Category Archives: Petroleum Engineering

Solution Gas Drive Mechanism

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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

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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

Figure 1 – Primary Recovery Continue reading →

Effective and Relative Permeability

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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

Where;

k = absolute permeability

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Capillary Pressure

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Capillary pressure is a force due to differentials between fluid densities in a rock that can force pull hydrocarbon through the pores of a rock so a transition zone between fluids occurs.

Let’s make it simple. If we put a small tube in water overlaid by oil, water will rise up into the tube due to capillary pressure (Pc). For this situation, the capillary pressure (Pc) is the difference in pressure across the curved interface between the fluids (oil and water) shown in Figure 1.

Figure-1-Capillary-Diagram

Figure 1 – Capillary Diagram Continue reading →

Rock Wettability

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Wettability is a tendency of fluid to stick to the surface of formation when other types of fluid are present. Wettability of rock is measured by a core analysis in a laboratory and typically a laboratory measure contact angle between the fluid and the rock.

Wettability of rock is classified by the angle of contact, which is divided into 3 categories.

Water wet – contact angle (θ) is less than 90 degrees

Figure-1---Water-Wet

Figure 1 – Water Wet Continue reading →