Several Canadian and U.S. onshore drilling contractors successfully reduced drilling-fluid losses during pipe-handling operations by installing mud-bucket/spill-tray assemblies, vacuum systems, integrated rig-floor troughs, and splash guards.
In addition to obvious environmental benefits including dry rig locations, cost control and safety issues continue to provide strong incentives for upgrading fluid-containment systems.
Tripping operations produce large losses of drillstring fluids when not captured and returned to the active mud system. For example, if a crew drills into gumbo shale, resulting in plugged drill-bit jets, the entire drillstring must be pulled "wet," resulting in fluid exiting out the bottom of each stand onto the rig floor.
This is much like placing a finger over the top end of a straw and holding it in a glass of water. Once you pull the straw out of the water, then remove your finger, the water will spill out, regardless of elevation.
Thus, as the column of drilling fluid discharges, it creates a very slick, wet, and dangerous work environment around the rotary table, especially if there are no mats to cover the steel floor plates.
Furthermore, fluid lost while pulling a wet string can result in unnecessary well costs. For example, a 10,000-ft drillstring made up of 41/2-in. ID, 3.826-in. OD drill pipe, can result in losses of 142 bbl (disregarding drill collars) if the rig is not set up to capture and return the runoff back into the active mud system (Fig. 1, Table 1). And with $500/bbl for oil-based mud, or $100/bbl for water-based mud, this can cost the operator as much as $71,000 if the fluid discharges are unrecoverable.
Additionally, over the life of a well, normal tripping activities can result in substantial drill-fluid losses. For example, assume 10 trips are made in increments of 1,000 ft to a TD of 10,000 ft. If drillstring fluid losses average 25% for each trip, 195 bbl of fluid can be lost (Table 1).
Finally, it is important to add in fluid spillage from connections. Again, a 10,000-ft well with about 333 connections will spill roughly 66 bbl (Table 1). This assumes a drilling-fluid loss of 20%, draining through a 40-ft kelly and 30-ft joint of pipe. Although the kelly cock can be closed during the connection, saving more than 60% of the fluid, this is a time-consuming and sometimes dangerous procedure that is not always taken.
The traditional mud bucket
Traditionally, most onshore rigs make use of an old-style mud bucket that clamps around the box and pin ends of drillpipe while tripping out of the hole. As each stand is unscrewed (broke out), the mud bucket encapsulates the free ends of the tool joints, redirecting the escaping fluids through a 4-in. return hose into the flow line or mud pit.
Patented in 1909 and limited to drill pipe applications, this piece of equipment weighs more than 200 lb and requires two floorhands to swing it back and forth from the well center.
Moreover, because the mud bucket often crosses the path of the tongs, leading to sudden and forceful impacts, the unit may be banged out of shape, resulting in leaking seals and a difficult-to-close clamping mechanism. And, the 4-in. return hose, which snakes across the rig floor, produces an additional foot hazard. Thus, inherent design problems with the old-style mud bucket result in only partial fluid reclamation and a dangerous work environment.
To reduce drillstring spillage, Dale Leitner, drilling superintendent for Ensign Drilling Co. in Canada, says his company uses a lightweight, free-standing mud bucket in conjunction with a collection tray mounted below the rotary table (Fig. 2). "While tripping out of the hole, we can save more than 90% of the drilling fluids using this equipment," he said. "This is extremely important since we use expensive oil-based muds in a lot of our operations." Out of Ensign`s fleet of 76 rigs, Leitner says 45 rigs now use this system.
In a drill-pipe tripping situation, the system, designed, developed, and patented by Katch Kan Ltd., works as follows:
- Driller sets slips.
- Floorhands attach makeup and breakout tongs to drillpipe.
- Driller breaks tool joints using tongs and initiates rotary table to unscrew stand.
- Roughneck places lightweight mud bucket around box and pin end of broken stand.
- Driller picks stand up 3-4 in.
- Drilling fluid rushes down stand into the lightweight mud bucket.
- Mud bucket encapsulates and redirects drilling flow downward through the base of the unit, which then drains through the rotary table into the collection tray.
- Collection tray, sealed to the top of the flow nipple below the drilling floor, drains fluid into active mud system through the flow line (Fig. 3).
As Fig. 4 shows, the 27-lb mud bucket can be clamped around the pipe by one person. Additionally, the unit can be used with service tubing, kellys, heavy-weight drill pipe, drill collars, and test tools by simply exchanging a seal or top steel plate.
The collection tray, in combination with a stripper rubber, forms a natural junk basket that prevents tools and foreign objects from falling down the hole.
According to Tracy McIntosh, operations manager for Katch Kan, 330 rigs have been retrofitted with this equipment since it was first installed on Akita Drilling Ltd.`s rig No. 38 in July 1995. Among the companies that use this technology are Nabors, Precision Drilling Corp., Jomax Drilling Ltd., Phelps Int. Ltd., and Helmerich & Payne Inc.
Another technology used by onshore drilling contractors includes a vacuum system that serves to "sweep up" runoff. Duane Mather, president of Nabors Canada, says a joint effort with Shell Canada led to the installation of a metal-dike complex (coaming bar) that is plumbed into a vacuum system through a series of plastic pipes.
He said the system is permanently installed in five areas across the rig including the mud pumps, mud system, centrifugal pumps, rig floor, and substructure. "We`re able to retain 99.7% of our drilling fluids," Mather noted.
So far, six of Nabors Canada`s 35-rig fleet, most of which are deep-drilling units, have installed these systems. In addition to the vacuum system, 20 of Nabors Canada`s rigs also use the mud-bucket/spill-tray equipment. Mather says Shell and Nabors collectively address and take action on safety and environmental issues each year, leading to continuous improvements in these areas.
Integrated rig design
Helmerich & Payne Inc. (H&P) tackled the issue of drillstring runoff by integrating a drainage system and splash guards into the substructure and floor of its new builds (OGJ, Dec. 14, 1998, p. 57, Fig. 5).
With this design, the cellar remains dry at all times. Grated troughs, located along the sides of the drill floor and racking board, serve as natural barriers to drillstring spillage, funneling drilling fluids into the active mud system or disposal tanks.
"It`s a people issue," said John Lindsay, vice-president of land operations for Helmerich & Payne. "If the (rig) is designed so that fluids are not dripping down through the sub, it`s not creating work involved with cleaning, and painting the substructure and BOPs. Plus, the hands aren`t slipping around the slick metal."
H&P recently built six new rigs with integrated mud-retention systems while another six rigs have been similarly modified. According to Dale Atwell, senior operations superintendent for H&P, these rigs are `built for near zero discharge."