This invention relates to an improved self-contained circulation sub used for passing drilling fluids from the interior of a drill string to its outside upon command from the surface. It particularly relates to an acoustically actuated electro-mechanical circulation sub.
Rotary drilling equipment used in the creation of oil and gas wells typically entails a long drill string which extends down into the borehole and is turned from the surface by a rotary table. The drill string itself is, simply stated, merely a series of detachable hollow pipe sections, nominally each of 30-foot length, terminated at the lower end by a drill bit. The drill string, which may be many thousands of feet long, is kept in tension to prevent its kinking. The drill bit must have some amount of downward force upon it or removal of downhole material fails to take place. The bit is weighted by drill collars which, in essence, are heavy, thick-walled sections of drill pipe. The drill collars are placed in the lower end of the drill string just above the drill bit.
Drilling through various strata of rock creates significant amounts of heat and drilling cuttings. Heat and cuttings typically are removed by drilling fluids or "muds" introduced into the drill string. The mud enters the hollow drill string at the surface, proceeds down through the drill pipe and drill collar sections, and exits the drill string through nozzles in the bit provided for that purpose. The mud cools the bit and carries the cuttings to the surface through the annular space formed by the borehole wall and the drill string.
Drilling mud performs a number of other functions during its circuit through the well. For instance, it controls subsurface pressures. Drilling a well of any significant depth requires the bit to traverse rock and sand strata of widely varying composition. Because of the variety of formation fluids contained in these strata, the pressure associated with them will vary widely. For instance, certain depleted Gulf Coast gas sands have localized pressures so low and porosities so great that a portion of the circulating mud disappears into the layers rather than returning to the surface. When this phenomenon reaches a critical point, a complete loss of mud circulation may occur. Clays, such as bentonite, can be added to the drilling mud to remedy the situation. Bentonite forms a thin, relatively impenetrable filter cake on the borehole wall in the region of the sand stratum. Proper addition of the bentonite to the mud will allow circulation of the mud to the surface to resume.
Another significant pressure problem occurs when a stratum bearing a high pressure fluid is penetrated. The localized downhole pressure may then exceed the liquid or hydraulic head of the drilling mud. If such occurs, a "kick" is observed and may result in a "blowout" of the drilling mud through the annulus at the surface. Muds having higher densities may be substituted for those already in the well. A higher density mud produces a correspondingly higher hydraulic head and can therefore keep the formation fluid in its stratum. However, until the higher density mud can be effectively introduced into the annular borehole space in the region of the offending stratum, the high pressure formation fluid entering the well must somehow be controlled.
An apparatus that keeps the high-pressure fluid in the well is known as a blowout preventer or, simply, as a "BOP". A BOP typically is a device that seals off the annular space found between the drill pipe and either the borehole wall or the well casing. It may be installed at the surface or, occasionally, downhole. The surface BOP is simple to install and operate in that size, power requirements, and control mechanisms are not overwhelming constraints. The downhole BOPs are more difficult to design because they must fit down inside a somewhat small borehole and still be operable from the surface. However, by thoughtful design of the equipment ancillary to the downhole BOP, an overall system having fast response to a "kick" and the versatility to easily circulate high density mud to the annulus above the high pressure stratum is possible.
A reliable downhole BOP is an especially large step forward in this art. A surface BOP will allow large downhole pressures to build up through the influx of formation fluids. The safety and integrity of a well can be compromised if the excessive pressures are allowed to linger. The downhole BOP suffers no such affliction since it isolates the flowing formation fluid in a small region near the drill bit.
BOPs are discussed in Ser. No. 219,631, filed Dec. 24, 1980, the entirety of which is incorporated by reference.
After a downhole BOP has been activated, the annular space and the pipe bore are closed to the circulation of mud. Since the only exit from pipe bore to annulus is by the nozzles in the now-isolated drill bit, some alternate route must be provided for circulating the drilling mud. A circulation sub provides that alternate route. It normally is placed in the drill string just above the downhole BOP. It provides controllable openings, known as circulation ports, so that drilling fluids can be passed from the inside of the drill string to the annulus.
Circulation subs in drill strings typically have been mechanically operated. A representative of this class of circulation subs is found in U.S. Pat. No. 3,941,190, to Conover. The Conover device, disclosed in conjunction with a downhole packer suitable for use as a blowout preventer, uses a spring-closed sleeve valve which prevents circulation through the ports until the device is actuated. Actuation of the circulation sub requires the insertion of a metal plug known as a "go-devil" into the pipe bore. The go-devil falls to a seat in the vicinity of the sleeve valve and utilizes the localized pressure to slide the seated go-devil and sleeve valve down and away from the mouth of the circulation ports. In order to re-start drilling operations, the driller must fish the go-devil out of the drill string along with some associated packer machinery which plugs the pipe bore.
As mentioned above, the inventive device may be acoustically actuated. Acoustically operated downhole devices are known. For instance U.S. Pat. Nos. 3,961,308; 4,073,341; and 4,129,184, all to Parker, discuss the use of acoustic waves to actuate the disaster valves often found in the tubing of production wells to shut off the flow of hydrocarbon fluids.
Another disclosure of acoustic actuation of a downhole tool is found in U.S. Pat. No. 3,233,674, to Leutwyler. In this instance an acoustic downhole receiver is used to trigger an explosive gas generator. The gas is used to inflate a resilient packer. These apparatus are obviously single use devices, the downhole portion of which must be replaced prior to each firing.
The theoretical use of torsionally propagated acoustic waves in the drill string to provide downhole communication was suggested in a paper entitled "A New Approach to Drill-String Acoustic Telemetry" by Squire and Whitehouse. This paper, SPE 8340, was presented at the Fall 1979 conference of the Society of Petroleum Engineers of AIME.
Certain other devices which bear a passing resemblance to a circulation sub are known. These devices are used either in testing formations for the presence of hydrocarbons after well completion or as disaster valves in production wells. They resemble circulation subs to the extent that they are suitable for remotely opening or closing a port in a pipe wall during the time that pipe is in a well.
Examples of such devices include the formation fluid sampling tools found in U.S. Pat. No. 3,664,415, to Wray et al and 3,850,250, to Holder, et al. Both use surface pumps to artificially enhance a downhole pressure used to open or close a sleeve valve covering the sample ports.
It is apparent that the prior art does not disclose or suggest using the pressure of the pipe bore drilling fluid as the motive fluid to open circulation ports in a circulation sub. Similarly the prior art does not show an electrically actuated control valve to control the disposition and use of such fluids. The use of a splined or slotted shaft in a control valve is not disclosed, nor is the use of an electric motor to move such a control valve. The combination of these elements with an acoustic receiver is, to our knowledge, unique.