Something I always try to remember when on a job site is that one cannot manage what one cannot measure. There is a reason maxi rig owners pay thousands to make pressure subs part of their bottom-hole assembly or pay hundreds in additional daily rates to steering tool service providers for annular fluid pressure data. For all HDD rig operators, from maxi rigs to mini rigs, knowing down hole annular fluid pressure is valuable information and is more often becoming a requirement. A downhole pressure sensor is another set of eyes providing insight not otherwise obtained and is now available to walk-over HDD locators.
During the HDD process drilling fluids circulate through the bore hole to:
- Clean the hole by suspending and transporting cuttings (gel strength). The liquified conveyor belt cannot stop.
- Hold the hole open via a filter cake and positive hydrostatic pressure. Keeping the drilling fluid in the hole is paramount to removing cuttings.
- Keep the downhole electronics cool. HDD transmitters are not cheap.
- Lubricate the hole, drill string and product pipe to reduce wear and torque requirements during the pilot shot and back ream.
Contractors need to be mindful of drilling fluid viscosity. Viscosity is a fluid’s resistance to flow.
Remember the old HDD adage – no flow, no go!
The higher the mud’s viscosity the higher the pump pressure which equates to higher downhole pressure. The pressurized fluid is contained by a thin-walled filter cake which holds the hole open. Limits must be placed on maximum fluid pressures in the annulus to prevent inadvertent releases (IRs). However, it is equally important that fluid pressures remain high to maintain borehole stability. The HDD industry is negatively impacted by IRs in multiply ways.
- Inadvertently disrupting the ground surface has social costs damaging HDD’s image.
- Increase in project costs.
- Opportunity cost associated with a project’s delayed completion.
Changes in annular fluid pressure indicate that conditions have changed downhole. A drop in downhole annular fluid pressure could mean:
- An Inadvertent Release (IR) has happened, not all IRs come to the surface. Pressurized fluid takes the path of least resistance. Particularly costly and dangerous when drilling near bodies of water.
- A cross bore has taken place, i.e., drilled into a sewer line.
- The fluid jets in the downhole tooling are plugged.
- A void or a notable change in soil type so the mud mix should be reformulated.
- Reduced fluid viscosity or low fluid volume compared to penetration rates.
- An increase in downhole fluid pressure could mean:
- The drilled tunnel has collapsed, so fluid and cuttings are no longer flowing out of the hole.
- Drilling fluid is too viscous, mud weights need to come down via mud dilution or better mud cleaning.
- Current mud mix is not adequately removing the cuttings.
- Oversteering, often seen when the drill head is steered up for the exit.
A gradual increase in mud pressure is expected as the pilot hole gets deeper and longer. Head pressure of clear water in a cylinder is approximately 0.5 psi for every foot of depth. Drilling fluids will yield higher pressures. Higher pressures will be needed to remove cutting as the pilot bore advances. Experienced users of the DCI’s Fluid Pressure Transmitter have a general idea what normal annular fluid pressures should be in each soil type, at given depths & distance, and when using a particular mud mix. It is common for drill rig operators to swab (retract and redrill) the last few drill rods to clean the tunnel invert as fluid pressures slowly climb. Clean the hole by removing the buildup of fines and watch fluid pressure come down.
The ultimate benefit of knowing drilling fluid pressure is lower cost per foot drilled. Knowing fluid pressure allows for faster completion with fewer problems.
- Reduced Costs / Higher Profit / Value Add
- Empirical data
- Improved mud program
- Soil identification
- Risk reduction
- Cross-bore identification
Mark Gallucci is a Customer Insight Manager