Abstract:
A hydraulic strain sensor for use with a downhole tool includes a housing having two chambers with a pressure differential between the two chambers. A mandrel is disposed in the housing. The mandrel is adapted to be coupled to the tool such that the weight of the tool is supported by the pressure differential between the two chambers. A pressure-responsive sensor in communication with the one of the chambers is provided to sense pressure changes in the chamber as the tool is accelerated or decelerated and to generate signals representative of the pressure changes.

Description:
This application is a continuation and claims the benefit under 35 U.S.C. §120 to U.S. patent application Ser. No. 09/663,372, filed on Sep. 12, 2000, now U.S. Pat. No. 6,389,890 issued on May 21, 2002, which is a continuation of U.S. patent application Ser. No. 09/267,498, filed on Mar. 12, 1999, which became abandoned on Oct. 27, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates generally to electrical downhole tools which are employed for various downhole oil-field applications, e.g., firing shaped charges through a casing and setting a packer in a wellbore. More particularly, the invention relates to a pressure-actuated downhole tool and a method and an apparatus for generating pressure signals which may be interpreted as command signals for actuating the downhole tool. 
     2. Background Art 
     Electrical downhole tools which are used to perform one or more operations in a wellbore may receive power and command signals through conductive logging cables which run from the surface to the downhole tools. Alternatively, the downhole tool may be powered by batteries, and commands may be preprogrammed into the tool and executed in a predetermined order over a fixed time interval, or command signals may be sent to the tool by manipulating the pressure exerted on the tool. The downhole pressure exerted on the tool is recorded using a pressure gage, and downhole electronics and software interpret the pressure signals from the pressure gage as executable commands. Typically, the downhole pressure exerted on the tool is manipulated by surface wellhead controls or by moving the tool over set vertical distances and at specified speeds in a column of fluid. However, generating pressure signals using these typical approaches can be difficult, take excessively long periods of time to produce, or require too much or unavailable equipment. Thus, it would be desirable to have a means of quickly and efficiently generating pressure signals. 
     SUMMARY OF THE INVENTION 
     In general, in one aspect, a hydraulic strain sensor for use with a downhole tool comprises a housing having two chambers with a pressure differential between the two chambers. A mandrel disposed in the housing is adapted to be coupled to the tool such that the weight of the tool is supported by the pressure differential between the two chambers. A pressure-responsive member in communication with one of the chambers is arranged to sense pressure changes in the one of the chambers as the tool is accelerated or decelerated and to generate signals representative of the pressure changes. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a downhole assembly for use in performing a downhole operation in a wellbore. 
     FIG. 2 is a detailed view of the hydraulic strain sensor shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings wherein like characters are used for like parts throughout the several views, FIG. 1 depicts a downhole assembly  10  which is suspended in a wellbore  12  on the end of a conveyance device  14 . The conveyance device  14  may be a slickline, wireline, coiled tubing, or drill pipe. Although running the downhole assembly into the wellbore on a slickline or wireline is considerably faster and more economical than running on a coiled tubing or drill pipe. The downhole assembly  10  includes a hydraulic strain sensor  16  and a downhole tool  18  which may be operated to perform one or more downhole operations in response to pressure signals generated by the hydraulic strain sensor  16 . For example, the downhole tool  18  may be a perforating gun which may be operated to fire shaped charges through a casing  19  in the wellbore  12 . 
     The hydraulic strain sensor  16  includes a sealed chamber (not shown) which experiences pressure changes when the downhole tool  18  is accelerated or decelerated and a pressure-responsive sensor, e.g., a pressure transducer (not shown), which detects the pressure changes and converts them to electrical signals. The hydraulic strain sensor  16  communicates with the downhole tool  18  through an electronics cartridge  20 . The electronics cartridge  20  includes electronic circuitry, e.g., microprocessors (not shown), which interprets the electrical signals generated by the pressure transducer as commands for operating the downhole tool  18 . The electronics cartridge  20  may also include an electrical power source, e.g., a battery pack (not shown), which supplies power to the electrical components in the downhole assembly  10 . Power may also be supplied to the downhole assembly  10  from the surface, e.g., through a wireline, or from a downhole autonomous power source. 
