Oil

Part 3, Part 3: The Fluids

Learning objectives

Read API gravity as the measure of how light an oil is, and its effect on the bulk modulus
Explain how dissolved gas, quantified by the gas-oil ratio, softens and lightens a live oil
Contrast dead-oil and live-oil moduli for the same API gravity at reservoir conditions
Place oil as a smear along the fluid ladder rather than a single point

Not One Substance

The middle rung of the ladder is the least honest word in the subject. Oil ranges from heavy tar barely distinguishable from the rock to a volatile fluid almost as light and soft as the gas dissolved in it. Two numbers organize the range. The first is API gravity, an inverse density scale: a higher API oil is lighter and less dense, and, as we will see, softer. The second is the amount of gas the oil carries in solution, and it matters even more than the first.

API and the Dead-Oil Modulus

Start with a dead oil, one that has released all its dissolved gas, so the gas-oil ratio is zero. A 40-API dead oil at the surface, 25 degrees Celsius, reads density about 0.822 g/cc, velocity 1337 m/s, and bulk modulus $K_{fl}$ fl near 1.47 GPa; carried to reservoir conditions at 75 degrees Celsius and 25 MPa it softens slightly to about 1.35 GPa. API gravity slides that value directly: at the same reservoir conditions a lighter 50-API dead oil reads about 1.18 GPa while a heavier 30-API oil reads about 1.56 GPa. Lighter oil is softer oil.

Live Oil: the Gas Dissolved In It

Now let the oil keep its gas. Dissolved gas, measured by the gas-oil ratio in liters of gas per liter of oil, both lightens and softens the liquid, and the effect is large. Charge that same 40-API oil to a gas-oil ratio of 100 at 75 degrees Celsius and 25 MPa and its density falls to about 0.69 g/cc and its bulk modulus to about 0.72 GPa, roughly half the dead-oil value. A live oil can slide down toward gas-like softness, which is exactly why oil is not one point on the ladder but a smear that runs from just under brine down toward the gas floor. Where a given oil lands depends on its API gravity and, more strongly, on how much gas it is carrying.

The reader who has been tracking the ladder can already see where this leads. The agent that softens a live oil is dissolved gas, and gas itself is the extreme rung, the lightest and softest and most pressure-sensitive fluid in the subsurface. It is the subject of the next section.

References

Mavko, G., Mukerji, T., & Dvorkin, J. (2009). The Rock Physics Handbook (2nd ed.). Cambridge University Press.
Batzle, M., & Wang, Z. (1992). Seismic properties of pore fluids. Geophysics, 57(11), 1396-1408.