Patent Publication Number: US-7708077-B2

Title: Retrieval of bottom hole assembly during casing while drilling operations

Description:
FIELD OF THE INVENTION 
   This invention relates in general to drilling boreholes with casing-while-drilling operations and in particular to methods for retrieving the bottom hole assembly. 
   BACKGROUND OF THE INVENTION 
   Casing-while-drilling is a technique that involves running the casing at the same time the well is being drilled. The operator locks a bottom hole assembly to the lower end of the casing. The bottom hole assembly has a pilot drill bit and a reamer for drilling the borehole as the casing is lowered into the earth. The operator pumps drilling mud down the casing string, which returns up the annulus surrounding the casing string along with cuttings. The operator may rotate the casing with the bottom hole assembly. Alternatively, the operator may employ a mud motor that is powered by the downward flowing drilling fluid and which rotates the drill bit. 
   When the total depth has been reached, unless the drill bit is to be cemented in the well, the operator will want to retrieve it through the casing string and install a cement valve for cementing the casing string. Also, at times, it may be necessary to retrieve the bottom hole assembly through the casing string prior to reaching total depth to replace the drill bit or repair instruments associated with the bottom hole assembly. One retrieval method employs a wireline retrieval tool that is lowered on wireline into engagement with the bottom hole assembly. The operator pulls upward on the wireline to retrieve the bottom hole assembly. While this is a workable solution in many cases, in some wells, the force necessary to pull loose the bottom hole assembly and retrieve it to the surface may be too high, resulting in breakage of the cable. 
   In another method, the operator reverse circulates to pump the bottom hole assembly back up the casing. One concern about reverse circulation is that the amount of pressure required to force the bottom hole assembly upward may be damaging to the open borehole. The pressure applied to the annulus of the casing could break down certain formations, causing lost circulation or drilling fluid flow into the formation. It could also cause formation fluid to flow into the drilling fluid and be circulated up the casing string. 
   SUMMARY OF THE INVENTION 
   In the method of retrieving a bottom hole assembly in a casing-while-drilling operation in this invention, a wireline to the bottom hole assembly, defining a retrievable unit. The density of the fluid in the casing string is lightened to a lesser density than the fluid in the annulus, thereby creating a pressure differential between fluid above the retrievable unit and fluid below the retrievable unit to exert an upward force on the retrievable unit. In addition, the operator pulls upward on the wireline to assist the upward force in moving the retrievable unit up the casing string. 
   In the preferred embodiment, a retrieval tool is conveyed down the casing string on the wireline and latched to the bottom hole assembly. The fluid in the casing string is preferably lightened by following the retrieval tool downward with a fluid having less density than the fluid in the casing annulus. As the heavier annulus fluid flows down the annulus and up into the casing string, the less dense flow up, causing an upward force the retrievable unit moves upward above the retrievable unit. 
   Preferably, the returning fluid passes through a restrictive orifice, and the operator controls the flow area of orifice as the retrievable unit ascends. In one embodiment, the operator monitors the flow rate of the returning fluid as the retrievable unit ascends. The operator also monitors the flow rate of the fluid flowing into the upper end of the annulus, and at least temporarily ceasing to pull upward on the wireline if the flow rates differ by more than a selected level. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view illustrating a drilling system for practicing a method of this invention and shown in a drilling mode 
       FIG. 2  is another view of the schematic of  FIG. 1 , showing a retrieval tool that has been pumped down into engagement with the bottom hole assembly with a less dense fluid than the fluid in the annulus. 
       FIG. 3  is an enlarged sectional view of the retrieval tool schematically illustrated in  FIG. 2 . 
       FIG. 4  is a side elevational view of the slips and spring employed with the retrieval tool of  FIG. 3 , and shown detached from the retrieval tool. 
       FIG. 5  is a sectional view of a retrieval tool of  FIG. 3 , taken along lines  5 - 5  of  FIG. 3 . 
       FIG. 6  is a further enlarged view of a portion of the retrieval tool of  FIG. 3  and shown engaging a bottom hole assembly, shown by dotted lines. 
       FIG. 7  is a graph illustrating energy required to cause heavier annulus fluid to push a bottom hole assembly upward in casing filled with a less dense fluid. 
       FIG. 8  is a graph illustrating effective borehole hydrostatic pressure during various stages of this invention. 
       FIG. 9  is another schematic view similar to  FIG. 2 , but showing the retrieval tool and bottom hole assembly moved partially up the casing string in response to the weight of the denser fluid in the casing annulus than the less dense fluid in the casing. 
       FIG. 10  is a schematic view similar to  FIG. 9 , but showing the bottom hole assembly and retrieval tool suspended by slips as the operator pumps less dense fluid down through the bottom hole assembly to refill the casing. 
