CLASSIFICATION OF RESERVOIR-AQUIFER SYSTEMS

Many gas and oil reservoirs are produced by a mechanism termed “water drive.” Often this is called natural water drive to distinguish it from artificial water drive that involves the injection of water into the formation. Hydrocarbon production from the reservoir and the subsequent pressure drop prompt a response from the aquifer to offset the pressure decline. This response comes in the form of a water influx, commonly called water encroachment, which is attributed to:

• Expansion of the water in the aquifer.
• Compressibility of the aquifer rock.
• Artesianan flow where the water-bearing formation outcrop is located structurally higher than the pay zone.

Reservoir-aquifer systems are commonly classified on the basis described in the following subsections.

Degree of pressure maintenance:

Based on the degree of reservoir pressuire maintenance provided by the aquifer, the natural water drive is often qualitatively described as:

1. The active water drive.
2. the partial water drive.
3. The limited water drive.

The term «active» water drive refers to the water encroachment mechanism in which the rate of water influw equals the reservoir total production rate. Active water drive reservoirs are typically characterized by a gradual and slow reservoir pressure decline. If during any long period the production rate and reservoir pressure remain reasonably constant, the reservoir voidage rate must be equal to the water influx rate.

Outer Boundary Conditions:

The aquifer can be classified as infinite or finite (bounded). Geologically all formations are finite but may act as infinite if the changes in the pressure at the oil-water contact are not «felt» at the aquifer boundary. Some aquifers outcrop and are infinite acting because of surface replenishment. By and large, the outer boundary governs the  behaviour of the aquifer and can be classified as follows:

• Infinite system indicates that the effect of the pressure changes at the oil/aquifer boundary can never be felt at the outer boundary. This boundary is for all intents and purposes at a constant pressure equal to initial reservoir pressure.
• Finite system indicates that the aquifer outer limit is affected by the influx into the oil zone and that the pressure at this outer limit changes with time.

Flow Regimes:

There are basically three flow regimes that influence the rate of water influx into the reservoir. These flow regimes are:

• Steady State.
• Semi (pseudo) Steady State.
• Unsteady State.

Flow geometries:

Reservoir-aquifer systems can be classified on the basis of flow geometry as:

1. Edge- weater drive
2. Bottom-water drive.
3. Linear-water drive.

RECOGNITIZION OF WATER INFLUX

Normally very little information is obtained during the exploration and development period of a reservoir concerning the presence or characteristics of an aquifer that could provide a source of water influx during the depletion period. Natural water drive may be assumed by analogy with nearby producing reservoirs, but early reservoir performance trends can provide clues. A comparatively low, and decreasing, rate of reservoir pressure decline with increasing cumulative withdrawals is indicative of fluid influx. Successive calculations of barrels withdrawn per psi change in reservoir pressure can supplement performance graphs.

However, if the reservoir limits have not been delineated by the developmental dry holes the influx could be from an undeveloped area of the reservoir not accounted for in averaging reservoir pressure. If the reservoir pressure is below the oil saturation pressure, a low rate of increase in produced GOR is also indicative of fluid influx. Early water production from edge wells is indicative of water encroachment. Such observations must be tempered by the possibility that the early water production is due to formation fractures, thin high-permeability streaks, or to coning in connection with a limited aquifer. The water production may be due to casing leaks.

Calculation of increasing original oil-in-place from successive reservoir pressure surveys by using the material balance and assuming no water influx is also indicative of fluid influx.