Oil RESERVOIR

There are basically six driving mechanisms that provide the natural energy necessary for oil recovery: • Rock and liquid expansion drive • Depletion drive • Gas cap drive • Water drive • Gravity drainage drive • Combination drive

The Depletion Drive Mechanism

This driving form may also be referred to by the following various terms: • Solution gas drive • Dissolved gas drive • Internal gas drive

In this type of reservoir, the principal source of energy is a result of gas liberation from the crude oil and the subsequent expansion of the solution gas as the reservoir pressure is reduced. As pressure falls below the bubble-point pressure, gas bubbles are liberated within the micro- scopic pore spaces. These bubbles expand and force the crude oil out of the pore space as shown conceptually in Figure below.

The Depletion Drive Mechanism

Solution Gas Drive Reservoir

OIL

OIL + GAS

Water Drive Oil Reservoir

WATER DRIVE RESERVOIR

Gas Cap Drive Reservoir

Gas-cap-drive reservoirs can be identified by the presence of a gas cap with little or no water drive as shown in Figure below. Due to the ability of the gas cap to expand, these reservoirs are charac- terized by a slow decline in the reservoir pressure.

Combination Drive Reservoir

Gas Cap Drive Reservoir

The Gravity-Drainage-Drive Mechanism

The mechanism of gravity drainage occurs in petroleum reservoirs as a result of differences in

densities of the reservoir fluids.

Factors that affect ultimate recovery from gravity-drainage reservoirs are: • Permeability in the direction of dip • Dip of the reservoir • Reservoir producing rates • Oil viscosity • Relative permeability characteristics

Gravity Drainage Reservoir

Gravity Drainage Reservoir

Injection Well Production

Well

Petroleum reservoirs are broadly classified as oil or gas reservoirs. These broad classifications are further subdivided depending on:

• The composition of the reservoir hydrocarbon mixture • Initial reservoir pressure and temperature • Pressure and temperature of the surface production

CLASSIFICATION OF RESERVOIRS AND RESERVOIR FLUIDS

Pressure-Temperature Diagram

Figure 1-1 shows a typical pressure-temperature diagram of a multi- component system with a specific overall composition. Although a dif- ferent hydrocarbon system would have a different phase diagram, the general configuration is similar.

Typical P-T Diagram of Hydrocarbon Fluids

Oil Reservoirs Depending upon initial reservoir pressure pi, oil reservoirs can be sub- classified into the following categories: 1. Undersaturated oil reservoir. If the initial reservoir pressure pi (as represented by

point 1 on Figure 1-1), is greater than the bubble-point pressure pb of the reservoir fluid, the reservoir is labeled an undersatu- rated oil reservoir.

2. Saturated oil reservoir. When the initial reservoir pressure is equal to the bubble-point pressure of the reservoir fluid, as shown on Figure 1-1 by point 2, the reservoir is called a saturated oil reservoir.

3. Gas-cap reservoir. If the initial reservoir pressure is below the bubble- point pressure of the reservoir fluid, as indicated by point 3 on Figure 1-1, the reservoir is termed a gas-cap or two-phase reservoir, in which the gas or vapor phase is underlain by an oil phase. The appropriate quality line gives the ratio of the gas-cap volume to reservoir oil volume.

Gas Reservoirs In general, if the reservoir temperature is above the critical tempera- ture of the hydrocarbon system, the reservoir is classified as a natural gas reservoir. On the basis of their phase diagrams and the prevailing reservoir conditions, natural gases can be classified into four categories: • Retrograde gas-condensate • Near-critical gas-condensate • Wet gas • Dry gas

Initial Conditions Development Conditions

1. Development of a heavy oil Field. 2. How to expoit a light oil Field. 3. How to exploite Deep Oil Reservoir. 4. Plan of Development of Oil Reservoir Field with Strong Water Drive.

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