Patent Publication Number: US-5896939-A

Title: Downhole measurement of depth

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing date of patent application Ser. No. 96109124.6 filed on Jun. 7, 1996 with the European Patent Office 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a method and an apparatus for the underground ascertainment of the depth of a bore sunk in underground formations. The underground ascertainment of depth is important to the execution of drilling projects in which the directional pattern of the bore hole is programme-controlled on a basis of data acquired underground. 
     SUMMARY OF THE INVENTION 
     For the underground determination of depth, it is already known (U.S. Pat. No. 5,341,886) to equip the drilling tool used with a sensing wheel which as drilling proceeds runs over the walls of the bore hole and makes the distance travelled available as a measured value to a computer accommodated in the housing of the drilling tool and serving as a central processor. Alternatively, the depth can also be obtained by magnetic marking of the bore hole wall and recognising the marking by a magnetically sensitive measuring device. In this respect the magnetic marking device is accommodated in the housing of the drilling tool at a predetermined distance upstream of the measuring device in the direction of drilling. Response of the measuring device to a magnetic marking consequently takes place each time the predetermined distance between marking and measuring device is travelled. 
     The invention is concerned with the problem of providing a method and an apparatus for underground ascertainment of depth and which guarantee reliable functioning combined with considerable simplicity. 
     The invention resolves this problem by a method having the features set out in claim 1 and by an apparatus having the features set out in claim 6. For fiber development of the method, reference is made to claims 2 to 5. 
     With a simple sensor, the invention ascertains one of the typical variations in the drilling parameters during attachment of a tubular rod part to the drilling rod, so that in conjunction with a programme-related preset of the length measurement of the pipe rod parts used, the underground central processor can at brief intervals be provided with an updated and precise depth value such as is required for the programmed control of the drilling process. The manner in which this is done is extremely simple, operationally reliable and have a long effective life. 
     BACKGROUND OF THE INVENTION 
     The invention is explained in greater detail hereinafter with reference to the accompanying drawing which diagrammatically and by way of example shows a drilling device with a drilling tool and comprising a device for underground ascertainment of depth. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is an illustration of the invention showing the above ground parts and the downhole device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As the drawing shows in greater detail, the above-ground portion of the drilling equipment comprises a conventional drilling tower 1 with a stock 2 of rods in which tubular rod parts (drill string segments) 3 of the same and predetermined length are housed. The tubular rod parts 3 are taken from the rod stock 2 by means of a hoist 4 and are screwed into place to become component parts of the drill line 5 to the bottom end ofwhich the drilling tool 7 illustrated diagrammatically in the broken-away part 6 is connected. The drilling tool disposed in the bore hole 5 in the underground formation 9 comprises a housing 10 with, accommodated in this housing, a drive motor (not shown) which drives a drill bit 12 via a drive shaft 11. As the central processor, there is in the housing 10 of the drilling tool 7 a computer 13 which is supplied with measured data acquired underground, the said computer determining the directional pattern of the bore hole, the bore hole profile. A computer 13 comprises a data memory 14 to receive predetermined data and is equipped with a programme which, in addition to other measured data such as the inclination of the axis of the drilling tool, evaluates and processes the switching pulses (signals) of a sensor 15 which responds to variations in the pressure or flow of drilling mud. In the usual manner, these pules are passed from the surface through the drill line 5 and the drilling tool 7 and fed to the drill bit 12, emerging from the drill bit 12 for cooling and flushing purposes and flows back to the surface through the annular space in the bore hole 8. 
     The sensor is shown only diagrammatically at 15 and, like the computer 13, is disposed in a part of the housing 10 which does not rotate in relation to the drill bit 12. 
     For depth ascertainment the longitudinal measurement of the tubular pipe parts 3 intended for use is ascertained and fed into the memory 14 of the computer 13 together with the starting depth of the bore hole 8. Before the drilling tool 7 is introduced into the bore hole 8 or upon the starting depth in the bore hole 8 being reached, the computer 13 is ascertained and receives from the sensor 15 pulses which are dependent on an interruption in the flow of drilling mud or which depend upon the pressure drop in the drilling mud, such as are typical of an interruption in the drilling process when a tubular rod part 3 is attached to the drill line 5. From the pulses received by the sensor 15 and while taking into account the data stored in the memory 14, the processing programme of the computer 13 calculates the current depth which is required as a control variable or the underground control of the drilling pattern. 
     If the computer 13 is in an inactive or deactivated state, a first switching pulse delivered by the sensor 15 causes activation of the computer 13 and in this case the drilling tool 7 is generally disposed on the bottom of a bore hole 8, the starting depth of which is held in the memory 14. By a predetermined coded sequence of switching pulses from the sensor 15, the computer 13 can be changed to a special operating mode and in fact independently of the processing of switching pulses of the sensor 15 which are used for depth ascertainment. Thus, it is for example possible to change the computer to a waiting condition or to supply it with data from the surface and this may for example cause a change of programmed. 
     The depth value calculated after every use of a tubular rod part 3 is preferably passed by the computer 13 to a direction controlling device 16 in the housing 10 of the drilling tool 7 which is supported in the housing 10 above or below the computer 13 and which causes a pivoting of a bottom tool part comprising, angled over in relation to the central axis through the outer housing 10, a drive shaft 11 for the drilling tool 12, when a change in direction is commanded by the processing programme of the computer 13. Various constructions of directional drilling tools with the direction controlling device integrated into the drilling tool are known (U.S. Pat. Nos. 5,215,151; 5,339,913; 5,311,952) and they do not therefore require to be explained in greater detail here.