Abstract:
A compensating system for an inflatable element is disclosed which can be responsive to a temperature increase or decrease and still regulate the inflate pressure of the inflatable element, despite fluctuations in pressures above or below the element. A compensating piston with an atmospheric chamber is used. The compensating piston is coupled to a balancing piston. The balancing piston is ported to receive pressure from above the element on one side, and below the element on the other side. When the apparatus is run in the hole, wellbore pressure causes the compensating piston to be in the collapsed position. Upon inflation, the compensating piston strokes. A positioning mechanism positions the compensating piston in the center to allow it to handle both temperature increases and decreases. Upon complete inflation of the element, the positioning mechanism releases the balancing piston to let it float and porting is opened from above and below the inflated element to the balancing piston. The balancing piston applies an opposite load on the compensating piston to counteract either a change in inject pressure from above or formation pressure from below.

Description:
FIELD OF THE INVENTION 
     The field of this invention relates to compensation devices for maintenance of inflate pressure on an inflatable element in a downhole packer device. 
     BACKGROUND OF THE INVENTION 
     Inflatable packers have been in use in the oilfield for many years. These packers include an inflatable element which expands under the application of fluid pressure into contact with the surrounding casing or tubular to effectively seal it off. Downhole conditions can change with regard to temperature. Downhole pressures can also fluctuate due to changes in the formation pressure or injection pressures applied in the annular space above the inflated element. The pressure and/or temperature fluctuations can be quite large. If the temperature of the element increases, the inflate pressure tends to increase. Conversely, if the temperature of the element decreases, the inflate pressure tends to decrease. If these fluctuations are large enough, an element rupture can occur. Alternatively, the element can release from the casing or tubular because of insufficient internal pressures. Temperature changes are frequently accompanied by applied pressure fluctuations. A cold fluid injected into the well or a zone that is shut off can cause the pressure and temperature effects on the inflated element described above. Experience shows that there are very few instances where a temperature change occurs without an accompanying pressure change in one direction or the other. 
     Compensation devices have been attempted in the past. One example is PCT application WO 98/36152 assigned to Tech Line Oil Tools A.S. In this design, a single floating piston, having two discrete piston areas with an atmospheric chamber in between, is employed. The purpose of this compensation device is to maintain the inflate pressure at a certain ratio above the well pressure, either above or below the element. This design, however, does not accommodate the discrete responses which occur due to pressure and temperature changes which occur contemporaneously. The compensator described by Tech Line is located below the element and attempts to inflate the element by way of compensation, depending on whether a cool-down or heat-up downhole is anticipated. In other words, the specific phenomenon must be anticipated before the tool is run in the wellbore so that the compensating piston will be in the appropriate position after inflation of the element. If cool-down is anticipated, the compensating piston of this design is completely stroked so that upon cool-down, the compensating piston can move uphole toward the element to maintain the internal pressure. Conversely, the compensating piston is not stroked at all if a heat-up is anticipated. In that manner, when the heat-up occurs, downhole movement of the compensating piston can occur to its opposing travel stop to avoid pressure build-up under the element in response to the surrounding heat-up. 
     However, where the compensator is below the elements as in the Tech Line design, and cool-down is expected, cold fluid is generally being injected from the surface. In these situations, the inject pressure is applied to the element, followed by subsequent cooling of the element. The inject pressure causes the element pressure to increase, and as the element cools, the inject pressure keeps the inflate pressure elevated and renders the compensator ineffective. This is because the compensator is placed in an initial fully stroked position, and while cool-down would bring it back toward the element, the applied inject pressure overcomes the cool-down effect and keeps the compensating piston bottomed against its travel stop, making the compensation system ineffective. This combination of forces causes the element to deform at the wall where the inject pressure is applied and substantially increases the risk of failure due to the possibility of kinking ribs which can cut the wall of the inflatable element. 
     Again, in the Tech Line design where the element temperature is expected to increase, an accompanying inflation pressure above the element results in fluid being squeezed out of the element so as to drive the compensating piston down. This occurs because due to the anticipated temperature increase, the compensating piston by design is against its travel stop closest to the element when the element is inflated. In that manner, the Tech Line compensator can compensate for temperature increases as the compensating piston moves away from the inflated element. However, temperature increases, coupled with applied pressures outside the element, add together to bring the compensating piston to its downward travel stop position, once again risking severe deformation and damage to the element. 
