The Black-Oil Model: Bo and Rs

Two Numbers for the Oil

The black-oil model describes the reservoir fluid with two pressure-dependent properties. Rs, the solution gas-oil ratio, is the volume of gas dissolved in each stock-tank barrel of oil; Bo, the oil formation volume factor, is the reservoir volume occupied by that barrel. Together they convert between reservoir and surface volumes and carry the dissolved gas.

Above and Below the Bubble Point

The bubble point is the pressure at which gas first comes out of solution. Above it the oil is undersaturated: all gas stays dissolved, so Rs is constant and Bo decreases as rising pressure compresses the oil. Below it the oil is saturated: free gas evolves, Rs falls, and the de-gassed oil shrinks. So Bo is largest exactly at the bubble point.

Why It Matters

These curves drive the material balance. As a field depletes and pressure falls toward and below the bubble point, Bo and Rs change, gas comes out of solution, and the reservoir and surface volumes diverge. The simulator needs the full Bo and Rs tables to turn what flows in the reservoir into barrels at the surface.