       FIG. 11  is a schematic view similar to  FIG. 9 , but showing the blowout preventer closed and the operator applying surface pressure to the drilling fluid in the annulus. 
       FIG. 12  is a schematic view similar to  FIG. 9 , but illustrating the operator employing a wireline or cable in addition to reverse circulating. 
       FIG. 13  is a schematic view illustrating an alternate arrangement of equipment at the rig for use in retrieving a bottom hole assembly. 
       FIG. 14  is a view similar to  FIG. 13 , but showing the retrieval tool returning to the surface. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a borehole  11  is shown being drilled. A casing string  13  is lowered into borehole  11 . An annulus  15  is located between the sidewall of borehole  11  and casing string  13 . One or more strings of casing  17  have already been installed and cemented in place by cement  18 , although the drawings shows only one casing string for convenience. Annulus  15  thus extends from the bottom of casing string  13  up the annular space between casing string  13  and casing  17 . 
   A wellhead assembly  19  is located at the surface. Wellhead assembly  19  will differ from one drilling rig to another, but preferably has a blowout preventer  21  (BOP) that is capable of closing and sealing around casing  17 . An annulus outlet flowline  22  extends from wellhead assembly  19  at a point above BOP  21 . An annulus inlet flowline  23  extends from wellhead assembly  19  from a point below BOP  21 . 
   Casing string  13  extends upward through an opening in rig floor  25  that will have a set of slips (not shown). A casing string gripper  27  engages and supports the weight of casing string  13 , and is also capable of rotating casing string  13 . Casing string gripper  27  may grip the inner side of casing string  13 , as shown, or it may alternately grip the outer side of casing string  13 . Casing string gripper  27  has a seal  29  that seals to the interior of casing string  13 . Casing string gripper  27  is secured to a top drive  31 , which will move casing string gripper  27  up and down the derrick. A flow passage  33  extends through top drive  31  and casing gripper  27  for communication with the interior of casing string  13 . 
   A hose  35  connects to the upper end of flow passage  33  at top drive  31 . Hose  35  extends over to a discharge port  36  of a mud pump  37 . Mud pump  37  may be a conventional pump that typically has reciprocating pistons. A valve  39  is located at outlet  36  for selectively opening and closing communication with hose  35 . The drilling fluid circulation system includes one or more mud tanks  41  that hold a quantity of drilling fluid  43 . The circulation system also has screening devices (not shown) that remove cuttings from drilling fluid  43  returning from borehole  11 . Mud pump  37  has an flowline inlet  45  that connects to mud tank  41  for receiving drilling fluid  43  after cuttings have been removed. A valve  46  selectively opens and closes the flow from mud tank  41  to an inlet of mud pump  37 . A centrifugal charging pump (not shown) may be mounted in flowline  45  for supplying drilling fluid  43  to mud pump  37 . Mud pump  37  may have an outlet that is connected to annulus fill line  23  for pumping fluid down casing annulus  15  and back up the interior of casing string  13 . 
   A bottom hole assembly  47  is shown located at the lower end of casing string  13 . Bottom hole assembly  47  may include a drill lock assembly  49  that has movable dogs  51  that engage an annular recess in a sub near the lower end of casing string  13  to lock bottom hole assembly  47  in place. Drill lock assembly  49  also has keys that engage vertical slots for transmitting rotation of casing string  13  to bottom hole assembly  47 . Dogs  51  could be eliminated, with the bottom hole assembly  47  retained at the lower end of casing string  13  by drilling fluid pressure in casing string  13 . An extension pipe  53  extends downward from drill lock assembly  49  out the lower end of casing string  13 . A drill bit  55  is connected to the lower end of extension pipe  53 , and a reamer  57  is mounted to extension pipe  53  above drill bit  55 . Alternately, reamer  57  could be located at the lower end of casing string  13 . Logging instruments may also be incorporated with extension pipe  53 . A centralizer  59  centralizes extension pipe  53  within casing string  13 . 
   During drilling, mud pump  37  receives drilling fluid  43  from mud tank  41  and pumps it through outlet  36  into hose  35 , as illustrated in  FIG. 1 . The drilling fluid flows through casing gripper  27 , down casing string  13  and out nozzles at the lower end of bit  55 . Drilling fluid  43  flows back up casing annulus  15  and through return flow line  22  back into mud tank  41 . 
