Why does oil viscosity decrease with decreasing pressure above bubble point?

Why does oil viscosity decrease with decreasing pressure above bubble point?

• A. The oil molecules are not forced as they are at higher pressures
• B. The temperature decreases
• C. Gas content increases
• D. Viscosity is independent of pressure

Answer: (A)

Viscosity is how badly a liquid resists flow, which comes from how tightly its molecules are forced together and how strongly they interact. When pressure is high, the oil is more compressed, free volume is smaller, and molecules push against each other more, making it harder for layers to slide past one another. Lowering the pressure relieves that compression, frees up space, and lets the molecules move more easily, so the liquid flows more readily. Above the bubble point, the oil stays as one liquid phase with dissolved gas, so the main effect here is the reduction in forcing on the molecules as pressure drops. Temperature isn’t changing in this scenario, and viscosity does depend on pressure, so the explanation that the oil becomes less viscous because the molecules are no longer being forced as at higher pressure fits best.

Viscosity is how badly a liquid resists flow, which comes from how tightly its molecules are forced together and how strongly they interact. When pressure is high, the oil is more compressed, free volume is smaller, and molecules push against each other more, making it harder for layers to slide past one another. Lowering the pressure relieves that compression, frees up space, and lets the molecules move more easily, so the liquid flows more readily. Above the bubble point, the oil stays as one liquid phase with dissolved gas, so the main effect here is the reduction in forcing on the molecules as pressure drops. Temperature isn’t changing in this scenario, and viscosity does depend on pressure, so the explanation that the oil becomes less viscous because the molecules are no longer being forced as at higher pressure fits best.