Coarse gravel and cobble are some of the most challenging soil conditions to drill through for both vertical and horizontal directional drilling contractors. Unless fluid-loss is controlled, the formation is constantly falling in, and large heavy gravel and cobble are difficult to suspend and carry out in drilling fluids returns.

Conventional wisdom entails adding more bentonite, adding lots of PAC (poly anionic cellulose) polymer for fluid-loss control, and the addition of xanthan gum for gel strength. The end result is higher annular pressures, while still having to deal with borehole stability issues, and a fluid that is marginally capable of adequately removing the heavy solids. Mixed metal oxide (MMO) drilling fluids can be a solution to drilling in such conditions, but those who use it will have to learn a whole new approach to drilling fluids while setting aside conventional thinking and testing equipment.

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The MMO product is sold in a powdered form and is added to a bentonite drilling fluid after the bentonite has thoroughly mixed. Similar to the platelet structure of sodium Wyoming bentonite, MMO has a layered crystal structure that is ten times smaller than a bentonite platelet, which averages about one micron in diameter. Unlike bentonite platelets, MMO has a positive or cationic charge, which is why MMO drilling fluids can lose its unique qualities when additives such as PAC or synthetic polymers (that are negatively charged) are added.

The ideal drilling fluid for any drilling application provides all of the required functions, such as gel strength and fluid-loss/filtration control, at the lowest possible viscosity in order to reduce pump pressures and annular pressures, and a MMO drilling fluid takes this principle to the extreme.

Although a MMO drilling fluid looks extremely thick at rest, when under shear (in motion) it is far thinner than a conventional bentonite drilling fluid.

The ideal drilling fluid for any drilling application provides all of the required functions, such as gel strength and fluid-loss/filtration control, at the lowest possible viscosity in order to reduce pump pressures and annular pressures, and a MMO drilling fluid takes this principle to the extreme.

An impressive demonstration for MMO drilling fluids is to have a clear tube or cylinder of the fluid, along with large rocks in it. When you shake the container, it moves and sounds like water, yet when you stop shaking it and hold it in a vertical position, the rocks are completely suspended.

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A MMO drilling fluid will penetrate into the formation and form a gel-like barrier that seals off the surrounding area and stabilizes the formation to prevent further fluid loss and hole collapsing. In a vertical well or horizontal bore, MMO drilling fluids move in a plug flow, as opposed to a laminar flow or turbulent flow like convention drilling fluids. Plug flow is much more efficient for transporting large drill cuttings to the surface. One of the downsides of a MMO drilling fluid is that conventional drilling fluids testing equipment, such as a viscosity cup and funnel or a rheometer/viscometer are pretty much useless for testing them.

Drilling Fluids

Do not panic: it’s supposed to look this way.

When mixed and yielded correctly, a good MMO drilling fluid will look like somebody totally messed up a tank of drilling fluid because of its pudding like texture. Don’t panic, it is supposed to look like that. The first thing most contractors want to do is to crank up the water and thin it out (or try to add a dispersant/thinner), but avoid making this knee-jerk reaction. MMO drilling fluids are extremely shear-thinning, therefore even though the fluid looks like pudding in a tank, pump pressure and annular pressures will actually be much lower, and pumping volume requirements may also be lower.

As previously mentioned, coarse gravel and cobble are some of the most challenging soil conditions to drill through for both vertical and horizontal directional drilling contractors, and MMO drilling fluids can be a great solution to drilling in such conditions. When running a MMO drilling fluid for the first time, consult your drilling fluids supplier to get proper mixing and utilization instructions.

George M. Dugan is territory sales manager at CETCO Drilling Products Group.

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