     What is needed is a compensating device that is fully functional for temperature increases or decreases which, at the same time, has the ability to respond to applied increases or decreases in pressure from above or below the element. One of the objects of the present invention is to isolate pressure effects, leaving the compensating device the ability to be fully responsive to increases or decreases in temperature, independent of fluctuations in pressures above or below the inflated element. Those and other advantages of the present invention will be more apparent to those skilled in the art by a review of the description of the preferred embodiment below. 
     SUMMARY OF THE INVENTION 
     A compensating system for an inflatable element is disclosed which can be responsive to a temperature increase or decrease and still regulate the inflate pressure of the inflatable element, despite fluctuations in pressures above or below the element. A compensating piston with an atmospheric chamber is used. The compensating piston is coupled to a balancing piston. The balancing piston is ported to receive pressure from above the element on one side, and below the element on the other side. When the apparatus is run in the hole, wellbore pressure causes the compensating piston to be in the collapsed position. Upon inflation, the compensating piston strokes. A positioning mechanism positions the compensating piston in the center to allow it to handle both temperature increases and decreases. Upon complete inflation of the element, the positioning mechanism releases the balancing piston to let it float and porting is opened from above and below the inflated element to the balancing piston. The balancing piston applies an opposite load on the compensating piston to counteract either a change in inject pressure from above or formation pressure from below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a-f  illustrate the compensator in the run-in position. 
     FIGS. 2 a-f  show the compensator in the fully inflated position of the element. 
     FIGS. 3 a-f  show the porting changed on the balancing piston which is now free to move. 
     FIG. 4 a-f  show the latch sub being removed from the inflation housing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 a-e , the compensating device C is installed adjacent to the inflatable packer P. However, shown in FIG. 1 e  is an inflate sub  10 , which is connected to an inflatable packer of a known design at thread  12 . The inflate sub  10  is connected to inflation housing  14  at thread  16 . Lower connector  18  is connected to inflation housing  14  at thread  20 . Outer housing  22  is connected to lower connector  18  at thread  24 . Filler plug housing  26  is connected to outer housing  22  at thread  28 . Upper housing  30  is connected to filler plug housing  26  at thread  32 . Shear sub  34  is connected to upper housing  30  at thread  36 . Spring housing  38  is connected to shear sub  34  at thread  40 . Lock sub  42  is connected to spring housing  38  at thread  44 . Thread  46  is used to connect to the bridge plug assembly  47 . Accordingly, the entire outer assembly of the compensating device C has been described. 
     The compensating device C has an interior wall assembly which, beginning in FIG. 1 e , comprises a multi-component mandrel made up of interconnected sleeves  48  and  50 , which is in turn connected to latch sub  52 , shown in FIG. 1 b . These sleeves  48  and  50 , as well as latch sub  52 , are collectively referred to as the mandrel  54 . Mandrel  54  is retained by collet assembly  56 , which is in turn secured to lock sub  42 . The collet assembly  56  retains a shoulder  58  on the latch sub  52  to hold it in place until the mandrel  54  is ready to be selectively removed. Removal of the mandrel  54  as shown in FIG. 4 will deflate the inflatable element. 
     Accordingly, what has been defined with the outer assembly and the mandrel is an annular space, generally described as  60 , which is broken into discrete areas based on the components located therein. Starting at the lower end or FIG. 1 d , an outer piston  62  is held in a stationary position due to tab  64  extending into groove  66 , which is defined between lower connector  18  and inflation housing  14 . Accordingly, the outer piston  62  is trapped against longitudinal movement. The outer piston  62  is a sleeve which defines an annular space  68  between itself and sleeve  50 . A compensating piston  70  is disposed in annular space  68  and further contains seals  72  and  74 , thus defining a discrete chamber using annular space  68 . Those skilled in the art will appreciate that movement of the compensating piston  70  will vary the volume of the annular space which is now a sealed chamber due to the presence of seals  72 ,  74  and  80 . Initially, atmospheric pressure is located in the space  68 , and it acts on surface  76  to put a very small uphole force on the compensating piston  70 , which varies as a function of its internal pressure. Outer piston  62  has a top end  78  (see FIG. 1 d ), which acts as a lower travel limit for the compensating piston  70 . Outer piston  62  further has a seal  80  in contact with sleeve  50  for complete isolation of the space  68 , which has an initial charge preferably of atmospheric pressure, but other pressures can be used without departing from the spirit of the invention. 
