Material Balance

As fluid is produced from the reservoir it doesn’t leave a void, something has to fill the space. Gas, oil, water and rock can all expand, to varying degrees, to fill up the space but it results in a decrease in the reservoir pressure. In addition, water can migrate into the reservoir area. The material balance equation was derived in order track the production of oil, gas and water; the expansion of existing fluid and rock; and the migration of water into the reservoir. The material balance provides reservoir engineers a great deal of insight in knowing the initial hydrocarbon in place, how much hydrocarbon can be produced at different pressures, the primary mechanism for reservoir production and the potential usefulness of varying enhanced recovery techniques. For a detailed derivation, please refer to Chapter 3 of Applied Petroleum Reservoir Engineering.

The material balance equation can be written as:

Oil Expansion + Gas Expansion + Formation and Water Expansion + Water Influx

=Oil and Gas Production + Water Production

General Material Balance Equationwhere

N   Initial reservoir oil, STB

Boi  Initial oil formation volume factor, bbl/STB

Np  Cumulative produced oil, STB

Bo  oil formation volume factor, bbl/STB

G   Initial reservoir gas, SCF

Bgi  Initial gas formation volume factor, bbl/SCF

Gf  Amount of free gas in the reservoir, SCF

Rsoi Initial solution gas-oil ratio, SCF/STB

Rp  Cumulative produced gas-oil ratio, SCF/STB

Rso Solution gas-oil ratio, SCF/STB

Bg  Gas formation volume factor, bbl/SCF

W   Initial reservoir water, bbl

Wp  Cumulative produced water, STB

Bw  Water formation volume factor, bbl/STB

We  Water influx into reservoir, bbl

cw   Water isothermal compressibility, psi–1

Swi  Initial water saturation

Vf   Initial pore volume, bbl

cf   Formation isothermal compressibility, psi–1

Terry, Ronald E., J. Brandon. Rogers, and B. C. Craft. Applied Petroleum Reservoir Engineering. Third ed. Massachusetts: Prentice Hall, 2014. Print.

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