     Referring to FIG. 2, the hydraulic strain sensor  16  comprises a hydraulic power section  22  and a sensor section  24 . The hydraulic power section  22  includes a cylinder  26 . A fishing neck  28  is mounted at the upper end of the cylinder  26  and adapted to be coupled to the conveyance device  14  (shown in FIG. 1) so that the hydraulic strain sensor  16  can be lowered into and retrieved from the wellbore on the conveyance device. With the fishing neck  28  coupled to the conveyance device  14 , the hydraulic strain sensor  16  and other attached components can be accelerated or decelerated by jerking the conveyance device. The fishing neck  28  may also be coupled to other tools. For example, if the conveyance device  14  is inadvertently disconnected from the fishing neck  28  so that the hydraulic strain sensor  16  drops to the bottom of the wellbore, a fishing tool, e.g., an overshot, may be lowered into the wellbore to engage the fishing neck  28  and retrieve the hydraulic strain sensor  16 . The fishing neck  28  may be provided with magnetic markers (not shown) which allow it to be easily located downhole. 
     A mandrel  30  is disposed in and axially movable within a bore  32  in the cylinder  26 . The mandrel  30  has a piston portion  34  and a shaft portion  36 . An upper chamber  38  is defined above the piston portion  34 , and a lower chamber  40  is defined below the piston portion  34  and around the shaft portion  36 . The upper chamber  38  is exposed to the pressure outside the cylinder  26  through a port  42  in the cylinder  26 . A sliding seal  44  between the piston portion  34  and the cylinder  26  isolates the upper chamber  38  from the lower chamber  40 , and a sliding seal  46  between the shaft portion  34  and the cylinder  26  isolates the lower chamber  40  from the exterior of the cylinder  26 . The sliding seal  44  is retained on the piston portion  34  by a seal retaining plug  48 , and the sliding seal  46  is secured to a lower end of the cylinder  26  by a seal retaining ring  50 . 
     The sensor section  24  comprises a first sleeve  52  which encloses and supports a pressure transducer  54  and a second sleeve  56  which includes an electrical connector  58 . The first sleeve  52  is attached to the lower end of a connecting body  62  with a portion of the pressure transducer  54  protruding into a bore  64  in the connecting body  62 . An end  66  of the shaft portion  36  extends out of the cylinder  26  into the bore  64  in the connecting body  62 . The end  66  of the shaft portion  26  is secured to the connecting body  62  so as to allow the connecting body  62  to move with the mandrel  30 . Static seals, e.g., o-ring seals  76  and  78 , are arranged between the connecting body  62  and the shaft portion  36  and pressure transducer  54  to contain fluid within the bore  64 . 
     The second sleeve  56  is mounted on the first sleeve  52  and includes slots  80  which are adapted to ride on projecting members  82  on the first sleeve  52 . When the slots  80  ride on the projecting members  82 , the hydraulic strain sensor  16  moves relative to the downhole tool  18  (shown in FIG.  1 ). A spring  82  connects and normally biases an upper end  84  of the second sleeve  56  to an outer shoulder  86  on the first sleeve  52 . The electrical connector  58  on the second sleeve  52  is connected to the pressure transducer  54  by electrical wires  88 . When the hydraulic strain sensor  16  is coupled to the electronics cartridge  20  (shown in FIG.  1 ), the electrical connector  58  forms a power and communications interface between the pressure transducer  54  and the electronic circuitry and electrical power source in the electronics cartridge. 
     The shaft portion  36  has a fluid channel  90  which is in communication with the bore  64  in the connecting body  62 . The fluid channel  90  opens to a bore  92  in the piston portion  34 , and the bore  92  in turn communicates with the lower chamber  40  through ports  94  in the piston portion  34 . The bore  92  and ports  94  in the piston portion  34 , the fluid channel  90  in the shaft portion  36 , and the bore  64  in the connecting body  62  define a pressure path from the lower chamber  40  to the pressure transducer  54 . The lower chamber  40  and the pressure path are filled with a pressure-transmitting medium, e.g., oil or other incompressible fluid, through fill ports  96  and  98  in the seal retaining plug  48  and the connecting body  62 , respectively. By using both fill ports  96  and  98  to fill the lower chamber  40  and the pressure path, the volume of air trapped in the lower chamber and the pressure path can be minimized. Plugs  100  and  102  are provided in the fill ports  96  and  98  to contain fluid in the pressure path and the lower chamber  40 . 
     When the hydraulic strain sensor  16  is coupled to the downhole tool  18 , as illustrated in FIG. 1, the net force, F net , resulting from the pressure differential across the piston portion  34  supports the weight of the downhole tool  18 . The net force resulting from the pressure differential across the piston portion  34  can be expressed as: 
     
       
           F   net =( P   lc   −P   uc )·A lc    (1)  
       
     
     where P lc  is the pressure in the lower chamber  40 , P uc  is the pressure in the upper chamber  38  or the wellbore pressure outside the cylinder  26 , A lc  is the cross-sectional area of the lower chamber  40 . 
     The total force, F total , that is applied to the piston portion  34  by the downhole tool  18  can be expressed as: 
     