     Referring to FIGS. 1 c-e , it can be seen that compensating piston  70  creates an annular space  82 , which extends from surface  84  down to the inflate sub  10 . Fluid communication with the inflatable element occurs through passage  86  into space  82 , all the way through to surface  84  on compensating piston  70 . Space  68  is, of course, isolated from the inflate pressure found in space  82  due to the presence of seals  72 ,  74 , and  80 . Accordingly, an increase in the inflate pressure of the element  27  is communicated through passage  86  into space  82  as a force against surface  84 . 
     Inner spacer  88  is mounted above surface  90  on compensating piston  70 . The area of surface  90  is designed to be larger than the area of surface  84 , with the preferred ratio being approximately 1.3:1. This results in a magnification of the net force applied to the underside of the inflated element due to pressure on surface  90  by a ratio of the areas of surface  90  divided by surface  84 . This neglects the area of surface  76  because the pressure acting on it is so low. In the run-in position shown in FIG. 1 c , the inner spacer  88  merely rests on surface  90 . 
     Compensating piston  70  defines an annular space  92  in which the inner spacer  88  is found. Filler plug housing  26  has a filler port  94 , which allows pressure in the annular space in the wellbore outside of filler plug housing  26  and above the inflated element  27  to be communicated into passage  92 . 
     Also located in space  92  is balancing piston  96 . Seals  98  and  100  mounted on opposite sides of balancing piston  96  effectively define the variable upper reaches of space  92 . Surfaces  102  and  104  are exposed to the pressure in space  92  and through port  94  to the pressure in the annulus in the wellbore above the set inflated element  27 . 
     In the run-in position, dog or dogs  106 , supported on a shear ring  108  and extending through an opening  110  in extension sub  112 , act as the upper travel limit for the balancing piston  96 . 
     Connected to extension sub  112  is spring piston  114 . A spring  116  bears on shear sub  34  on one end and on shoulder  118  on spring piston  114 . Resisting the uphole bias of spring  116  is a series of locking segments  120 . Locking segments  120  are preferably in quarter sections featuring an external groove  122  within which is located a band spring  124 . In the run-in position shown in FIG. 1 a , the locking segments  120  engage shoulder  126  on lock sub  42 . Accordingly, upward movement of the spring piston  114 , responsive to the bias force of spring  116 , is resisted by contact with shoulder  126  by locking segments  120 . 
     Spring housing  38  has a port  128 . Spring piston  114  has a recess  130  opposite port  128  in the run-in position shown in FIG. 1 a . Seal  132 , in conjunction with seal  134 , defines an annular space  136  above spring piston  114 . During run-in, mandrel  54  is obstructed at its lower end to allow element inflation. As a result of inflation and subsequent release of the bridg plug, mandrel  54  allows communication from below the element to port  138 , while above port  138  the mandrel  54  is obstructed. A port  138  extends through the mandrel  54  at sleeve  50  to allow fluid communication from the formation below the inflated element up to and above spring piston  114  at annular space  136 . In the run-in position, downward movement of spring piston  114  is limited by shoulder  150 . Annulus pressure outside of port  128 , in the run-in position, cannot communicate with space  136  due to the presence of seals  132  and  134 . However, the presence of recess  130  allows annular pressure through port  128  to communicate down to balancing piston  96  at surfaces  140  and  142 . Since the same annulus pressure at port  128  is also present at port  94 , and the surface areas of surfaces  102  and  104  are equal to surface areas of surfaces  140  and  142 , the balancing piston  96  is in pressure balance during the run-in procedure. 
