What term describes the comparison of a fluid's ability to move through another fluid or to displace it?

What term describes the comparison of a fluid's ability to move through another fluid or to displace it?

• A. Mobility Ratio
• B. Viscosity Contrast
• C. Capillary Number
• D. Reynolds Number

Answer: (A)

Mobility ratio is the concept at play here. It describes how easily one fluid can move through another in a porous medium by comparing their mobilities. A fluid’s mobility is defined as lambda = k/μ, where k is permeability and μ is viscosity. The mobility ratio then compares the displacing fluid’s mobility to the displaced fluid’s mobility: M = (k/μ_displacing) / (k/μ_displaced) = μ_displaced / μ_displacing. This ratio tells you how stable the displacement will be. If the displacing fluid has much higher mobility than the fluid it's displacing (a large M), the front tends to become unstable and viscous fingering develops, wasting sweep efficiency. If the mobilities are similar (M around 1) or the displacing fluid has lower mobility (M < 1, a favorable scenario), the front remains more uniform, improving displacement efficiency. The term that captures this comparison is mobility ratio, as it directly links how easily each fluid moves through the same medium. The other terms describe different concepts: capillary number balances viscous and capillary forces, Reynolds number compares inertial to viscous forces, and viscosity contrast notes a difference in viscosities but not how permeability interacts with flow to influence displacement stability.

Mobility ratio is the concept at play here. It describes how easily one fluid can move through another in a porous medium by comparing their mobilities. A fluid’s mobility is defined as lambda = k/μ, where k is permeability and μ is viscosity. The mobility ratio then compares the displacing fluid’s mobility to the displaced fluid’s mobility: M = (k/μ_displacing) / (k/μ_displaced) = μ_displaced / μ_displacing.

This ratio tells you how stable the displacement will be. If the displacing fluid has much higher mobility than the fluid it's displacing (a large M), the front tends to become unstable and viscous fingering develops, wasting sweep efficiency. If the mobilities are similar (M around 1) or the displacing fluid has lower mobility (M < 1, a favorable scenario), the front remains more uniform, improving displacement efficiency. The term that captures this comparison is mobility ratio, as it directly links how easily each fluid moves through the same medium.

The other terms describe different concepts: capillary number balances viscous and capillary forces, Reynolds number compares inertial to viscous forces, and viscosity contrast notes a difference in viscosities but not how permeability interacts with flow to influence displacement stability.