New solids control system reduced oil on cuttings

T.P. Wilkinson
Oiltools (Europe) Ltd. Aberdeen

A new solids control system, consisting of four new shakers and a dryer in parallel all discharging into another dryer, significantly reduced the oil on the cuttings in a nine-well offshore drilling program (Fig. 1 [56065 bytes]).

Cleaned, slurrified cuttings were then discharged overboard.

In November 1994, Oiltools (Europe) Ltd. received contracts to upgrade the solids control systems on Sedco Forex's Sedco 711 and Sovereign Explorer semisubmersible drilling vessels. Sedco Forex required systems that would meet the reduced oil-on-cuttings (OOC) disposal limit of less than 80 g/kg set by the operator, while staying efficient and economical to operate and maintain. In addition, all solids were required to be slurrified for pumping overboard to ensure dispersal away from the subsea center.

This article highlights the equipment used and the savings realized on the Sovereign Explorer after the first three wells of a nine-well program.

Solids control equipment

A Flo-Divider was designed by Oiltools and Derrick Equipment Co. in response to Sedco Forex's criticism of the existing header tank design. The Flo-Divider eliminated the requirement for a header tank altogether by distributing solids and liquids evenly to each shaker.

The small unit (1,800 3 1,270 3 1,100 mm) is unaffected by rig heave because of its size and totally enclosed design. It allows better management of fluid onto the shakers (finer screens can be fitted), has no moving parts, and can be remotely located in the shaker house.

The Flo-Line Cleaner used for this contract is an adaptation of the three-panel Flo-Line Cleaner Plus. The original design incorporated a header tank system which was removed to accommodate new Flo-Divider technology. These four shakers were added together with a four-panel Hi G Dryer (operating in shaker mode).

The shaker screens were new Pyramid screens (Sidebar) rather than the traditional flat, linear type. The Pyramid Plus screens allowed a 70% increase in fluid handling capacity, or approximately 2-3 screen sizes finer than possible with flat screens. These screens also permitted more effective use of the last screen panel because of their ability to absorb fluid and cuttings surges, because the fluid end point could be increased without fear of fluid losses.

The Hi G Dryer was setup for use as either a conventional shaker or cuttings dryer. On the Sovereign Explorer, two Hi G units were added: one as a shaker with a standard sump and one as a dryer with a 10-bbl sump. The Hi G Dryer has a single four-panel screen deck with 42.4 sq ft of screen area. The screen deck can be angled up to 10 to reduce fluid loss.

A DE-1000 (FHD) centrifuge was added. Its rotating assembly is based on a 14-in. diameter 3 49-in. long, 10 contoured cylinder with variable epicentric effluent ports, and a 4.25-in. single lead radial flow scroll. The maximum bowl speed of 4,000 rpm generates a 3,180 G-force. The power train consists of a single electric motor delivering 50 hp to either the scroll or bowl drive system. A built-in control system allows automatic feed control, automatic speed boost of scroll to prevent overloading, and automatic unit shut down. The entire system is mounted on a single skid. The centrifuge has a maximum flow rate of 190 gpm.

The slurrification system consists of two 50-bbl tanks and two modified 6 3 5 centrifugal pumps. The manifolding is designed such that the cuttings are processed in batch mode, one tank at a time. The entire system is skid mounted and semiautomated to reduce personnel requirements.

All four shakers and the four-panel Hi G Dryer (operating as a shaker) were installed in traditional parallel mode. The second Hi G Dryer was installed downstream of the shakers to process all cuttings, thus ensuring environmental compliance. The screen underflow from the second dryer is transferred directly to the active system or through the centrifuge system for fine solids removal processing.

The drill cuttings from the second dryer are then transferred to the slurrification system for discharge.

Results

The dryer ensured environmental compliance with regulations and also limited losses below the operator's target of 80 g/kg. A total of 534 bbl of oil-based mud was recovered, and OOC figures where significantly reduced (Fig. 2 [45838 bytes]). The OOC target set by the operator was met even though the majority of screens used on the shakers were 180-230 mesh. The Pyramid screen technology allowed greater conductance from the same size shaker meshes without increasing OOC levels. During the first three wells, fluid losses overboard equated to 0.086 bbl/ft.

The 3D screen technology allowed finer screens to be used at a much earlier stage of the well, improving solids control without adversely affecting the OOC figures. This setup contributed to the very low mud costs. No new mud was added to the system during the first three wells. Screen usage was acceptable, with an average of 41 screens consumed for each well. Average screen costs for the three wells was 0.61/ft (about $0.93/ft).

The Flo-Divider performed well, distributing solids and liquid evenly to each of the five shakers. No problems were encountered, and no fluid was lost from rig heave.

During the course of the first three wells, the solids control equipment had no downtime. The lack of downtime can be directly attributed to routine maintenance practices, rugged equipment construction, and the use of integral vibrators (no belts) and an automatic lubrication system (self timing).