     As shown in FIG. 1 b , a shear release ring  144  is held by a shear pin  146 . The shear release ring  144  abuts the spring piston  114  to prevent its downhole movement until a predetermined force exists in annular space  136 , as will be explained below. 
     In the run-in position, another annular space  148  is defined above the balancing piston  96  and extends from surfaces  140  and  142  and on both inside and outside of extension sub  112  and spring piston  114  up to seals  132  and  134  on spring piston  114 . In the run-in position, port  128  aligns annulus pressure around the compensating device C into annular space  148 . Seals  132  and  134  effectively isolate space  136  from space  148 . 
     The key components of the compensating device having been described, its operation after run-in will now be reviewed in more detail. 
     Inflate pressure is applied through the mandrel  54  to the inflatable element. As the pressure inside of the mandrel  54  rises, the pressure in space  136  rises as well due to the open communication because of port  138 . Due to ports  128  and  94 , communication of external annulus pressure occurs in the area around recess  130  and against surfaces  102  and  104  on balancing piston  96 , respectively. Since the annulus pressure remains constant and the internal pressure in the mandrel  54  is building up, a sufficient force imbalance occurs on the assembly of spring piston  114  and extension sub  112 . Eventually, the shear pin  146  is broken, allowing the assembly of spring piston  114  and extension sub  112  to move downwardly, compressing spring  116 . Downward motion continues until the shear release ring  144  bottoms on shoulder  150 . As that movement occurs, the dogs  106  may push the balancing piston  96  downwardly if it happens to be adjacent at that time. At the same time, a rise in the inflate pressure brings the pressure up in passage  86 , communicating to annular space  82 , thus increasing the pressure seen by surface  84 . In view of port  94 , the pressure seen at surface  90 , which is opposite surface  84  on compensating piston  70 , remains the annulus pressure outside the compensating device C. Accordingly, with a build-up of pressure in annular space  82  against a reference pressure of annulus pressure in space  92 , the compensating piston  70  moves uphole, taking with it inner spacer  88 . The pressure required to initiate this movement in the preferred embodiment where the ratio of surfaces  90  to  84  is 1:1.3 is 30% above annulus pressure. This assumes that the initial pressure in chamber  68  is atmospheric or a negligibly small pressure. Eventually, inner spacer  88  contacts surface  104  on balancing piston  96 , as shown in FIGS. 2 b  and  2   c . FIGS. 2 b  and  2   c  also show the balancing piston  96  somewhat downwardly shifted, with the bottoming of shear release ring  144  on shoulder  150 . 
     As shown in FIG. 2c, the compensating or movable piston  70  is disposed approximately midway between top end  78  of outer piston  62 , which comprises the lower travel stop, and shoulder  152 , which comprises the upper travel stop. Shoulder  152  is on filler plug housing  26 . The spacer  88  dictates the position of compensating piston  70  when it contacts balancing piston  96 . 
