Dewatering and Zero Discharge Solids Control

Rigsite dewatering of a mud removes fine solids and recovers usable water. The process usually involves adding acid to the mud to obtain a pH of 6 to 8, followed by a coagulant and a chemical floc-culant (if needed) to cause the less than 2-micron solids to form aggregates large enough to be removed by centrifugation.

The water that is recovered can be almost clear or it can be murky or muddy as controlled by the amount of flocculant used and as dictated by the type of disposal or reuse requirements. For disposal into a local stream or into a water treatment system, clear water will likely be required, with low chlorides, neutral pH and no soluble metals or other contaminants. For disposal by injection into a well, the water may also need to be clear and polymer free to avoid plugging of the permeable zone. Water can be murky or muddy, if pumped into a fractured formation.

Solids separated by the unit's centrifuge will usually feel fairly dry but may still contain 20 to 40% liquid by volume. This will depend on the efficiency of the machine and how it is set to operate. Dampness may be acceptable for disposal into a local dump site or for on-site burial. Here again, salt and heavy metal content of the solids must be low.

Dewatering is usually undertaken to comply with local, state or federal environmental requirements. However, it can prove to be economically beneficial by keeping disposal volumes low, saving cost of water and minimizing future environmental liability.

Dewatering involves the following steps using the equipment shown in the diagram of Figure 19.

Figure 19

Diagram of Dewatering Equipment Layout

Figure 19

Diagram of Dewatering Equipment Layout

Solids Control Layout

Step

  1. Mud from the active system is drawn into a tank or taken directly into the injection manifold. This mud has previously been tested by the dewatering operator to determine volume of dilution water, amount of acid (to lower pH) and amount and kind of flocculant (polymer) or coagulant (alum or ferric salt) to be used. This testing involves a series of bottle-shake tests after which the dewatering operator compares the degree of clarification with what is desired.
  2. Using the bottle-shake test results as a guide, the operator sets up the injection rate to give the desired concentration of water, acid and flocculant. The treated mud is then circulated in the holding tank to allow time for coagulation to occur before passing the fluid on to the next step, the centrifuge.
  3. The treated fluid goes next into the clarification centrifuge, with dilution water if needed. Inside the machine, the solids are thrown outward and discarded. Water exits the machine as clear, murky or muddy water, which can be fine-tuned by the injection rate of coagulant and flocculant pumped into the manifold.
  4. The water from the centrifuge is stored in a neutralizing tank. This is where the dewatering operator or mud engineer will test and adjust its chemistry before it is put back into the mud. The usual adjustment is to raise pH to offset the effects of the acid added upstream and treat out hardness, if needed. In some cases, this tank is partitioned for the pre-hydration of bentonite or polymers.

In summary, the dewatering process is an important aspect of a zero-discharge or closed-loop solids control system. This must work in conjunction with the normal solids removal devices on a rig. The chemicals for dewatering must be compatible with chemicals used in the mud - otherwise they must be removed from the water before reuse in the mud.

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