       
           F   total   =m   tool ( g−a )+ F   drag    (2)  
       
     
     where m tool  is the mass of the downhole tool  18 , g is the acceleration due to gravity, a is the acceleration of the downhole tool  18 , and F drag  is the drag force acting on the downhole tool  18 . Drag force and acceleration are considered to be positive when acting in the same direction as gravity. 
     Assuming that the weight of the sensor section  24  and the weight of the connecting body  62  is negligibly small compared to the weight of the downhole tool  18 , then the net force, F net , resulting from the pressure differential across the piston portion  34  can be equated to the total force, F total , applied to the piston portion  34  by the downhole tool  18 , and the pressure, P lc , in the lower chamber  40  can then be expressed as:                P     l                 c       =       1     A     l                 c              [         m     t                 o                 o                 l       ·     (     g   -   a     )       +     F     d                 r                 a                 g       +       P     u                 c       ·     A     l                 c           ]               (   3   )                                
     From the expression above, it is clear that the pressure, P lc , in the lower chamber  40  changes as the downhole tool  18  is accelerated or decelerated. These pressure changes are transmitted to the pressure transducer  54  through the fluid in the lower chamber  40  and the pressure path. The pressure transducer  54  responds to the pressure changes in the lower chamber  40  and converts them to electrical signals. For a given acceleration or deceleration, the size of a pressure change or pulse can be increased by reducing the cross-sectional area, A lc , of the lower chamber  40 . 
     In operation, the downhole assembly  10  is lowered into the wellbore  12  with the lower chamber  40  and pressure path filled with a pressure-transmitting medium. When the downhole assembly  10  is accelerated in the upward direction, the total force, F total , which is applied to the piston portion  34  by the downhole tool  18  increases and results in a corresponding increase in the pressure, P lc , in the lower chamber  40 . When the downhole tool  18  is accelerated in the downward direction, the force, F total , which is applied to the piston portion  34  by the downhole tool  18  decreases and results in a corresponding decrease in the pressure, P lc , in the lower chamber  40 . The downhole assembly  10  may also be decelerated in either the upward or downward direction to effect similar pressure changes in the lower chamber  40 . The pressure changes in the lower chamber  40  are detected by the pressure transducer  54  as pressure pulses. Moving the downhole assembly  10  in prescribed patterns will produce pressure pulses which can be converted to electrical signals that can be interpreted by the electronics cartridge  20  in the downhole tool  18  as command signals. 
     If the downhole assembly  10  becomes stuck and jars are used to try and free the assembly, the pressure differential across the piston portion  34  can become very high. If the bottom-hole pressure, i.e., the wellbore pressure at the exterior of the downhole assembly  10 , is close to the pressure rating of the downhole assembly  10 , then the pressure transducer  54  can potentially be subjected to pressures that are well over its rated operating value. To prevent damage to the pressure transducer  54 , the fill plug  100  may be provided with a rupture disc  108  which bursts when the pressure in the lower chamber  40  is above the pressure rating of the pressure transducer  54 . When the rupture disc  108  bursts, fluid will drain out of the lower chamber  40  and the pressure path, through the fill port  96 , and out of the cylinder  26 . As the fluid drains out of the lower chamber  40  and the pressure path, the piston portion  34  will move to the lower end of the cylinder  26  until it reaches the end of travel, at which time the hydraulic strain sensor  16  becomes solid and the highest pressure the pressure transducer  54  will be subjected to is the bottom-hole pressure. Instead of using a rupture disc, a check valve or other pressure responsive member may also be arranged in the fill port  96  to allow fluid to drain out of the lower chamber  40  when necessary. 
     If the downhole assembly  10  becomes unstuck, commands can no longer be generated using acceleration or deceleration of the downhole assembly  10 . However, traditional methods such as manipulation of surface wellhead controls or movement of the downhole assembly  10  over fixed vertical distances in a column of liquid can still be used. When traditional methods are used, the pressure transducer  54 , which is now in communication with the wellbore, will detect changes in wellbore or bottom-hole pressure around the hydraulic strain sensor  16  and transmit signals that are representative of the pressure changes to the electronics cartridge  20 . It should be noted that while the downhole assembly  10  is stuck, pressure signals can still be sent to the downhole tool  18  by alternately pulling and releasing on the conveyance device  14 . 
     The invention is advantageous in that pressure signals can be generated by simply accelerating or decelerating the downhole tool. The pressure signals are generated at the downhole tool and received by the downhole tool in real-time. The invention can be used with traditional methods of pressure-signal transmission, i.e., manipulation of surface wellhead controls or movement of the downhole tool over fixed vertical distances in a column of liquid. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous variations therefrom 
     without departing from the spirit and scope of the invention.