Determine Compressibility of Gases

This article will demonstrate how to determine gas compressibility by using simplified equation of state.

At a low pressure condition, gas similarly behaves like ideal gas and the equation for the ideal gas law is listed below;

PV = nRT

Where;

P = pressure

V = gas volume

T = absolute temperature

n = number of moles of gas

R = gas constant

Gas constant (R) is different because it depends on the unit system used in the calculation. The R in the different unit system is demonstrated in the below table.

Table 1 – Gas Constant (R) at Different Unit

Volumetric Method To Estimate Volume In Place and Reserves

Volumetric is a method to estimate fluid in reservoir based on volume of pore space in a rock and water saturation.

Volume of Oil Initially In Place (OIIP)

To estimate oil initially volume in place, the following formula is a volumetric calculation for oil.

Where;

STOIIP = stock tank oil in place, stb

A= area, acre

h = reservoir thickness, ft

ɸ = rock porosity, %

Swc =connate water saturation, %

Boi = oil formation volume factor, rb/stb

Note: the stock tank condition is a standard surface condition of oil and gas at 60F and 14.7 psia. Continue reading

Material Balance for a Water Drive Mechanism

This article will demonstrate a material balance equation in a natural water drive mechanism.  A full material balance equation is shown below:

Figure 1 – Full Material Balance Equation

Assumptions

• Reservoir pressure above the bubble point (Pb). Above a bubble point, Rs = Rsi.
• No water injection
• No gas injection
• No gas cap
• Neglect formation and connate water compressibility (Cf and Cw have little effect for a gas cap drive mechanism.)

Figure 2 – Material Balance Equation with Assumption for a Natural Water Drive Mechanism

Material Balance for Gas Cap Drive Mechanism

Figure 1 – Full Material Balance Equation

Assumptions

• No water production
• No water injection
• No gas injection
• No water influx
• Neglect formation and connate water compressibility (Cf and Cw have little effect for a gas cap drive mechanism.)

Solution Gas Drive Mechanism Explained in Material Balance Equation

Material balance equation can be applied for any drive mechanism and this article demonstrates how to apply the material balance equation in a solution drive mechanism. For a solution drive mechanism, there are 2 cases. The first case is when reservoir pressure is above a bubble point and the second case is when reservoir pressure is below a bubble point.

Solution Gas above Bubble Point