Water Formation Volume Factor ()Water formation volume factor is defined as the ratio of the volume of water at reservoir (in-situ) conditions to that at stock tank (surface) conditions. It is occupied by 1 stock tank barrel (STB) of formation water plus its dissolved gas. It represents the change in volume of the formation water as it moves from reservoir conditions to surface conditions. The formula is as below.

=

Effect of Pressure on Formation Volume Factor of Water The graph below shows a typical plot of water formation volume factor versus the pressure. As the pressure is decreased to the bubblepoint (), the increases as the liquid expands. At pressures below the bubblepoint, gas is liberated. But in most cases, the still will increase because the shrinkage of the water resulting from gas liberation is insufficient to counterbalance the expansion of the liquid. This is the effect of the small solubility of natural gas in water.

Fig. 1: A graph of versus pressure

Measuring Water Formation Volume FactorThe most accurate source of is from the laboratory data. It can also be calculated from density correlations if the effects of solution of gas have been accounted properly. Eq. 1 is used to estimate if solution of gas is included in the laboratory measurement.

.....(1)

Where :Vrc=volume occupied by a unit mass of water at reservoir conditions (weight of gas dissolved in water at reservoir or standard conditions is negligible), ft3,Vsc=volume occupied by a unit mass of water at standard conditions, ft3,=density of water at standard conditions, lbm/ ft3,=density of water at reservoir conditions, lbm/ ft3.The density correlations and the methods of estimating and were described previously. The can be less than 1 if the increase in volume resulting from dissolved gas is not great enough to overcome the decrease in volume caused by increased pressure. The value of is seldom higher than 1.06.There are also another three effects involved in . Firstly is liberation of gas from water as pressure is reduced. Secondly, the water undergo expansion as the pressure is reduced and lastly, the shrinkage of water as the temperature is reduced.

ReferencesMcCain, W.D. Jr.: McCain, W.D. Jr. 1990.The Properties of Petroleum Fluids, second edition. Tulsa, Oklahoma: PennWell Books.McCain Jr., W.D. 1991. Reservoir-Fluid Property Correlations-State of the Art (includes associated papers 23583 and 23594).SPE Res Eng6(2): 266-272. SPE-18571-PA.http://dx.doi.org/10.2118/18571-PAhttp://www.fekete.com/SAN/TheoryAndEquations/WellTestTheoryEquations/Water_Formation_Volume_Factor.htmhttp://petrowiki.org/Produced_water_formation_volume_factor#Effect_of_pressure_on_formation_volume_factor_of_water

Chew, J., and Connally, C.A.: /1959/,"A Viscosity Correlation for Gas Saturated Crude Oils",Trans. AIME 216, 23 (1959).