Drilling fluids recycling systems can provide horizontal directional drilling (HDD) contractors with a multitude of benefits and savings. These benefits and savings include a reduction in drilling fluids products required for a project, lower shipping costs to get these products to the jobsite, a cleaner jobsite, less water needed for mixing drilling fluids, a reduction in disposal cost of drill spoils, along with less labor and trucking to haul drill spoils away.

Like the drill, a drilling fluids recycling system is only as good as the manner in which it is operated. Without a thorough understanding of how the equipment works or what drilling fluids products to use with a recycling system, and/or how to properly test and maintain drilling fluids properties, even the best recycling system in the world is a wasted investment and improper use of this equipment can cost a contractor to lose a bore.

Fundamentals of Drilling Fluids Recycling


A drilling fluids recycling system is also a mixing system, and includes a centrifugal mixing pump, mixing hopper and mixing jets located on the bottom of the tanks/compartments to keep the drilling fluid agitated. As clean drilling fluid is returned to the drill via a centrifugal charge pump during drilling operations, drilling fluids return from the entry and/or exit pit are picked up via a trash pump and discharged over a scalping shaker that makes the initial coarse cut, removing large drill solids that can either settle to the bottom of the tanks or cause plugging problems with hydrocyclones.

Drilling fluid from the scalping shaker drop into a compartment under the scalping shaker and is picked up by a centrifugal pump and fed into a series of hydrocyclones that utilize the centrifugal force of spinning fluid to remove finer cuttings.

The hydrocyclone underflow discharge of fine cuttings also include a significant amount of liquid, therefore, the material is discharged on to a vibratory screening deck (shaker) equipped with ultra-fine screens in order to facilitate the recovery of additional drilling fluid and provide a dry discharge of drill cuttings. The combination of hydrocyclones mounted over a shaker is commonly referred to as a mud cleaner.

Drilling fluids recycling systems should be sized to handle at least one and a half times the maximum pumping rate of a drill’s circulating system in order to maintain a clean drilling fluid and to minimize wear on the rigs drilling fluids circulating equipment (especially the reciprocating pump). When in operation, drilling fluid from the clean compartment should cascade back into previous compartments (lower bypass gates should always be closed during recycling).

Avoid using high-molecular weight/long chain synthetic polymers with drilling fluids recycling equipment, because the long polymer strands/chains can create a sheeting effect on shaker and mud cleaner screens, which will bind the screens and cause excessive amounts of drilling fluid to run off of the screens.

Avoid using high-molecular weight/long chain synthetic polymers with drilling fluids recycling equipment, because the long polymer strands/chains can create a sheeting effect on shaker and mud cleaner screens, which will bind the screens and cause excessive amounts of drilling fluid to run off of the screens.


Drilling Fluids and Recycling Equipment


There is a misconception with many drilling contractors regarding the use of polymers and additives with drilling fluids recycling equipment. Polymers and additives can be used with drilling fluids recycling equipment as long as the correct products are used. Avoid using high-molecular weight/long chain synthetic polymers (normally used as clay inhibitors) with drilling fluids recycling equipment, because the long polymer strands/chains can create a sheeting effect on shaker and mud cleaner screens, which will bind the screens and cause excessive amounts of drilling fluid to run off of the screens. Low molecular weight/short chain clay inhibitors are available through all of the major drilling fluids companies.

Also avoid using drilling detergents/surfactants that have a tendency to foam up. Excessing foaming in recycling will cause pump cavitation and shut down a drilling operation.

Another good recommendation is to avoid excessively high drilling fluid viscosities.

Excessive viscosities not only lead to excessive annular pressures down hole; excessive viscosities have a negative effect on the efficiency of a recycling system in regards to the equipment’s ability to clean the drilling fluid.

PAC polymers (modified natural cellulose polymers used for filtration control), and gel-strength enhancers (xanthan gum) work great with drilling fluids recycling equipment. Just introduce these products slowly through the mixing hopper so that un-yielded globules of polymer do not blind the mud cleaner screens. PAC polymers are also available in liquid form and can be slowly introduced into the clean compartment (no need to clog up a hopper) with fewer problems regarding the product blinding up screens.

