
In the majority of reservoirs, the production of fluids is not controlled by one but often by several drive mechanisms in combination. In such situations the response of the reservoir to production is less predictable.
Recovery factor for oil reservoir varies in the range at different drive mechanism presented in table below. Oil recovery factor is the fraction of the volume of produced oil from the amount of volume of original oil in place (OOIP).