Total Oil FVF vs Single Phase Oil Phase

I found a great explanation about total oil formation volume factor or FVF (Bt or Bto) and single-phase oil FVF (Bo) in this article in Top Dog Engineer website.

When multiple phases exist, it becomes a little more difficult to characterize fluid production at the surface and keep track of the volumes. Luckily, petroleum engineers developed the two-phase oil formation volume factor to account for both oil and free gas. An illustration of the two phase formation volume factor is shown below:

The relations between Bto and Bo is as follows: Bto = Bo + Bg(Rsb-Rs), where Bg is single-phase gas FVF, Rs solution gas-oil ratio (SGOR), and Rsb solution gas-oil ratio at bubblepoint pressure.

The figure below shows how the two-phase oil formation volume factor compares to the oil formation volume factor as a function of pressure at constant reservoir temperature