Screen cost figures will become more accurate as more wells are drilled and more data become available. Typically, the initial costs are high because of stocking the rig, which inflates the average cost of screens during the first few wells (Figs. 3 [43037 bytes] and 4 [47291 bytes]). These costs should decrease over time as the screens are repaired and reused. Further efforts are being made to reduce the OOC and screen consumption. These im provements should be realized as personnel become more familiar with the equipment and drilling conditions improve.

Before the advent of dryer technology, the traditional method for control of OOC was to install a cuttings wash/cleaning system. This type of system was expensive and in some cases very inefficient.

Another major benefit of the dryer technology is the elimination of whole mud losses from the shakers. Historically, because of mud losses, the shakers were never run at their optimum capacity, and thus the solids control system efficiency was affected. The use of a dryer downstream allows the shakers to be run close to maximum capacity without the concern of whole mud losses.

CORRUGATED screens can improve shaker efficiency, allowing more drilling fluid throughput and less waste of whole mud, according to Mark Crabbe with Derrick Equipment Co. in Houston. Because of the increased area relative to flat screens, finer mesh sizes can be used.

During the past 2 decades, there have been significant advances in solids control equipment. The mud cleaner was developed in the 1970s, and linear-motion shakers were developed in the 1980s.

Shaker manufacturers adopted linear motion with an uphill screening deck to create a continuous fluid column over about two-thirds of the screen deck. Solids control efficiency was dramatically increased by fine screening at the flow line. In 1983, linear motion shakers, in combination with plated sandwich screens, established a new benchmark of performance.

With improvements in linear motion shakers in the past decade, the use of finer screens (150-200 mesh and finer) became possible. Many operators began stipulating the use of fine screens in the weighted and slim hole sections.

In the 1990s, new trends in drilling created tougher demands for fine screening. Two factors were especially important: the popularity of more expensive drilling fluids and increased hydraulic demands for bit cleaning, solids transport, and motor rotation. These factors required an increase in the number of linear motion shakers per rig so operators could take advantage of the new advances in technology.

In 1993, 3D screen technology was developed, raising the standards for fine screening by two to three sizes. These corrugated screens have rows of parallel peaks and valleys.

A Pyramid screen consists of a standard "sandwich" construction of two fine-mesh cloths layered with a coarse backing cloth. The three are bonded together, then the sandwich is corrugated and bonded to a perforated plate. The resulting corrugations, 0.8-in. tall, add about 40% more screen area compared to a standard perforated plate panel. Typical conventional flat screens have an area about 5.3 sq ft, compared to 7.6 sq ft for a Pyramid screen. Field use has shown flow capacity gains of 80-100%.

The 3D construction allows gravity to force oncoming solids down into the corrugation troughs and away from the peaked area. The upper portion of each corrugation is free of solids and therefore provides improved liquid throughput. The peaks do not become blinded, allowing an increased flow capacity (Fig. 1).

Cuttings size, shape, and composition always play a role in screen selection and use. Too often, shaker screen selection and use are based only on mud weight and flow rate. Hole size, types of formations, and expected penetration rate are equally important, because oversized solids occupy usable screen area.

The solids, or moving solids bed, become a permeable medium. The resistance to liquid passing through the solids (conductance) becomes the primary performance variable; thus, decreasing the blinded area of a screen is important. The enhanced permeability of the first two screen panels submerged in fluid helps explain the gain in fluid-handling capacity of Pyramid screens.

Most linear-motion shakers have a crown in the screen frame which provides hoop tensioning to secure the screen panel to the screen frame. The result of this design is a tendency of flow to concentrate on the outside edges of the discard screen, thus creating a "horseshoe" effect.

By dividing solids and liquid into individual streams and not allowing a stream to run to the side, Pyramid screens provide more uniform distribution of fluid and solids across the entire screen panel. This uniform distribution greatly reduces the horseshoe effect and increases shaker screen utilization. By lengthening the fluid end point and increasing pool depth on the shaker basket, the third (discard) screen panel can be used more effectively.

Because the greatest advantage of Pyramid screens is achieved with the screen submerged in the pool, research was directed at finding the optimum screen corrugation height. Practical design limits the screen height to that of pool depth.

In field trials, various lengths of corrugated screen were bonded on 40-in. wide perforated panels. Data from 6 months of lab and field testing indicated optimum performance with 80 in. of corrugated screen cloth bonded on the 40-in. wide panel.

This design uses a corrugation height of 1.5 in., increasing screen area for a 48-in. 3 30-in. screen to 10.6 sq ft. These Pyramid Plus screens allow shaker capacity to be further increased one to two screen-mesh sizes finer.

In a field trial in Hardin County, Texas, a Pyramid Plus screen handled 580 gpm of 8.9-ppg mud from the 121/2-in. hole, whereas standard flat screens handled less than 290 gpm.

Copyright 1996 Oil & Gas Journal. All Rights Reserved.