     Eventually, sufficient pressure is applied inside of mandrel  54  to fully set the element on the inflatable packer with the pressure being built up high enough for an ultimate release from the packer. As an example, the element could inflate at approximately 400 psi within mandrel  54 . A further pressure increase to around 600 psi would be used to break shear pin  146 , with the release mechanism from the packer being actuated at about 3000 psi. Subsequent to that release, the pressure inside of the mandrel  54  decreases, which allows the spring  116 , shown in FIG. 3b, to expand, pushing up spring piston  114 . Upward movement of spring piston  114  takes seal  134  past surface  154 , which is on the outside of the mandrel assembly  54 . The upward movement of spring piston  114  in effect aligns port  138  to annular space  148 . Thus, the pressure below the set inflatable packer is communicated through the mandrel  54  into port  138  to above the balancing piston  96  within annular space  148 . At the same time, the upward movement of spring piston  114  shifts recess  130  sufficiently so as to bring seal  156  in juxtaposition with surface  158 , effectively closing off port  128  by virtue of seals  132  and  156  which straddle port  128  on spring piston  114 . Therefore, in the position shown in FIGS. 3 a-e , the balancing piston  96  is now freely floating, with surfaces  102  and  104  in annular space  92  exposed to annulus pressure above the set inflatable through port  94 , while opposing surfaces  140  and  142  are exposed to the formation pressure below the set inflatable by communication through the mandrel  54  and port  138 . The ability of the balancing piston to float occurs because the upward movement of spring piston  114  pulls the dogs  106  off of shear ring  108 , as shown in FIG. 3 b . Accordingly, the new upper travel stop of the balancing piston  96  once the dogs  106  retract inwardly, as shown in FIG. 3 b , is surface  160  on shear sub  34 . During inflation, the element is inflated to well above the annulus presure so that the internal pressure exceeds the annulus pressure by more than the 30% area difference in the surfaces  90  and  84 . Upon release of balancing piston  96 , the inflate pressure in chamber  82  will decrease as piston  70  moves up slightly until the pressure in chamber  82  is about 30% higher than the pressure in chamber  92 . Again, this balance is dictated by the area ratios of surfaces  90  and  84 , neglecting surface  76  because pressure in chamber  68  is presumed negligible. In the ideal situation, upon the conclusion of inflation of the element in the packer, the downward forces on surfaces  140  and  142  should offset the upward forces on surface  84  so that very little net residual movement of balancing piston  96 , spacer  88 , and compensating piston  70  occurs. Depending on the area difference between surfaces  140  and  142  on one hand, and surface  84  on the other hand, there may be a slight shifting of compensating piston  70  immediately after inflation. However, despite this slight shifting, the compensating piston should be close to its mid-point in its available travel range between top end  78  of outer piston  62  and surface  152  on filler plug housing  26 . 
     If purely thermal loads are applied with no pressure changes experienced, the compensator works to adjust by moving. Thus, if the temperature decreases, the compensating piston  70  moves downwardly toward top end  78  of outer piston  62 . Conversely, if the temperature increases, the opposite movement of compensating piston  70  occurs toward shoulder  152 . Upward movement toward shoulder  152  by compensating piston  70  will move balancing piston  96  with it. Opposite movement by compensating piston  70  toward top end  78  of outer piston  62  will simply allow the entire assembly, including balancing piston  96 , to shift downwardly. Thus, without any pressure changes occurring downhole, the compensating device C of the present invention functions in response to increasing or decreasing temperatures by virtue of translation between its travel stops  78  and  152 . 
     It may occur that there is injection pressure applied outside the compensating device C at the same time as a temperature change is occurring. If the injection pressure in the annular space outside the compensating device C increases, the pressure in annular space  92  will also increase. The formation pressure below the set packer will remain the same and the pressure will be communicated through port  138  into annular space  148  on the other side of balancing piston  96  from annular space  92 . Thus, an unbalanced force will occur on balancing piston  96 , tending to drive it uphole. At the same time, the increased injection pressure in the annular space, communicated through port  94  into annular space  92 , will be applied to surface  90 . Since surface  90  is larger than surface  84  by some predetermined ratio, a boost force is applied to passage  82  and, in turn, through passage  86  to under the element to keep it from collapsing under the increased injection pressure in the annular space outside the compensating device C. The net result should be a small movement of compensating piston  70 , thus still leaving it between its travel stops  78  and  152  so that it is continually able to compensate for increases or decreases in temperature. It should be noted that upon increase in the pressure of the annular space outside the compensating device C, the residual pressure in annular space  68 , which started at a predetermined value such as atmospheric, also acts to move the compensating piston  70  upwardly by exerting a very small force on surface  76 . 
