Drilling with less than normal pressure fracture gradients has led to the necessity, design and implementation of novel systems delivering equivalent mud weights from 4 to 8 pounds per gallon (ppg). These fluids are not light weight fluids like air or foam, but are not as dense as heavier conventional fluids. This combination of a small amount of equipment plus a novel water base fluid system has resulted in reduced pressures while drilling with a simple adjustment in air flow. These middle weight fluids have been field tested with a low pressure cavitation mixer, and minimal air supply, similar to what a rig air system supplies.

An inhibitive low solids fluid weighing 8.7 pounds per gallon (ppg) arrived at the standpipe at less than 8.0 ppg equivalent, due to the novel air injection system used between the pits and the rig pump. The fluid circulated through the standard triplex rig pump, then the air broke out just before the fluid flowed over the shaker screens, with no problems. Conventional solids control equipment was used without adjustment.

Just less than 8 pounds per gallon was maintained for several hours while drilling ahead horizontally. Brief periods of equivalent density as low as 5 pounds per gallon were possible, and can be reasonably expected after equipment changes are made. The wellbore remained slick and stable, hole cleaning was good. Standpipe pressure was approximately 400 pounds per square inch less (1600 pounds per square inch versus 2000 pounds per square inch) compared to conventional unaerated fluid.

The fluid formulation included a small amount of potassium formate, clay/shale stabilizers, xanthan gum and an air dispersant. Fluid chemistry facilitated even smooth air entrainment with instant breakout at surface. The formulation was solids free, with no intent to build a filter cake. While conventional solids control equipment was used to remove drill solids, supplementary solids control equipment was identified as necessary to extend the ability to maintain the equivalent density at less than 8 ppg.

The cavitation mixer injects air into the liquid under pressure, just before the fluid flows into the rig pump. The field trial began to identify air and liquid flow supply requirements for consistent middle weight densities.


Drilling in areas with fracture gradients less than normal pressure gradient has presented tough challenges with expensive solutions. Light weight fluids such as air or foam can deliver an equivalent 1 or 2 ppg density, often with as much rig site footprint devoted to air compression equipment as used by the drilling rig itself. Personnel and daily costs for the compression services also rival those of the drilling services. "Heavy" weight fluids meaning conventional fluids may or may not be lost when drilling through weak or depleted zones, possibly requiring higher than normal volumes of drilling fluid, specialized lost circulation materials (LCM) added to the drilling fluid, cross link treatments to plug loss zones, modified cement programs, even modified casing programs, all options that work in some areas and may not work in others.

"Middle" weight fluids are specialized water base fluids that can incorporate, disperse then release air (or nitrogen) in such a manner that a much smaller footprint is used than a light weight setup to deliver an equivalent density of four to eight ppg. Traditional LCM treatments used with conventional fluids are avoided because the conventional minimum density of 8 or 9 ppg is reduced as needed with air. Concentric casing strings might be an unnecessary cost with middle weight fluids, since air is pumped down the drill string, and available throughout the annulus, rather than only at or above a certain casing point. So in many cases the least expensive alternative will be these middle weight fluids.

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