1. Field of the Invention
The invention relates to the field of measuring while drilling (MWD) and in particular to the communication of information from downhole within a borehole to the surface by selective manipulation of the angle or speed of rotation of the drill string.
2. Description of the Prior Art
The performance of measurements while drilling have long been practiced in petroleum technology and has become of greater commercial importance. Early activity in MWD technology generally involved communication of a single or few simple parameters between the well surface and at the bottom of the borehole. Typical prior art mechanisms for communicating to the well surface from the downhole location include: a wide variety of mud pulsing techniques for transmission of acoustical waves in the hydraulic column contained within the drill string or in the drill string itself; and radio communication down the drill string or in another types of electrical communication networks built into the drill string including direct wire and distributed networks. Generally, such prior art techniques have been unsatisfactory on account of loss of signal quality and poor data transmission rates, as is typical in the case of mud pulsing systems, or in case of radio and electrical communication systems where complex and delicate communication transmission lines are often unreliable.
For example, Lamel et al, "Telemetry System For Oil Wells," U.S. Pat. No. 3,790,930 shows a typical prior art acoustical communication system for transmitting information by establishing modular torsional acoustic waves in the drill pipe. Lamel shows both the modulation of existing acoustical waves which naturally occur in the drill string and the generation of modulated acoustal waves which are introduced into the drill string. However, as is characteristic of all acoustical transmissions, the system is noise prone, is characterized by a low data transmission rate, and requires sophisticated and delicate transducers or operation.
Park, "Method And Apparatuses for Transmission Of Data From The Bottom Of The Drill String During Drilling Of A Well," U.S. Pat. No. 3,788,136 similarly shows an acoustic system in which torque pulses in the drill string are generated by selectively creating a torsional drag downhole.
Stelzer et al, "Method And Apparatus For Transmission Of Data From Drill Bit In Wellbore While Drilling," U.S. Pat. No. 3,805,606 similarly generates torsional pulses within the drill pipe by engaging a bit through a mechanically, electrically or hydraulically operated clutch which couples and decouples the drill bit from the drill string.
What is needed is a method and apparatus for MWD telemetry which is simple, extremely rugged, uses proven downhole signal sources and which is capable of high fidelity with high data transmission rates.