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Drilling fluid was used in the mid-1800s in cable tool (percussion) drilling to suspend the cuttings until bailed from the drilled hole.1 With the advent of rotary drilling in the water well-drilling industry, drilling fluid was well understood to cool the drill bit and to suspend drilled cuttings for removal from the wellbore. Clays were being added to the drilling fluid by the 1890s and by the time Spindletop was discovered in 1901, it was considered necessary to have suspended solids (clays) in the drilling fluid to support the walls of the borehole. These solids (clays) resulted from the disaggregation2 of formations penetrated by the drill bit.2 If the penetrated formations failed to yield sufficient clay in the drilling process, clay was mined on the surface from a nearby source and added to the drilling fluid. These were native muds created either by "mud making formations" or, as mentioned, by adding specific materials from a surface source.
Drilling fluid was recirculated and water was added to maintain the best weight and viscosity for specific drilling conditions. Cuttings, or pieces of formation (small rocks) that were not dissolved by water, required removal from the drilling fluid to continue the drilling operation. Under the sole discretion of the driller or tool pusher, a system of pits and ditches was dug on-site to separate cuttings from the drilling fluid by gravity settling (gravity forced the cuttings to deposit in the pits and ditches). This system included a ditch from the well, or possibly a bell nipple, settling pits, and a suction pit from which the "clean" mud was picked up by the mud pump and recirculated.
2 Bold-faced words are defined in the Glossary, pages 276-329.
Mud was circulated through these pits, and sometimes a partition was placed in the settling pits to accelerate removal of unwanted sand and cuttings. This partition extended to within a foot or two of the bottom of the pit, thereby forcing all the mud to move downward under the partition and up again to flow into the ditch to the suction pit. Much of the heavier material settled, by gravity, in the bottom of the pit. With time, the pits filled with cuttings and the fluid became too thick to pump because of the finely ground cuttings being carried along in the drilling fluid. To remedy this problem, jets were placed in the settling pits to move the unusable mud to a reserve pit. Then, water was added to thin the mud and drilling resumed.
In the late 1920s, drillers started looking to see how other industries resolved similar problems. It was discovered that ore dressing plants and coal tipples were using:
The latter two methods were adopted for removing cuttings from drilling fluids.
The revolving drum, or barrel-type screens (called trommel screens), were widely used with the early low-height substructures. These units could be placed in the ditch or incorporated into the flow line from the wellbore. The mud flowing into the machine turned a paddle wheel that rotated the drum screen, through which the drilling fluid passed. The screen used at this time was very coarse, or 4 to 12 mesh. These units were quite popular because no electricity was required and the settling pits did not fill up as quickly. Today, revolving drum units have just about disappeared.
The vibrating screen, or shaker, became the first line of defense in the solids removal chain and for many years was the only machine used. Early shakers were generally used in dry sizing applications
Earthen Pit Design. The earthen pit system was designed as a settling pit that overflowed through a ditch into the suction pit. A duplex mud pump suction is shown at the far end of the suction pit. Drilled solids levels were poorly controlled. Mud treatment consisted of dilution to decrease the drilled solids concentration and viscosity. Fortunately, the well was normally pressured and was easy to drill.
Earthern Mud Pits. A drilling rig in East Texas drilling a 2,800-foot well using earthen mud pits. Cuttings, for geological examination, were collected on a piece of hardware cloth at the end of the flow line. Solids control equipment was not used on this well.
and went through several modifications before arriving at a basic type and size for drilling. The first modification reduced the size and weight of the unit for transporting between locations. The name "shale shaker" was adopted to distinguish the difference between shakers (classifiers) used in mining and shale shakers used in oil well drilling since both were obtained from the same suppliers.
Other modifications included a 4' x 5' hook strip screen that tensioned from the sides with tension bolts. Motion was elliptical, which made a down-slope necessary to move cuttings off the screen. Screen mesh was limited to 20- to 30-square mesh (838 to 541 microns). This unit was the workhorse of the industry until the late 1960s. Even though superseded by circular motion and linear motion shakers, standard shale shakers are still in demand and being manufactured today.
