DRILLING WASTE TREATMENT – DEWATERING

Dewatering is the art and science of chemically enhanced centrifuge separation. Dewatering is the final step of a closed loop system and follows the separation process after shale shakers, hydrocyclones, and centrifuges. While high-speed centrifuges remove particles 2 to 3 microns and larger, dewatering can remove all colloidal particles down to clear water.

Dewatering has become common in many instances, especially as technology has advanced and the units have become more compact and less expensive. At first, dewatering units were introduced where only stringent environmental conditions existed. However, dewatering has now become economical where freshwater is scarce or disposal sites for off-spec fluid are too far from the drill site, making transportation costs expensive. Dewatering units have also found application in drilling through fragile clay formations.

The presence of oils or lubricants does not affect the dewatering of water-based fluids. Once the colloidal solids are removed, the oils, lubricants, or organics separate out with the liquid and tend to float on top of the water. All water-based fluids can be dewatered, although some are easier to dewater than others. Dewatering oil-base fluids is not easy and requires a prior treatment with a demulsifier to break the oilwater emulsion. However, today, even cement-contaminated fluids can be dewatered on location in order to reduce disposal costs by requiring only the solids to be removed from the site.

Dewatering units can be compact systems located on the rig, or they can be mobile, trailer-based systems parked next to the rig. Dewatering units are also sometimes set up in a central fluid plant serving a cluster of rigs. A dewatering unit on location allows a portion of the off-spec fluid to be dumped into a storage tank and then processed into solids and fluid. Depending on the dewatering process and the chemical treatment applied, the fluid can be recycled as is or further treated before recycling. The water generated from the first step generally tends to be clear but sometimes requires pH adjustment prior to being recycled. However further treatment may be required to bring the water up to local standards before disposal.

Use of an on-location dewatering unit during operations has become mandatory in some parts of the world, such as the Arctic, jungles, and rain forests and in close proximity to urban environments. Additionally, on-location dewatering units may be mandated when drilling near freshwater sources, near sensitive fishing areas, or where concern for protecting ocean species is very strong or regulated.

With the pH of fluid typically being between 7.0 and 10.0, the colloidal particles in the fluid tend to be negatively charged. The negative charges repel the particles, preventing them from clumping together to form larger particles. To remove these submicron colloids is difficult, even with a high-speed centrifuge with 2000 g force. Therefore, to remove these tiny particles in the fluid, it is first necessary to treat the fluid with chemicals to agglomerate the solids to make them large enough to be removed by a high-speed centrifuge.

The process of agglomeration to create large, dense clusters requires three steps:

• Destabilize the submicron particles so they no longer repel each other. This is easily achieved by lowering the pH from 7.0-10.0 to approximately 5.5.
• Coagulate or bring together the fine solids—create an attraction between the particles.
• Flocculate, bundle, or wrap together to create large dense clusters.

These three steps can be accomplished by sequentially adding three chemicals; or sometimes only one or two chemical additives can accomplish all three steps. To maximize dewatering efficiency and effectiveness, enough time between the addition of each of the three chemicals should be allotted. This enables the chemicals to fully react with the particles and therefore ensures that all chemical treatment affects primarily the solids, with negligible amounts of chemical remaining in the liquid phase. This helps generate a reusable fluid (water) or one that can comply with local norms if being disposed of.
Typical dewatering systems include:

• A holding tank with mixing to create a homogeneous waste fluid that
  is ready to be processed.
• Small storage mud tanks for the chemical additives with controllable feed
  pumps to calibrate minute levels of treatment—one tank each for acid
  and coagulant and two tanks for flocculant. Since flocculant makedown
  takes time, having two allows one for makedown while the
  other one is in use.
• Pumps to feed the treated fluid under steady pressure and at constant
  flow to a centrifuge.
• A manifold with inline mixers that will allow the fluid time to react
  with the chemical additives before reaching the centrifuge.
• A high-speed centrifuge in which the desired g force can be attained
  to remove/discard the coagulated solids and discharge clear fluid.
• A storage tank for clear fluid exiting the centrifuge before recycling
  to the active system or disposal.
• Skimmers to remove any oil or lubricants present. The oil remains
  with the cleaned water and floats above the water phase, where it can
  be removed.

A simple explanation of the dewatering process is that it diverts fluid through a manifold system prior to pumping to the centrifuge. It is in the manifold that the chemicals are mixed sequentially, inline with the fluid. As the fluid flows through the manifold, the acid is first added, the fluid continues through the manifold to where coagulants are mixed in, and finally it arrives at where the flocculant is added. The chemicals should be properly metered in order to allow for minimum dosage. With a properly designed manifold, one that is sufficiently long and includes inline mixers, the chemicals will have time to react individually with the fluid, producing the desired result of larger clustered solids that are more easily removed by centrifugal force.

When the fluid exits the manifold system, it is ready move into the high-speed centrifuge. The particles in the fluid have been clumped together into relatively large ones, with a high enough density for a centrifuge to remove. After being processed by the centrifuge, the clear effluent (water) is stored in a tank and the solids are conveyed to a separate pit or tank for disposal. A pump takes the fluid back to the fluid system or to a separate storage tank.

Using clean chemistry, buffered phosphoric acid, and coagulants and flocculants approved by the National Sanitation Foundation can usually generate clean water that is good for recycling or disposal with minimal treatment. Depending on the nature of solids from the formation and fluid additives, with the proper treatment chemistry it is possible to generate solids with minimal chemical content that can be disposed of like other cuttings.

Dewatering oil-base fluids generally follows the same procedures as that for water-based fluids. Oil-based fluid, however, must first be treated to break the emulsion. This can be accomplished by adding acid and additional water or by use of a demulsifier. The oil on separation rises to the surface, where it is removed by a skimmer. The remaining solids in the water are then treated in the same manner as water-based fluids. Careful treatment with adequate dilution can ensure that the remaining oil content in the sludge is well within acceptable environmental norms. Figure 1 shows a schematic of a dewatering set-up. The process can be quite complex.