Yukos, Big E employ near closed-loop systems.

Drillers can eliminate most drilling fluid wastes, decrease the need for dilution, and reduce the cost for mud preparation with near closed-loop systems (NCLSs).

Yukos Oil Co., Moscow, which has adapted seven Siberian rigs to include centrifuges and flocculation-coagulation units, and Big E Drilling Co., Houston, which has constructed a new rig with a purpose-built system, show clear examples of how solids control systems can be tailored to accommodate local drilling practices.

System design

For most drilling rigs, solids-control equipment design must consider the following sections:

1. Removal section--sand trap, degasser, desander, desilter, centrifuge, and flocculation unit.

2. Onsite cuttings and fluid dump--reserve pit.

3. Additions section--agitators, hoppers, and mud guns.

4. Suction and testing section--suction tank, agitators, slug tank, and trip tank.

5. Wellbore section--borehole, drillstring, casing, and drill bit.

With an NCLS, however, the reserve pit, basically a large excavation lined with plastic (Fig. 1), has been entirely removed. Instead, drillers continuously remove cuttings in the removal section, then reprocess the waste-free effluent in the additions section.

From here, the reclaimed drilling fluid undergoes final preparation in the suction and testing section so that it can be recirculated back into the borehole.

Concurrently waste solids, extracted in relatively dry form, are continuously collected and transported during the drilling operation for off-site treatment and disposal. Systems set up to discard these cuttings include augers, positioned near the sand trap for continuous conveyance of the cuttings, cuttings containment boxes, cranes, and dump trucks.

Removal section

In many systems that include a full complement of NCLS equipment, drilling contractors arrange mud-tank compartments in a linear fashion so that shale shakers, degassers, desanders, desilters, centrifuges, and coagulation-flocculation units can be arranged sequentially, each with a dedicated tank (Fig. 2).

In this way, solids are treated from one tank to the next in order of diminishing particle size (see box: Near closed-loop system).

Thus, with a properly designed removal section, each piece of equipment will discharge fluids into the next compartment downstream of its suction compartment.

"No mud should enter the suction compartment in the removal system," said Mark Morgan, technical service manager for Derrick Equipment Co., "except directly from equipment immediately upstream or from an equalizing backflow. In this way, you won't mix dirty and clean fluid together.

To implement an efficient solids-control fluid path properly, each compartment, excluding the sand trap, should be set up so that drilling fluids flow backward from the downstream compartment into the upstream compartment (Fig. 3).

"Backflow occurs by processing more fluid through the solids-control equipment than what is being circulated by the rig's main mud pumps," …