In the late 1920s and early 1930s, larger oil companies organized research laboratories and began exploring oil well drilling problems. They began to understand that the smaller cuttings, or particles, left in drilling fluid were also detrimental to the drilling process and another ore dressing machine was introduced from the mining industry—the cone classifier. This machine, combined with the concept of a centrifugal separator taken from the dairy industry, became the hydrocyclone desander. The basic principle behind separating heavier and coarser materials from the drilling fluid is the centrifugal action of rotating the volume of the sand-laden mud to the outer limit, or periphery, of the cone. The heavier particles exit the bottom of the cone, and the cleaner drilling fluid rises to the top and exits as the effluent. The desander, ranging in size from 6 to 12 inches in diameter, removes most solids larger than 30 to 60 microns. Desanders have been considerably refined through the use of more abrasion-resistant materials and more accurately defined body geometry and are an integral part of most solids separation systems today.
After the development of the oil-field desander, it became apparent that side wall sticking of the drill string on the borehole wall was generally associated with soft, thick filter cakes. Using the already existing desander design, a 4-inch desander was introduced in the early 1960s. Results were better than anticipated and included longer bit life, reduced pump repair costs, increased penetration rates, and lower mud costs. These smaller hydrocyclones became known as "desilters" since they remove a much smaller particle, called silt (15 to 30 microns), which is smaller than "API sand."
As barite and other compounds were developed to improve drilling, drilling fluid became very complex. Also, the liquid phase for carrying solids was being reduced by the addition of barite and other compounds. The shaker removed the larger cuttings (larger than 541 microns, or 30 mesh), and the desanders and desilters removed the smaller particles (60 to 15 microns). However, the intermediate-size particles (from 541 to 60 microns) were still left in the drilling fluid.
Intermediate-size particle removal led to the development of circular motion, or tandem, shakers. Development was slow for these fine-screen, high-speed shakers for two reasons. First, screen technology was insufficiently developed for screen strength, so screen life was short. There was insufficient mass in the screen wires to properly secure the screens without tearing. Second, the screening basket required greater development expertise than required for earlier modifications in solids removal equipment.
During this time, major oil company research recognized the problems associated with ultrafines (colloidals) in sizes of 10 microns or less. These ultra-fines "tied up" large amounts of liquid and created viscosity problems that could only be solved by adding water (dilution). Centrifuges had been used in many industries for years and were adapted to drilling operations in the early 1950s. They were first used on weighted muds to remove and discard colloidals—fine particles smaller than 2 to 4 microns—and to save larger particle-size barite (weighting material) and some drilled solids.
In recent years, a centrifuge was applied to unweighted drilling fluids to reduce and discharge fine solids in the active mud system. This application saves the more expensive liquid phase of the mud for reuse. Dilution is minimized, thereby reducing mud cost; however, these machines are quite expensive and require a great deal of care.
In the early 1970s, the mud cleaner was developed as an addition to the desander and desilter for reducing loss in the expensive liquid phase. Hydrocyclones discard a slurry, including the liquid phase, which can be expensive over time. The mud cleaner takes the underflow from a bank of hydrocyclones and introduces the slurry to a very fine, pretentioned vibrating screen. The expensive liquid phase and most of the barite pass through the screen and back into the system while the larger solids are discarded. This was the first successful application of a screen, bonded to a rigid frame, using very fine screens. Many mud cleaners have screen cleaners, or sliders, which are circular plastic pieces that vibrate against the bottom of the screen to prevent screen blinding. In weighted muds, screens of 200 mesh (74 microns) can be used, which is the upper size for commerical barite. For unweighted muds, the smallest practical size is 250 mesh (58 microns) for economical operation.
A Mud Cleaner In the First Field Application. The mud cleaner consisted of a bank of 20, 4-inch Pioneer hydrocyclones mounted above a stainless steel, double-deck, round Sweco shaker. The mud cleaner was used during 40 days of drifing from 11,000 to 16,000 feet, with 11 ppg mud, through gas sands with formation pressures equivalent to 2.2 ppg mud weight. Stuck pipe and lost circulation experienced on earlier wells was eliminated. The mud cleaner was deactivated for drilling from 16,000 to 16,200 feet to "find" a caseing seat. Torque and drag on the drill string increased significantly, and wiper trips with the drill string were required between logging runs. The mud cleaner was reactivated and "cleaned" the drilling fluid for one circulation through a 150 x 150 mesh screen, followed by one circulation using a 2C0 x 200 mesh screen before running 9f-inch casing in a 12-¿■-inch hole. No problems were encountered whie running the casing or during the cementing operation.
A more recent development, introduced in the 1980s, is the linear shaker. Developments in screen technology have made it possible for pretensioned screens to be layered for obtaining very precise cuts while still maintaining an economical screen life. Linear motion is the best conveying motion to move the solids off the screen, and it is possible to convey cuttings "uphill." Screens can be elevated to retain the cuttings longer to obtain a dryer-reduced liquid content discharge. Also, finer screens, with smaller openings, can be used on the linear motion shaker. One application of linear shakers is to screen the underflow from desanders and desilters rather than using a mud cleaner. This device is called a "mud conditioner."
Present technology includes liquid salvage— dewatering or solids flocculation—that strips the liquid phase from solids and returns an almost clear stream of water into the mud system. This process includes a decanting centrifuge with pre-
mixed polymers injected into the feed line of the centrifuge causing flocculation. The solids are coalesced inside the centrifuge resulting in separation of solids from the liquid, and the solids are then discarded.
A recent innovation for environmental purposes and more liquid retention, is the dryer. The discharge from linear shakers, desanders, and desilters is flowed across another linear shaker with even finer screens (down to 450 mesh, or 32 microns) and usually a larger screening surface. Any liquid that escapes can be retained in the sump. The sump pump returns the liquid to the active system, usually to the centrifuge feed tank.
These systems, or combinations of the various items discussed above, meet most environmental requirements and conserve expensive liquid phases. The desirable effect is to close the loop on liquid discharges, leaving a damp, semi-dry solid mass to remove for disposal.
A Mud Cleaner in the First Commercial Application. This installation, in Pecan Island Field in Louisiana, occurred within one year after the patent was submitted. The mud cleaner consisted of a manifold arranged for eight, 4-inch hydrocyclones mounted above a single-deck, round Sweco shaker. This was the first well in this field that casing could be reciprocated while cementing.
The First Mud Cleaner Installation, in 1971, Power Rig #10 was drilled M. J. Foster in South Louisiana. The mud cleaner concept had previously been tested in a 1,900-foot well near Houston, Texas. This unit consisted of a Sweco shale shaker, five feet in diameter, receiving solids from the underflow of two, 12-inch desanders and twenty, 4-inch desilters while circulating a potassium chloride drilling fluid The rig shown has a degasser mounted on the mud tank in front of the mud cleaner skid and a centrifuge mounted on top of the tank after the mud cleaner. The mud cleaner skid was covered by a tin roof with additional lighting to facilitate sampling.
The Second Field Model. The mud cleaner included 2 open Cop, single-deck Sweco shale shakers processing underflow from sixteen, 4-inch Magcobar desilters.
Disposal, in some locations, consists of a cuttings injection system that blends cuttings into a slurry inside a specially modified centrifugal pump (shear pump). This slurry is pumped down the well annulus between two strings of casing, or the casing and wellbore.
An innovation recently made available on the Gulf coast is the "gumbo chain," or gumbo screen belt. It is used to discard gumbo and large, pliable cuttings typical of coastal and offshore drilling. The gumbo chain is a special conveyor, in a channel or trough, which drags gumbo and large, pliable cuttings out of the drilling lluid. The gumbo screen belt is an endless belt of 5-10 mesh synthetic cloth that moves gumbo up out of a liquid pool. This operation reduces the severe screen loading problems caused by gumbo in typical screening operations. The devices can be mounted in the flow line or as an accessory unit in the solids control system.
In summary, specific requirements for a given drilling fluid system and prevailing economic factors, will dictate the need for using specific items of solids removal equipment. The solids removal system may be simple using one shale shaker, or very complex using a multiple of one or more different items discussed above for complete separation of solids from liquid. Factors controlling the items used in a drilling fluid system include economics and the ever-increasing requirements of the environmental sector.
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