Directional drilling has been around for some decades but it was only in the 1990s that directional drilling started being used for underground construction. Drilling underground can be complicated in several ways, and most importantly in maintaining an open borehole 'til pipe pullback.

This is where drilling fluid comes into play, and the quality of drilling mud can be the difference between the life and death of a trenchless project. (Read The Planning Process: How to Prepare for a Trenchless Project.) The role of drilling fluid has been viewed with mixed feelings when it comes to recognizing the important role it plays in trenchless construction.

Jim Watland, a veteran in horizontal directional drilling (HDD), says that it was only some years ago that he realized the importance of drilling fluid the hard way, after using mostly water and occasionally water mixed with polymer for drilling.

Drilling Fluid

Drilling fluid, also known as mud or slurry, is used in all drilling operations by default. It is a mix of fluids and solids in suspension that assist in drilling sub-surface boreholes. The mix is either water-based or oil-based, and is combined with appropriate additives conducive to geological and subsurface soil conditions.

Drilling fluid consists of water, bentonite, and appropriate additives to give the right mix.

The preferred pH for water is between 8.5 and 9.5; calcium content should be below 100 ppm. The deciding factors for the type of drilling fluid to be used are the formation through which the bore will pass, and environmentally-friendly fluid disposal post-use.

Additives in the drilling fluid are added to impart:

  • Filtration and suspension characteristics to the drilling fluid.
  • Prevent frac-out.
  • Prevent settlement or heaving of ground surface during drilling.
  • Prevent failure during pipe pullback.

Drilling Fluid Systems

A drilling fluid system is a complete unit that includes a mud mixing tank, mixing pump, mud hopper, agitator, storage and settlement tanks, and slurry lines. Proper mud mixing is the key to a successful drilling operation because it can be controlled to manage borehole problems. (Read Mixing the Mud: The Science of Drilling Fluid in HDD.)

A standard mixing system consists of a mixing hopper, an agitation pump and a pump to bring the fluid to the drill bit. Bentonite is introduced through the hopper into the water or mud where the agitation system keeps the constituents in suspension. Where large volume of fluid is required, reclaimers consisting of shakers, screens and cones are used. This removes the solids or cuttings from the fluid, leaving the bentonite and the additives in the solution ready to be re-circulated for further drilling.

The drilling fluid system is where important factors such as density, viscosity, gel strength, solid content, and fluid loss control are controlled and monitored. Every system is uniquely modified for the specific project to maintain correct formation pressure. With the need to follow environmental regulation for drilling fluid disposal, self-contained mud systems that clean and re-circulate drilling fluid through the borehole are preferred for small projects.

These units reduce the time and cost required to dispose drilling fluid to specific locations approved by the Environmental Protection Agency (EPA). Some units also have automated controls, full-fluid transportation, and leveling screens.

Importance of Drilling Fluid

Drilling fluid performs some important functions in a trenchless operation:

  • Cleans the borehole by transporting drill cuttings out of the borehole.
  • Balances formation pressure in the borehole and prevents damage to the formation.
  • Stabilizes borehole walls for casing to be set or pipe to be pulled back.
  • Cools and lubricates the drill bit and prevents cuttings from getting stuck in the drill bit.
  • Transmits hydraulic horsepower to the bit.
  • Keeps the cuttings in suspension in case of trips or when downhole assembly needs to be withdrawn for maintenance or change of drill bit.
  • Keeps the borehole from getting clogged.

Drilling fluid plays a crucial role right from the beginning to the finish of a trenchless project. Selecting the right drilling fluid is therefore necessary, and it is recommended that drilling fluid selection and management should start even before bidding on a job.

On some jobs, over half of the contractor’s expenses will be in the drilling fluid because of additional expenses associated with disposal. However; compromising on the quality or quantity of drilling fluid can also prove to be disastrous for the project.

Calculations for quantity of drilling fluid required for a particular project are available for contractors to estimate their drilling fluid requirements. Controlling costs by reducing the amount of drilling fluid or additives could mean insufficient fluid downhole, resulting in excessively thick mud which will be difficult to pump out of the exit or launch pit.

It can also cause inadvertent return, ground heave, and stuck or damaged downhole tools. (Read How can I identify an inadvertent return and how can I fix it?)