If you’re involved in drilling operations, you know how important it is to understand the concept of “bottom hole pressure.” This term is critical for drilling supervisors, drillers, drilling engineers, and all rig personnel, as it directly impacts well control. In fact, if the BHP drops below the formation pressure, it can cause a kick in the hole – which is why staying on top of this important metric is crucial. For this reason, you will find that all well control courses give big value for explaining the bottom hole pressure.
What is the Bottom hole Pressure?
The bottom hole pressure is the summation of all pressures that are exerted on the bottom of the hole. Let’s assume you are doing a flow check, the BHP will equal the hydraustatic pressure of the mud column. Now, let’s assume that we are drilling and the mud pump is running. The bottom hole pressure will increase as additional pressure is caused by the pump on the bottom of the well.
BHP & Formation Pressure
There are three situations related to BHP & formation pressure which are:
- Balanced Situation: This situation happens when the formation pressure is equal to the bottom hole pressure
- Overbalanced Situation: The bottom hole pressure is greater than the formation pressure, causing an overbalance. This is a common situation in drilling.
- Underbalanced Situation: When the Bottom hole pressure is less than the formation pressure. This is where you will suffer from a kick in the well.
Bottom Hole Pressure Formula During Static
BHP = Hydraustatic Pressure + Surface Pressure
We can normally calculate the bottom hole pressure from the drill pipe and the annulus side.
BHP = Hydrostatic Pressure in the DP + The surface pressure In the pipe
BHP = Hydrostatic Pressure in the annulus + the surface pressure in the annulus
BHP = 0.052 x TVD x Fluid Density + Surface Pressure
In some well control situations, we may have different densities of fluid columns in both pipe and annulus. So, It is better to calculate it from the side of the uniform fluid column.
Bottom Hole Pressure During Circulation
During the circulation, there will be a pressure loss in the annulus. This pressure loss comes mainly from the friction force that was produced due to the mud circulation. This force will generate a back pressure, which will be exerted on the bottom of the well. Accordingly, the BHP formula will be changed to be as follows:
BHP = Hydrostatic Pressure in the DP + Annular Pressure Loss
So once there is a need to increase mud pump SPM, the BHP will increase due to an increase in the friction force and APL. The figure below shows an example of how to calculate the BHP during circulation.
In Flowing Wells
When wells flow naturally through the tubing, the pressure at the bottom of the tubing can be calculated by adding the pressure at the casing head between the tubing and the casing pipe, and the pressure due to the weight of the column of gas. However, there is always a possibility of error caused by fluid being present in the annular space above the bottom of the tubing.
Risks Of High Bottom Hole Pressure
BHP is an important term that we must watch. Increasing it too much beyond formation pressure will cause problems such as lost circulation. Furthermore, It may exceed the formation fracture pressure, causing fractures and complete loss. It may also increase due to poor hole cleaning that will increase the amount of cuttings in the annulus.
In some cases, we may encounter a circulation loss problem that will cause a mud level drop in the annulus. This will cause a drop in the hydrostatic pressure and the BHP. In other situations, a sudden failure in the casing float valve will cause mud in the annulus to fill inside the casing. This will also cause a drop in the mud level in the annulus.
In any of the above situations, there will be a possibility that the BHP will be lower than the formation pressure, which will cause a kick.