In the drilling of deep boreholes such as oil wells, it is desirable to monitor certain downhole conditions, and to transmit information on these conditions to the surface for the guidance of the driller. Typical parameters of interest include temperature, pressure, borehole orientation or deviation from vertical, and a variety of other geophysical data relating to the formation being penetrated. The variables of interest are typically monitored by various forms of transducers which convert the parameter (e.g. pressure) into an electrical signal for transmission directly to the surface in a wired system, or which is used to command some form of wireless telemetry system for transmitting a surface-detactable signal.
Direct or "hard-wired" transmission systems present significant difficulty in that the cable or similar conductors are susceptible to damage and are awkward to manipulate during drilling. A more satisfactory approach which has found considerable acceptance in the drilling industry involves generation of drilling-fluid pressure pulses which are propagated to the surface through the flowing column of drilling fluid or "mud." These systems are wireless transmitters, and the pressure pulses typically represent binary-coded digital signals analogous to the variable being measured.
The drilling mud is pumped at high pressure from the surface downwardly through the interior of interconnected lengths of drill pipe (the "drill string") to cool and carry rock chips away from a drill bit at the bottom of the borehole. The cutting-laden mud then returns to the surface in an annular flow passage between the borehole wall and the exterior of the drill string. The mud pressure in the annulus is much lower than in the interior of the drill string above the drill bit due to the pressure drop across mud nozzles or jets in the bit.
Two styles of "mud pulse" telemetry are presently known to the industry. Positive-pulse systems use a downhole valve which restricts or briefly blocks mud flow to the bit to produce a surface-detectable pressure increase or positive pulse. Negative-pulse systems bypass the pressure drop across the bit by opening a flow passage to the annulus from the drill-string interior above the bit, thus producing a surface-detectable pressure decrease or negative pulse.
U.S. Pat. Nos. 2,925,251, 3,958,217, 3,964,556 and 3,983,948 disclose positive-pulse systems. Negative-pulse systems are disclosed or discussed in U.S. Pat. Nos. 3,983,948 and 4,078,620, and in British patent application No. 2,009,473A published on June 13, 1979. The broad concept of opening a bypass channel to the annulus to signal a downhole condition is shown in U.S. Pat. No. 2,887,298. The disclosures of these publications are incorporated herein by reference to the extent that they show operating features of the overall mud-pulse telemetry system in addition to specific styles of mud valves.
The valve of this invention is for use in negative-pulse systems. In contrast to prior-art negative-pulse valves which are bi-stable (and hence remain in either an open or closed position until electrically commanded to change state), the new valve is monostable in that it will "fail safe" to a closed position in the event of most power failure or other command-system breakdowns. The risk of the valve sticking open is minimized at the expense of a relatively small consumption of electrical power needed to maintain a pilot valve in a position which hydraulically holds open the main valve. This fail-safe property is helpful in avoiding loss of necessary mud flow to the drill bit, and to avoid possible drill-string washout over a period of time.
The valve has a horizontally oriented main closure member which is mounted for movement in a direction perpendicular to the drill-string axis to minimize the influence of vertical acceleration on the valve. The closure member is hydraulically urged against a floating seat, and both the closure member and seat are replaceable in the field without dismantling of the overall valve structure. Electrical power requirements are minimized by the pilot-valve system which enables most of the energy needed to actuate the closure member to be extracted from the flowing high-pressure mud stream.
Some known mud-pulse telemetry systems require temporary stopping of a rotary-drilling operation and the attendant flow of high-pressure mud to the drill bit. These systems depend on a static column of mud for transmission of a signal to the surface. The valve of this invention, however, is suitable for logging-while-drilling ("LWD") systems which are capable of sending surface-detectable pulses through a flowing (and hence turbulent and noisy) mud column, thus avoiding the recognized problems attendant to even short-term cessation of bit rotation and mud flow.