# Quick Guide to Easily Understand the Behavior of Your Reservoir Fluids

Updated on October 2021

How would your PVT properties respond to the decrease in reservoir pressure during production? This post would help you describe your reservoir fluids behavior below and above the bubble point pressure.

Here is the behavior of six black oil properties as a response to the decrease in your reservoir pressure:

## Solution Gas-Oil Ratio

At pressures greater than bubble point pressure, the solution gas-oil ratio remains constant despite decreasing reservoir pressure. That’s because the oil phase holds the maximum amount of the solution gas.

When the pressure decreases below the bubble point pressure, the solution gas-oil ratio decreases rapidly. That’s because more gas is liberating from the liquid phase leaving the oil with less gas in solution.

## Oil Formation Volume Factor

At pressures greater than bubble point pressure, the oil formation volume factor increases gently as the reservoir pressure decreases. That behavior occurs because the oil phase, holding the maximum amount of gas in solution, expands gently due to the pressure decrease.

When the pressure decreases below the bubble point pressure, the oil formation volume factor decreases rapidly.​ That is because more gas is liberating from the liquid phase, making the oil to shrink rapidly.

## Oil Viscosity

At pressures greater than bubble point pressure, the oil viscosity decreases gently as the reservoir pressure decreases. That is because the oil-phase, holding the maximum amount of gas in solution, responds to the pressure decrease and becomes relatively less viscous.

When the pressure decreases below the bubble point pressure, the oil viscosity increases rapidly.​ That is because more gas is liberating from the liquid phase, making the oil much more viscous.

## Oil Density

At pressures greater than bubble point pressure, the oil density decreases gently as the reservoir pressure decreases. That behavior occurs because the oil phase, holding the maximum amount of gas in solution, respond to the pressure decrease and becomes relatively less dense.

When the pressure decreases below the bubble point pressure, the oil density increases rapidly.​ That is because more gas is liberating from the liquid phase, making the oil much denser.

## Oil Compressibility

At pressures greater than bubble point pressure, the oil compressibility increases gently as the reservoir pressure decreases. That is because the oil phase, holding the maximum amount of gas in solution, becomes relatively more compressible due to the decrease in pressure.

When the pressure decreases below the bubble point pressure, the oil compressibility increases rapidly.​ That is because more gas is liberating from the liquid phase, making the system much more compressible.

## Total Formation Volume Factor

At pressures greater than bubble point pressure, the total/two-phase formation volume factor increases gently as the reservoir pressure decreases. That behavior occurs because the oil phase, holding the maximum amount of gas in solution, expands gently due to the pressure decrease.

When the pressure decreases below the bubble point pressure, the total/two-phase formation volume factor increases rapidly.​ That’s because more gas is liberating from the liquid phase, making the system to expand rapidly.

## Conclusion

This table aims to summarize the behavior of your PVT properties as the pressure decreases in the reservoir.