Cobble and gravel are undoubedtly the worst soil conditions to have to drill through, be it horizontal directional drilling (HDD) or vertical drilling. When soil conditions go beyond the capabilities of conventional drilling fluids and additives, mixed metal oxide (MMO) drilling fluids can get the job done.
The arduous task of maintaining borehole stability, suspending, and transporting drill cuttings out of the borehole in such soil conditions can challenge even the most seasoned drilling contractors. Many contractors refuse to even bid on projects in these conditions.
Fluids such, as water or oil, are Newtonian because their viscosity or thickness are not affected by shear, but changes with temperature. As the temperature increases in a Newtonian fluid, the viscosity or thickness decreases. Bentonite drilling fluids are non-Newtonian fluid because viscosity or thickness of the fluid changes with shear. The faster a bentonite drilling fluid moves, the thinner the fluid becomes making it easier to pump.
When the fluid is at rest it forms gels that keep cuttings in suspension. This is made possible because of an imbalance of negative (anionic) charge within microscopic bentonite platelets. Gel strength is a measure of the ability of a bentonite drilling fluid to form gels that enable drilling fluid to suspend and transport drill cutting out of a borehole.
One of the easiest ways for HDD contractors to determine if they have adequate gel strength is to take a clear container (one can cut the top of a water bottle), partially fill with the fluid they are drilling with, and go where the entry or exit pit was dug and grab a sample of the soil they are drilling through. Dump some of the soil into the fluid, stir and let it set for 15 minutes. If the material settles to the bottom, that is a warning sign of inadequate gel strength and problems ahead if not corrected. There are two ways to increase the gel strength of a drilling fluid. The first is to simply add more bentonite, which dramatically raises the viscosity (the resistance to flow) of the drilling fluid, resulting in increased annular pressure that can cause problems such as inadvertent returns (frac-outs) and/or raised roadways. A typical alternative to adding more bentonite is using xanthan gum that can increase gel strength with a minimal increase in viscosity.
Like xanthan gum, mix metal oxide (MMO) additives are designed to increase gel strength without paying a penalty of higher viscosities and higher annular pressures, but that is where the similarities end. MMO drilling fluids take non-Newtonian principles to an extreme because they can be pumped and flow as easy as water yet suspend solids that conventional bentonite drilling fluids are not capable of suspending. A MMO fluid will look like pudding in a mix tank, but flow with pumping pressures that are around half of what a conventional bentonite drilling fluid would be.
MMO drilling fluids work off an imbalance of positive (cationic) charges and require specific mixing instructions that must be carefully followed. Care must be taken not to use additives such as synthetic polymers used for inhibiting clays, or polyanionic cellulose (PAC) polymers that are used for fluid-loss control. These polymers are negative (anionic) charge and will cancel out the benefits of a MMO fluid. MMO drilling fluids move through the bore in a plug flow as opposed to conventional drilling fluids that move in a laminar flow, which enhances the solids carrying capacity of the fluid and helps in controlling fluid loss into the formation.
As previously noted, a mix tank of MMO drilling fluid will have the physical appearance like pudding and first-time users will tend to want to thin the fluid back with water, but that would be a mistake. Appearance is deceiving with a MMO drilling fluid, which will provide superior gel strength and lower annular pressures.
Conventional means of testing viscosity (the Marsh Funnel and Cup) will not work on MMO drilling fluids because of the unique flow characteristics. One of the best tools to determine if a MMO drilling fluid is up to the job is simply taking a cup of the fluid and setting a marble on the surface. If the marble, which has a density similar to rock, stays on the surface, the fluid is ready for drilling. MMO drilling fluids also work great with drilling fluids recycling systems. One can take a large screen strainer and dump MMO returns into it, and it will stay in the screen without leaking (good fluid loss control for borehole stability). With just a shake of the screen strainer, the MMO drilling fluid will quickly fall out leaving just drill cuttings in the strainer.
It is important that contractors using MMO drilling fluids follow the manufacturers mixing instructions which may include using a specific grade of bentonite, more soda ash than normally used in conventional drilling fluids, and additional mixing time. When utilizing a drilling fluids recycling system, it may be a good idea to have an additional mixing system on site so that additional MMO drilling fluid can have time to mix and be available to add to the active system when needed.
Extreme soil conditions call for extreme measures to ensure success. MMO drilling fluids are not designed for everyday drilling applications, but will provide the borehole stability, and suspension in gravel and cobble that no other drilling fluid can provide. MMO drilling fluids will increase drilling efficiency for soil conditions that are beyond the capabilities of conventional drilling fluids.