     Another possible scenario is that the annulus pressure drops outside the compensating device C. When this occurs, there is a net unbalanced downward force on the balancing piston  96  because the formation pressure remains constant, as does the pressure in annular space  148  which acts on surfaces  140  and  142 . However, with the outer annular pressure dropping and communication occurring with surfaces  102  and  104  through port  94 , the balancing piston  96  is urged downwardly. When contact is made with the inner spacer  88 , the unbalanced downward force on balancing piston  96  is transferred to compensating piston  70 . However, with the decrease in the annulus pressure, the pressure in annular space  92  is also decreasing. The pressure under the inflatable element, communicated to annular space  82 , creates a net upward force on compensating piston  70 . These two forces in opposite directions offset, perhaps with minor movement of the assembly due to the area differences of surfaces  102  and  104  compared to surface  84 . This is because the pressure from below, communicated and applied to surfaces  140  and  142 , results in a force which is offset by the inflate pressure under the inflatable element acting on the area of surface  84 . Thus, when the compensating piston  70  in the circumstance of decreasing external annular pressure finds its equilibrium position, the ratio of the inflate pressure under the inflatable element and the formation pressure below is equal to the area of surfaces  140  and  142  divided by the area of surface  84 . Ideally, the area of surfaces  140  and  142  should be between the areas of surface  84 , on the one hand, and  90 , on the other hand, and slightly larger than surface  84 . For the purposes of simplification of the analysis, the area of surface  76  exposed to the annular space  68  is ignored. Thus, the force balance is as follows: The formation pressure below acts downwardly on surfaces  140  and  142 . Surfaces  140  and  142  are equal in cross-sectional area to surfaces  102  and  104 . Thus, there is an upward force on the surfaces  102  and  104  by virtue of the outer annulus pressure. The inflate pressure under the element acts on surface  84  upwardly, while the annulus pressure through port  94  acts downwardly on surface  90 . Surface  90  is identical in area to surfaces  102  and  104  together or  140  and  142  together. The force balance simplifies to the formation pressure from below the inflatable element acting on an area such as surfaces  140  and  142  equals the inflation pressure under the inflatable element acting on the area of surface  84 . From that the relationship is derived where the inflation pressure under the element equals the formation pressure below the element times the ratio of the areas of, for example, surface  90  divided by surface  84 . 
     In the event of an increase in pressure from the formation, the annulus pressure above the inflated element and outside of the compensating device C remains the same. However, the increase in the formation pressure is communicated through port  138  onto the balancing piston  96 . Since the pressure above the balancing piston  96  is increasing while the outer annulus pressure remains constant, there is a net downward force on balancing piston  96 . This is communicated through spacer  88  to the compensating piston  70 . At the same time, the rising formation pressure tends to increase the inflate pressure, which presents an offsetting force in annular space  82  acting on surface  84 . Thus, because the formation pressure increases and such pressure is communicated to above the balancing piston  96 , any tendency to increase the inflate pressure, due to a rise in formation pressure, creates an offsetting uphole force on compensating piston  70 . The increased inflate pressure acts on surface  84 , thus offsetting the downhole increased force applied by a pressure increase from the formation acting in annular space  148  on the balancing piston  96 ,. Since the areas of surfaces  140  and  142  on the one hand are only slightly larger than area  84 , the assembly of the balancing piston  96  and compensating piston  70  finds a new equilibrium position while still leaving the compensating piston  70  between its travel stops  78  and  152 . In that position, it can still further respond to thermal effects, regardless of the increase in formation pressure. 
     Those skilled in the art can appreciate that a drop in the annulus pressure outside the compensating device C and above the inflated element causes the same reaction as pressure increase in the formation below the inflated element. Similarly, the situation of additional pressure applied to the annulus outside the compensating device C is similar to a reduction in the formation pressure below the inflated element. 
     FIGS. 4 a-f  illustrate the removal of the mandrel  54  which causes the breaking of shear pin  160  attached to shear ring  108 . In order to accomplish this, the collets  56  release shoulder  58  so that the mandrel assembly  54 , including the latch sub  52 , can be pulled out. This action deflates the element. 
     Accordingly, the compensating device C of the present invention is able to continue functioning to compensate for thermal variations upward or downward, despite the overlay of pressure changers whether those are increases or decreases and whether their origin is in the formation below the inflated element or in the annular space above the inflated element. The design is simple and compact and can prevent failure or release as an anchor which was possible with some of the prior art designs, such as the Tech Line design described in the background of the invention. 
     Although the preferred embodiment shows the assembly of pistons above the element, they both can be below the element and still function identically to compensate for pressure and temperature effects. The compensating piston  70  would have one end exposed to the formation pressure and the balancing piston  96  would have one end exposed to the annular space. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.