Abstract:
A device to monitor and quantify the tension and compression forces acting on a well logging instrument string during deployment. The device eliminates the undesirable effects of downhole hydrostatic pressure on the sensors, and eliminates the need for a costly, complex, and high maintenance hydraulic pressure equalizing system in the force gage assembly. The device provides improved measurement accuracy, provides enhanced reliability and longer life of the sensors, and allows lower cost of manufacture and maintenance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a measuring device and relates in particular to a device for measuring deployment and operating forces on a well logging instrument. 
     2. Description of the Related Art 
     In the deployment of well logging instruments and devices in wells, it is desired to remotely monitor and quantify the forces applied to the instrument string by the various deployment means such as wire line/armored cable with or without assistance of well tractor, caterpillar, worm, crawler, mule, or other push/pull devices; pipe conveyed; or coiled tubing conveyed. A downhole force gage is used for sensing and monitoring the forces applied to the instrument string. 
     Existing downhole force gages, also called cable head tension sensors, typically employ strain gage sensors to monitor the mechanical strains induced by deployment forces. The strain gages are mounted on a high strength body which is housed in a sealed internal cavity of the gage assembly. The strain gages are attached and bonded with adhesive or other techniques to the strain gage body and configured electrically as a balanced bridge circuit. Mechanical strain proportional to the applied tension or compression load is induced into the strain gage body. With the bridge circuit powered by a constant, regulated d.c. voltage (typically 10 volts), the strain gage bridge outputs a signal (typically in millivolts) proportional to the applied loads. 
     When submerged in a fluid filled borehole, hydrostatic pressure impinges on the downhole instrument string and force gage assembly, and produces an external differential pressure force which acts upon the force gage assembly. These hydrostatic pressure forces induce undesired proportional offsets in the strain gage output, so a pressure equalizing system is utilized to eliminate the effects of hydrostatic pressure. 
     A typical force gage assembly is configured with a suitable floating piston (or an elastic bellows), and the internal cavity of the assembly is filled with a suitable hydraulic fluid. The floating piston (or elastic bellows) moves to accommodate any changes in the volume of the hydraulic fluid in the internal cavity due to changes in hydrostatic pressure or due to changes in temperature. By this means the internal cavity of the force gage assembly is thus pressure-equalized to external hydrostatic pressure, and also by this means the internal cavity, together with the strain gage bridge circuits and wiring, are protected from direct contact with the borehole fluids. 
     However, the typical configuration is complex, has relatively high cost of manufacture, has relatively high cost of maintenance, and requires hydraulic fluid filling of the force gage assembly. The strain gages are in contact with hydraulic fluid which can be a path of electrical leakage, and over time the hydraulic fluid can attack and degrade the strain gage adhesive bonds. The strain gages also are exposed to hydrostatic pressure which induces some inaccuracy in the output signal. Therefore, there is a demonstrated need for a force gage that eliminates the effects of downhole pressure while maintaining the sensing elements in a gas filled chamber. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above-noted and other deficiencies in the prior art and provides a downhole force gage for measuring both compression and tension forces on a well logging instrument string. 
     This invention provides more accurate load measurement by isolating the strain sensing elements from all effects of downhole pressure. The sensing elements reside in an atmospheric pressure chamber. The strain sensing member is attached to a load rod which is pressure balanced by suitable selection of multiple seal diameters such that the external pressure loads on the load rod are canceled out. Compression and tension loads are transferred to the sensing member by a plurality of load links. 
     In one aspect of the invention, strain gages are adhesively bonded to the sensing member to form a conventional bridge circuit. 
     In another embodiment, strain gages are vacuum deposited on the sensing member. 
     Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein: 
     FIG. 1 show a schematic diagram of a well logging instrument being deployed in a wellbore. 
     FIG. 2 shows a schematic diagram of a load measuring tool according to one embodiment of the present invention. 
     FIG. 3 shows a schematic diagram of a load rod according to one embodiment of the present invention. 
     FIG. 4 show a schematic diagram of a seal body according to one embodiment of the present invention. 
     FIG. 5 show a schematic diagram of the forces imposed on the load rod according to one embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic showing of a well logging instrument string  45  suspended in a borehole  65  at the end of a braided wireline  70 . The braided wireline  70  runs over pulleys (not shown) at the surface and winds on a surface winch (not shown) allowing the instrument string  45  to be moved along the borehole  65 . The instrument string  45  comprises a cable head  50  at the top end, which terminates the wireline  70  at the top; a well logging tool  60  at the bottom end; and, a force sensing instrument  55  disposed between the cable head  50  and the well logging tool  60 . When run with wireline as shown in FIG. 1, the force sensing instrument  55  measures the tension force on the instrument string  45 . In other deployment configurations (not shown) the instrument string  45  may be run into the borehole  65  using coiled tubing or jointed pipe. In these situations, the force-sensing instrument  55 , measures both tension and compression forces on the instrument string  45  as it is pushed into the hole using the coiled tubing or jointed pipe. In addition, certain wireline deployment schemes use devices such as well tractors, crawlers, and other devices to push the instrument string  45  through highly deviated or horizontal boreholes. These pushing devices result in compression forces being imposed on the instrument string  45 . 
