permeable zone. The water-producing zones can be plugged by use of (1) low-viscosity plastics or (2) silicon tetrafluoride gas. A catalyst injected with the plastic causes the plastic to begin to solidify when it contacts the hot formation. Silicon tetrafluoride gas reacts with the formation water and precipitates silica in the pore spaces of the rock. Best results are obtained when the water-producing formation is isolated for fluid injection by use of packers. Also, sufficient injection pressure must be used to exceed the formation pressure. Since this technique requires expending a considerable amount of rig time, the cost of isolating numerous water zones tends to offset the drilling rate improvement.
Both air and natural gas have been used as drilling fluids. An air compressor or natural gas pressure regulator allows the gas to be injected into the standpipe at the desired pressure. An example rig circulating system used for air drilling is shown in Fig. 1.31. The injection pressure usually is chosen so that the minimum annular velocity is about 3,000 ft/min. Also shown are small pumps used to inject water and surfactant into the discharge line. A
rotating head installed below the rig floor seals against the kelly and prevents the gas from spraying through the rig floor. The gas returning from the annulus then is vented through a blooey line to the reserve pit, at least 200 ft from the rig. If natural gas is used, it usually is burned continuously at the end of the blooey line. Even if air is used, care must be taken to prevent an explosion. Small amounts of formation hydrocarbons mixed with compressed air can be quite dangerous.
The subsurface equipment used for drilling with air is normally the same as the equipment used for drilling with mud. However, in a few areas where the compressive rock strength is extremely high, a percussion tool may be used in the drillstring above the bit. A cutaway view of an example percussion device is shown in Fig. 1.32. Gas flow through the tool causes a hammer to strike repeatedly on an anvil above the bit. The tool is similar in operation to the percussion hammer used by construction crews to break concrete. Under a normal operating pressure of 350 psia, the percussion tool causes the bit to hammer the formation about 1,800 blows/min in
addition to the normal rotary action. Penetration rates in extremely hard formations have been improved significantly by use of this tool.
The rotary system includes all of the equipment used to achieve bit rotation. A schematic diagram illustrating the arrangement and nomenclature of the rotary system is shown in Fig. 1.33. The main parts of the rotary system are the (1) swivel, (2) kelly, (3) rotary drive, (4) rotary table, (5) drillpipe, and (6) drill collars.
The swivel (Fig. 1.34) supports the weight of the drillstring and permits rotation. The bail of the swivel is attached to the hook of the traveling block, and the gooseneck of the swivel provides a downward-pointing connection for the rotary hose. Swivels are rated according to their load capacities.
The kelly is the first section of pipe below the swivel. The outside cross section of the kelly is square or hexagonal to permit it to be gripped easily for turning. Torque is transmitted to the kelly through kelly bushings, which fit inside the master bushing of the rotary table. The kelly must be kept as straight as possible. Rotation of a crooked kelly causes a
whipping motion that results in unnecessary wear on the crown block, drilling line, swivel, and threaded connections throughout a large part of the drillstring.
A view of a kelly and kelly bushings in operation is shown in Fig. 1.35. The kelly thread is right-handed on the lower end and left-handed on the upper end to permit normal right-hand rotation of the drillstring. A kelly saver sub is used between the kelly and the first joint of drillpipe. This relatively inexpensive short section of pipe prevents wear on the kelly threads and provides a place for mounting a rubber protector to keep the kelly centralized.
An example rotary table is shown in Fig. 1.36. The opening in the rotary table that accepts the kelly bushings must be large enough for passage of the largest bit to be run in the hole. The lower portion of the opening is contoured to accept slips that grip the drillstring and prevent it from falling into the hole while a new joint of pipe is being added to the drillstring. A lock on the rotary prevents the table from turning when pipe is unscrewed without the use of backup tongs.
Power for driving the rotary table usually is provided by an independent rotary drive. However, in some cases, power is taken from the drawworks. A hydraulic transmission between the rotary table and
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