Downhole pressure is one of the most important and least understood aspects of HDD. Comprehending its effects and how to manage them can mean the difference between a job that makes money and one that doesn’t. Thus it’s essential for HDD contractors to know how downhole pressure works on job sites.
What is Downhole Pressure?
Bottomhole pressure (BHP) (or downhole pressure) is the pressure measured at the bottom of the hole. It’s the sum of the various pressures acting downhole or at the drilled hole’s bottom.
It’s commonly measured in pounds per square inch (psi), and there are three components to downhole pressure: hydrostatic, lithostatic and bottom-hole.
Hydrostatic Pressure
Hydrostatic pressure, also referred to as formation pressure, is the weight of water above a wellbore. Water can have different densities, which means that it can exert different pressures depending on what depth it’s at.
This fluctuation in water density makes hydrostatic pressure very difficult to predict during hdd drilling operations. It’s more complicated than that, too. Water has a relatively low density at the surface, but it rapidly gains weight with depth, which means hydrostatic pressure can change dramatically within a very small range of depth.
Lithostatic Pressure
Lithostatic pressure is simply lithostatic (rock-compressing) pressure acting on any given depth. This type of downhole pressure comes from the weight of the rock above a wellbore. As you might expect, lithostatic pressure increases as you go deeper into the ground, and it becomes significantly higher than hydrostatic pressure at depths below 1,500 feet or so.
Bottom-Hole Pressure
Bottom-hole pressure is essentially downhole pressure at the bottom of a borehole. The deeper you go, the greater the bottom-hole pressure will be. It’s typically measured in psi.
How Downhole Pressure Affects Productivity and Profitability on HDD Jobsites
Understanding the effects of downhole pressure is crucial for understanding how HDD works and how to get the most out of it. These effects primarily affect the productivity and profitability of HDD jobsites, so it’s important for contractors to know what they mean and what to do about them:
- Coriolis force affects drill pipe rotation.
- Temperature increases can cause tool string damage.
- High-pressure fluid events can cause fluid loss.
Downhole pressure affects more than just your equipment, too. It also has implications for the following on HDD job sites:
- Safety
- Logistics
- The environment
Coriolis Force Affects Drill Pipe Rotation
Coriolis force is the apparent deflection of moving objects in a rotating frame of reference.
Example: The Coriolis Effect
Imagine you’re standing on top of an escalator. If you hold out your left arm, you’ll feel like it’s pointing to the right, which means that if your arm is actually straight, it will appear to be crooked. This same principle applies to drill pipe rotation in HDD locations.
Coriolis force affects drill pipe rotation in HDD locations because rotary steerable systems (RSS) rotate around a vertical axis. If the top of your drill pipe doesn’t rotate in sync with the RSS, you’ll experience an apparent deflection of the drill pipe when you try to steer.
How Drill Pipe Rotation Can Be Affected by Downhole Pressure
This process is complicated, but it’s crucial that you understand it because doing so can help you minimize the negative effects of Coriolis force on drill pipe rotation during HDD jobs.
To start, the greater the differential pressure (DP) across your drill string, the more likely it is that you’ll experience poor or unwanted drill pipe rotation. Remember, bottom-hole pressure determines DP, and if that’s high enough to deflect your drill pipe when you’re steering, you could face difficulties.
Temperature Increases Can Cause Tool String Damage
Tool string damage is another one of the most significant effects of downhole pressure on HDD jobs. It’s important to know that temperature increases are directly correlated with downhole pressure increases in HDD locations. You’ll see more pronounced effects at greater depths, but even shallow job sites can see a rise in tool string temperature if downhole pressure is high enough.
When you’re going deeper into the ground, the increase in temperature becomes even more pronounced because heat transfer increases with depth. At depths below 1,500 feet or so, it’s not uncommon for tool string temperatures to reach 350-400 degrees Fahrenheit.
If you’re conducting hdd drilling job at these depths, it’s crucial to know what the expected temperature increases will be so you can plan for them.
If your tool string temperatures are approaching the recommended maximum for your operation, you may find it necessary to submit a high-pressure fluid event report.
High-Pressure Fluid Events Can Cause Fluid Loss
When conducting an HDD job, the fluids that help lubricate and cool down your tool string can also cause problems if they enter the well. These fluids can be classified as either a high-pressure fluid event (HPFE) or a loss circulation material (LCM).
The difference between the two is that HPFEs impede downhole drilling progress while LCMs simply affect your ability to complete the job. An example of an HPFE is when formation fluid enters into the wellbore (whether that’s during the drilling stage or when you’re tripping in after the fact). The loss of return circulation is an example of LCM.
If you experience either one of these, it will affect your productivity and profitability on the HDD job site. For example, if formation fluid enters into the wellbore when you’re drilling, your tripping speed will be slower than usual, which will reduce the number of hours you can use to complete the job.
A loss of return circulation during the tripping operation could cause you to lose all fluid returns while tripping in. This, too, would affect productivity and profitability because it may force you to wait until the next available mud pit to complete the trip if you want to avoid an LCM on your way out.
