Why does increasing the imposed wellhead pressure typically result in a proportionally larger increase in bottomhole pressure?

Why does increasing the imposed wellhead pressure typically result in a proportionally larger increase in bottomhole pressure?

• A. Because gas would be re-dissolved, increasing the density of the fluid in the wellbore.
• B. Because the fluid temperature increases with pressure.
• C. Because the viscosity decreases with pressure.
• D. Because the hydrostatic head is independent of pressure.

Answer: (A)

The main idea is that bottomhole pressure is driven by the hydrostatic weight of the fluid column, which depends on the fluid’s density. When you raise the wellhead pressure, gas dissolves into the drilling fluid more readily. This increases the fluid’s density along the column. Since hydrostatic pressure equals density times gravity times depth, a denser fluid column means a larger pressure increase at the bottom for the same depth. So the bottomhole pressure rises more than the surface (wellhead) pressure increase. The other ideas don’t explain this effect: temperature changes with pressure aren’t the primary driver here, viscosity changes don’t set the hydrostatic component, and the hydrostatic head isn’t independent of pressure—it changes with density, which is affected by dissolved gas.

The main idea is that bottomhole pressure is driven by the hydrostatic weight of the fluid column, which depends on the fluid’s density. When you raise the wellhead pressure, gas dissolves into the drilling fluid more readily. This increases the fluid’s density along the column. Since hydrostatic pressure equals density times gravity times depth, a denser fluid column means a larger pressure increase at the bottom for the same depth. So the bottomhole pressure rises more than the surface (wellhead) pressure increase. The other ideas don’t explain this effect: temperature changes with pressure aren’t the primary driver here, viscosity changes don’t set the hydrostatic component, and the hydrostatic head isn’t independent of pressure—it changes with density, which is affected by dissolved gas.