Material Balance Equation in Reservoir Engineering

Material balance is a mathematic way to express mass conservation in a reservoir and a simple key principle is “what reservoir is produced must be replaced by other mass.

Volume Produced = Volume Replaced

Volume Produced comes from Gas Production, Oil Production, and Water Production.

Volume Replaced comes from volume expansion, water in flux and water/gas injection.

Figure 1 shows the relationship of the material balance.


Figure 1 – Concept of Material Balance

Let’s take a look at each component of equation. Continue reading

Gas Cap Drive Mechanism

Some reservoirs have a gas cap, which provides energy from gas expansion to help production from a wellbore; therefore, this is called “gas cap drive”. When oil is being produced, the gas cap expands and pushes oil downwards to a producing well (Figure 1).


Figure 1 – Gas Cap Drive

For this type of drive mechanism, it is imperative to keep gas within a reservoir as long as possible since it is an excellent energy source of the reservoir. Wells which are drilled into a high structure area where the gas cap is located must be closely monitored because this well will have more of a chance to produce gas. Continue reading

Solution Gas Drive Mechanism

Solution gas drive is a mechanism by which dissolved gas in a reservoir will expand and become an energy support to produce reservoir fluid. Solution gas drive has other name, such as dissolved gas drive or depletion drive.

When reservoir pressure is more than the bubble point, no free gas presents in a reservoir and this is called “under saturated reservoir.” At this stage, the drive comes from oil and connate water expansion and the compaction of reservoir pore space. Because compressibility of oil and rock is very low, only a small amount of fluid can be produced and typically the volume is around 1-2% of oil in place.

When reservoir pressure reaches a bubble point, oil becomes saturated and free gas will present in a reservoir. The expansion of gas is a main energy to produce reservoir fluid for the solution gas drive. At the beginning, the produced gas oil ratio will be slightly decline because free gas in a reservoir cannot move until it goes over the critical gas saturation. Then gas will begin to flow into a well. In some cases, where vertical permeability is high, gas may migrate up and become a secondary gas cap, which helps oil production. Continue reading

Three Types of Reservoir Recovery

Reservoir drive mechanism is the manner in which various energy sources in a reservoir  provide energy to flow fluids in reservoir to surface. Recovery of reservoir fluid is categorized into three categories (primary, secondary, and tertiary recover).

Primary Recovery

This is the first mechanism which is carried out by natural energy in a reservoir.


Figure 1 – Primary Recovery Continue reading

Effective and Relative Permeability

When there is only one type of fluid flowing through porous media, the permeability for this case is called “absolute permeability.” However, when there is more than one type of fluids present in a rock, a permeability of each fluid to flow is decreased because another fluid will be moving in the rock as well.  A new term of permeability called “effective permeability” is a permeability of a rock to a particular fluid when more than one type of fluid is in a rock.

Reservoir consists of three fluids (gas, oil, and water) so these are commonly used abbreviations for effective permeability for each fluid.

kg = effective permeability to gas

ko = effective permeability to oil

kw = effective permeability to water

Normally, it is common to state effective permeability as a function of a rock’s absolute permeability. Relative permeability is defined as a ration of effective permeability to an absolute permeability of rock. The relative permeability is widely used in reservoir engineering. These functions below are the relative permeability of gas, oil, and water.

Relative permeability to gas – krg = kg÷k

Relative permeability to oil – kro = ko÷k

Relative permeability to water – krw = kw÷k


k = absolute permeability

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