Oil Formation Volume Factor (Bo)

Oil formation volume factor  is defined as the ratio of the volume of oil at reservoir (in-situ) conditions to that at stock tank (surface) conditions.  This factor, is used to convert the flow rate of oil (at stock tank conditions) to reservoir conditions.  It is defined as:

 

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In pressure transient analysis the flow rate used in the calculations is defined assuming reservoir conditions.  Since the oil flow rate is generally measured at the surface, in stock tank barrels, this rate must be converted to reservoir conditions by multiplying the surface rate by the oil formation volume factor.

 

Below the bubble point pressure, the oil formation volume factor increases with pressure.  This is because more gas goes into solution as the pressure is increased causing the oil to swell.  Above the bubble point pressure, the oil formation volume factor decreases as the pressure is increased, because there is no more gas available to go into solution and the oil is compressed.

 

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The value of the oil formation volume factor is generally between 1 and 2 Rbbl / stbbl (R m3 / st m3).  It is readily obtained from laboratory PVT measurements or may be calculated from correlations such as Vasquez and Beggs.  It is recommended that the value be evaluated at reservoir conditions at the average reservoir pressure at the time of the test.

 

Note that when calculating the oil formation volume factor from correlations, the solution gas oil ratio (Rs) has a significant effect on the value.