Oil Material Balance

Introduction

The generalized oil material balance equation (MBE) is shown below:

+ + = + + +

Where:

B =

C =

D =

E =

F =

G =

The table below provides some insight on how to use the general MBE

Initial Condition	Detail	MBE Reduction/Form
Pi > Pb	P_i > P_b (No Gas Cap) No W_p or W_e	A = D
Pi > Pb	P_r < P_b (Gas Cap Formed) No W_p or W_e	A + B = D + F
Pi = Pb	P_i = P_b + Initial Gas Cap No W_p or W_e	A + B = D + E + F
Pi < Pb	Pi < Pb + Initial Gas Cap	A + B = D + E + F
Pi < Pb	Pi < Pb + Initial Gas Cap Both W_p or W_e	A+ B + C = D + E + F + G
Pi < Pb	Pi < Pb + Initial Gas Cap Both W_p or W_e	A+ B + C = D + E + F + G Modify: Rp = Gp/Np Gp = Gp - Ginj

For further information on the derivation of the MBE, follow the steps sequentially presented below:

Bubble Point

Bubble Point (Pb) is a a key parameter that distinguishes between saturated and understaturated conditions as described above.

Solution Gas Ratio

Different crudes and oils have different amounts of gas dissolved within them. For instance, a heavy oil can have less dissolved gas, when compared to a light oil. The amount of gas dissolved in the oil depends on the pressure, temperature, and composition of the reservoir fluid.

Crude	Oil API Gravity	Oil FVF	Solution Gas Oil Ratio
Light Oil	High (40 to 55)	Large (1.8 to 2.2)	Large (300 to 400)
Heavy Oil	Low (10 to 25)	Small (1.0 to 1.4)	Small (0 to 100)

To relate this volume of gas to the stock tank of oil, a “Solution-Gas-Ratio” is used. It is shown below:

One should always be aware of the temperature and pressure condition at which Rs is quoted.

References:

Northern Alberta Institute of Technology, Course Notes, PETE 2320, 2020.