Operating Drilling Fluids Recycling Equipment


Drilling fluids recycling systems are designed to start off with clean fluid and have dirty fluid introduced into the system at a reasonable rate, determined by the capacity of the unit and size of the drill. Many contractors try to have one recycling system set up in a central location and haul drill spoils to the unit from various locations to be cleaned and re-conditioned. If an empty recycling system is filled with drill spoils (dirty drilling fluid) it will not perform well and take a lot longer to get the desired results.

When operating a recycling system, always maintain proper tank levels and do not let the fluid level go down below half in the dirty compartment before introducing more drill spoils into the system. It is best to try to balance the flow of the spoils pump for a constant feed into the system, or at least operate the dirty/scalping compartment tank between full to half. Letting the dirty/spoils compartment go down to almost empty can temporarily overload the system with solids and the overall end-result is more solids going back to the drill’s reciprocating pump, leading to shorter pump life.

Keep in mind that once drilling is under way and additional mud mixing is needed, a portion of un-yielded bentonite will be lost off the end of the mud cleaner screens, therefore try to keep up and mix the bentonite slowly. Those who manufacture drilling fluids recycling systems can almost eliminate this problem by incorporating an in-line mud shear/static mixer to speed up the mixing process.

Determining when to add polymers and additives (including soda ash) can be based on the consumption of bentonite. For example, if a contractor is using approximately 25 lbs of bentonite per 100 gals of drilling fluid, adding one 50-lb sack of bentonite is like mixing 200 gals of drilling fluid. Bentonite, polymers, and additives, should be viewed as expendable items that are constantly being deposited on the walls of the bore-hole for filtration control, and some of this material is being carried off with the drill cuttings.

Drilling fluids must be constantly tested, when using a recycling system, to ensure that the drilling fluid is capable of performing all of the required functions. Many recycling system operators feel that they simply can look at a drilling fluid and determine the viscosity, when after 39 years of experience in working with drilling fluids and recycling systems, this author has learned through experience to never trust one’s eyes. A 10-second difference in viscosity, for example, can make the difference between getting a product line back or getting stuck. Viscosity should be checked frequently, and tested especially when one notes a visual change in the consistency of the drilling fluid.

Sand content is another test that is important to maintaining a good quality drilling fluid. Sand ranges in from 74 to 200 microns in size and the screen on a sand content test kit is 200 mesh, which makes a 74 micron cut point; 200 mesh or finer is also what should be used on desliting mud-cleaners in order to maintain the lowest sand content possible.

It is not unusual for a knowledgeable operator running a good quality drilling fluids recycling system (also well maintained) to average below 0.25 percent sand content for the duration of a project. If the percentage of sand increases, check for problems such as torn mud-cleaner screens or open lower compartment gates. Also make sure that clean fluid is cascading from the clean compartment into previous/dirty compartments.

When drill cuttings are recirculated, they get ground down into smaller particles (becoming a bigger problem), and solids smaller than 74 microns will not show up in a sand content test. Therefore, a mud balance scale is absolutely necessary when operating a drilling fluids recycling system. Clean drilling fluid will weigh around 8.5 lbs per gal, and when the weight of a drilling fluid rises above 9 lbs per gal, it is time to look at displacing a portion of the drilling fluid (haul off to be disposed of) with fresh/clean drilling fluid to reduce the weight/solids content. Ultra-fine solids are still abrasive and will cause excessive wear on the rig’s circulating system components and also slow down the penetration rate of the drill. Drilling fluids containing high solids do not perform well in regards to filtration control and gel-strength.

The advantages of drilling fluids recycling are only gained through a good understanding of the equipment, as well as the drilling fluid that is being recycled. Operators should be experienced as opposed as to throwing a new hire off of the street on a system in which the success of a drilling project is dependent upon.

For additional training, contractors should consult with the manufacturers of the recycling equipment and or a drilling fluids company representative who is knowledgeable in drilling fluids recycling.
George Dugan is technical sales manager at CETCO Drilling Products Group.

See Discussion, Leave A Comment