     FIG. 2 is a schematic of the force-sensing instrument  55 . The lower sub  25  is threadably adapted on its lower end to connect to the well logging instrument  60 . A connector  22  is mounted in lower sub  25  and provides electrical connection to a mating connector in the logging instrument  60 . Alternatively, the connector  22  may include provision for both electric wire and optical fiber connections. The connector  22  has typical O-ring seals  23  and  24  to seal the lower end of sub  25  against wellbore fluid intrusion. The upper end of lower sub  25  is threadably adapted to connect to strain gage sub  18 . O-rings  19  seal out wellbore fluid in the connection. Strain gage sub  18  has a reduced cross-section  32  on which strain gages  35  are disposed in a standard strain gage bridge arrangement. Strain gages  35  may be bonded gages or vapor deposited gages. Both methods are known in the art and are not described herein. Wires (not shown) from the strain gages  35  are fed through holes  37  and  38  and fed to the connector  22 . 
     The strain gage sub  18  is coupled with threads to a lower housing  15 , and the coupling joint is sealed with o-rings  19 . Lower housing  15  has a large internal bore at one end to provide clearance for the strain gaged section of strain gage sub  18 . A smaller seal bore is at the other end to allow passage of the load rod  14  and o-ring  17  seals the lower housing  15  against fluid intrusion. The load rod  14  is inserted through the bore and joined with threads to the strain gage sub  18 , and functions to transfer external forces to the strain gage body. The internal cavity  42  containing the strain gages  35  is thus sealed and isolated from the external environment in contrast to the typical oil-filled systems. The internal cavity  42  contains air, but may alternatively contain dry nitrogen or any chemically inert gas. 
     The load rod  14 , is configured with features critical to functional performance, as shown in FIG.  2  and FIG.  3 . The thread  14   a  is provided and suitably designed to connect the load rod  14  to the strain gage sub  18 , and to withstand the applied external forces. The diameters  14   b  and  14   d  function as pressure sealing surfaces, and are also designed and proportioned to effect a balance of hydrostatic pressure forces applied to the load rod  14 . The diameter  14   c  is sized to provide mechanical shoulders as a means to transfer the external tension and compression forces. The internal diameter  14   e  provides for mechanical clearance, and the diameter  14   f  provides passage for electrical wiring and optical fibers. 
     The seal body  10 , (see FIG.  2  and FIG. 4) functions as an extension of the lower housing  15 , and provides a seal for the upper end of the load rod  14  and the top sub  1 . The critical design features of the seal body, shown in FIG. 4, are: the axial bores  10   a ,  10   b ,  10   c , the two external parallel flats  10   d , the two external windows  10   e  which are perpendicular to the two flats, and the o-rings  10   f  and  10   g . The bore  10   a  is sized to clear the outside diameter of the pull rod. Together with the o-rings  10   f  and  10   g , the bores  10   b  and  10   c  are proportioned to effect a pressure seal on the pull rod diameter  14   d  and the top sub diameter  1   d , respectively. Parallel flats  10   d  and external windows  10   e  are proportioned and arranged to provide clearance for the tension links  13 , and access to the load rod  14 . 
     As a major point of novelty as compared to other systems, the bores and o-rings are proportioned and arranged to produce a balance of hydrostatic forces acting on the load rod  14 , as shown in FIG.  5 . It can be shown that, considered as a free body, the load rod  14  is affected by hydrostatic pressure force vectors F 2 , F 1 , and F 3 . For free body equilibrium along the central axis, force vector F 2  must be equal to the sum of force vector F 1  and force vector F 3 , but opposite in direction. The interactions of the seal body  10 , the load rod  14 , and the lower housing  15 , cause the force vector F 2  to oppose the force vector F 1 . To enable the summation of force vector F 1  and force vector F 3 , a pair of tension links  13  are incorporated. 
     The tension links  13  are designed to pass through the windows  10   e  of the seal body  10  to engage the respective shoulders on the load rod  14 , and top sub  1 . This is shown in FIG.  2  and FIG.  5 . The load rod  14  is thus maintained in a state of hydrostatic equilibrium. 
     The pair of tensile links  13  are suitably proportioned to transmit the force vector F 3  and the external tension and/or compression force vectors. With the force vector F 3  applied, the load rod  14  is maintained in a state of hydrostatic equilibrium, and only the tension and/or compression force vectors are transmitted to the strain gage assembly  18 . 
     In addition to the primary function, (to monitor and quantify the external tension and/or compression forces), the strain gage sub  18  is a structural member of the instrument. 
     Referring to FIG. 2, the upper housing  9  slides over the top sub  1  and the tension links  13  and threads into the lower housing  15 . As shown in FIG. 2, the inner diameter of upper housing  9  constrains the tension link  13  to remain engaged in the notches in the seal body  10  and in the top sub  1 . In FIG. 2, anti-rotation pin  8  fits through elongated slot, in the upper housing  9  and screws into top sub  1 , preventing rotation of the top sub  1  relative to the strain gage sub  18 . This prevents torque loading of the load rod  14  and the strain gages  35  and allows measurement of only the tension and compression loads on the system. Split collars  2  clamp around top sub  1 , as shown in FIG. 2, and are fastened together by screws (not shown) in threaded holes  3 . The split collars  2  are adapted to mate with threads in the cable head  50 . O-rings  4  seal out wellbore fluid. Electrical connector  6  is inserted in top sub  1  and provides for electrical and optical fiber connection with a similar connector in the cable head  50 . Threaded pin  5  fastens the connector  6  in position in top sub  1  and seal  7  provides a seal against fluid intrusion. 
     The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes.