Category Archives: Material Balance

Drive Mechanisms

Primary production of hydrocarbons from petroleum reservoirs are due to three forces . These are typically referred to as drive mechanisms. They are depletion drive, segregation drive, and water drive.

Depletion drive (DDI) is the volumetric expansion of the oil. This drive harnesses the energy of the oil that has been compressed due to the high initial reservoir pressure.

Segregation (gas cap) drive (SDI) is the volumetric expansion of the gas. Like depletion drive, it harnessed the energy of the compressed gas. As gas is much more compressible the oil, this drive often has a greater impact. Segregation drive includes both to the expansion of the original gas in place and the evolution of gas from the oil as the pressure declines.

Water drive (WDI) is the bulk inflow of water from outside the boundaries of the reservoir, typically from an adjacent aquifer.

Not all reservoirs experience all three types of drive. Water drive applies only to reservoirs with an attached aquifer of sufficient magnitude. Depletion drive does not apply to single-phase gas reservoirs. Segregation drive applies to all reservoir types.

The relative magnitude of each drive on the overall production can be measured using the material balance equation rearranged in the form of a drive index as shown below. It can be summarized as DDI + SDI + WDI = 1. Using this method, the contribution of each drive to a field can be quantified and used for selecting appropriate strategies to increase production.

Eq 3.11

Calculating Gas in Place Using the Material Balance

The material balance can be simplified depending on what type of reservoir we are interested in analyzing. The single-phase gas reservoir is the most simple. We’ll cover calculating gas in place using the material balance. The material balance equation can be simplified, as all the terms for oil production, liquid expansion and rock expansion can be neglected. See Chapter 3 of Applied Petroleum Reservoir Engineering Third Edition for the derivation. See Material Balance for Nomenclature.

Simplified Gas Reservoir Material BalanceSingle-phase gas reservoirs have two possible drive mechanisms: segregation (gas cap) drive and water drive. Segregation drive will occur in all reservoirs, while water drive will only occur in select reservoirs. We’ll cover segregation drive.

Segregation Drive

Under segregation drive, the equation can be simplified even further, first by neglecting water influx and water production and then by substituting in expressions for the gas formation volume factor.

Depletion Drive Volumetric Reservoir Material Balance EquationStraight-line Depletion Drive Material Balance Equation

As pi, zi and G are constants for a given reservoir, a plot of early production data for p/z versus Gp gives us a straight line. The initial gas in place (G) is the intersection with the x axis. Please note that neglecting to correct the pressure term by the gas compressibility factor results in an incorrect extrapolation. In the same manner, a reservoir experiencing water drive will have a slower decline, as the water influx assists in stabilizing the pressure. Using this method for reservoirs with a water drive will also result in an incorrect extrapolation.

Comparison of theoretical vales of p/z plotted versus cumulative production from a volumetric gas reservoir

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

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.