   The schematic of  FIG. 1  shows also a valve  61  and a flow meter  63  located in annulus inlet flowline  23 . During normal drilling operations, as shown in  FIG. 1 , no flow will be flowing through annulus inlet  23 . Another tank  65 , this one containing a less dense fluid  67 , is shown in  FIG. 1 . Less dense fluid  67  has a lower density than drilling fluid  43  and is used during the retrieval process. For example, less dense fluid  67  may be water, which has a lesser density and weight per gallon than typical drilling fluid  43 . The inlet line  66  to less dense fluid tank  65  connects to hose  35 . A flow meter  69  is preferably located in inlet line  66 . Also, a choke  71  is preferably located in inlet line  66 . Choke  71  has a restrictive, variable diameter orifice. Chokes of this nature are commonly used for drilling and well control in general. A valve  76  may be located between mud hose  35  and choke  71  to block flow to choke  71 . Tank  65  has an outlet line  68  that contains a valve  70  and which leads to an inlet of mud pump  37 . 
   A fill-up pump  72 , which is normally a centrifugal pump, may be connected in a fill-up lines extending from mud tank  41  and casing annulus  15 . A valve  74  may be located in the fill-up line between fill-up pump  72  and casing annulus  15 . The outlet of fill-up pump  72  preferably enters casing annulus  15  above BOP  21  since fill-up pump  72  is not used to apply surface pressure to the fluid in annulus  15 . 
   Referring to  FIG. 2 , a retrieval tool  73  is shown in engagement with bottom hole assembly  49 , Retrieval tool  73  preferably has a seal  75  that seals to the inner diameter of casing string  13 . This arrangement allows the operator to pump retrieval tool  73  down casing string  13  and into engagement with drill lock assembly  49 . Alternately, seal  75  could be omitted and retrieval tool  73  conveyed down casing string  13  by gravity. If seal  75  is employed, it need not form a tight seal against casing string  13 . The retrieval tool  73  latches to drill lock assembly  49  and also releases dogs  51  to allow bottom hole assembly  47  to be retrieved.  FIG. 2  illustrates retrieval tool  73  after being pumped down with less dense fluid  67  drawn from tank  65  and pumped by mud pump  37  through hose  35 . 
   Referring to  FIG. 6 , the dotted lines schematically illustrate that drill lock assembly  49  has optionally a set of seals  77  that enable drill lock assembly  49  to be pumped down along with extension pipe  53  and drill bit  55  ( FIG. 1 ). Alternately drill lock assembly  49  could have been installed in casing string  13  while casing string  13  is being made up. Seals  77  may comprise cup seals that face both upward and downward and engage the inner diameter of casing string  13  ( FIG. 1 ) for sealing against upward as well as downward pressure. It is not necessary that seals  77  form tight sealing engagement with casing string  13 , as some leakage past would be permissible. 
   Drill lock assembly  49  also has a mandrel  78  that moves upward and downward relative to an outer housing of drill lock assembly  49 . When mandrel  78  is in the lower position shown in  FIG. 6 , dogs  51  retract. When in the upper position, dogs  51  will extend out and engage a recess in casing string  13 . Furthermore, drill lock assembly  49  has a check valve  79 , shown schematically in  FIG. 6 . Check valve  79  will allow downward flow through drill lock assembly  49  but prevent upward flow. 
   Referring to  FIG. 3 , an example of retrieval tool  73  is shown. Seals  75 , if employed, may be similar to seals  77  ( FIG. 6 ); that is, seals  75  are preferably cup-shaped, with the upper seal facing downward and the lower seal facing upward. Seals  75  will slidingly engage and seal to the inner diameter of casing string  13  ( FIG. 2 ), but need not seal tightly. 
   Retrieval tool  73  has a body  80  formed of multiple pieces that has a flow passage  81  extending through it. A check valve  83  is located within flow passage  81 . Check valve  83  may be constructed similar to check valve  79  ( FIG. 6 ). In this embodiment, check valve  83  has a spring  82  that urges a valve element  84  against a seat. Check valve  83  allows downward flow in passage  81  but not upward flow. 
   A plug  85  is mounted in flow passage  81 . Plug  85  moves between a closed position shown in  FIG. 3  and an open position shown in  FIG. 6 . In the closed position, flow through passage  81  is blocked, both in an upward and in a downward direction. When moved downward to the open position, flow can circulate around an annular recess through flow ports  87  and down passage  81 , Plug  85  is preferably initially held in the closed position by a plurality of shear pins  88  ( FIG. 5 ). Downward acting fluid pressure on plug  85  of sufficient magnitude will shear the shear pins  88 . 
   Retrieval tool  73  also has a release member  89  that is employed to release drill lock assembly  49  ( FIG. 6 ) from the locked position. In this instance, release member  89  comprises an elongated tube that extends downward and into drill lock assembly  49  as retrieval tool  73  lands on drill lock assembly  49 . Release member  89  contacts mandrel  78  and pushes it downward to the released position. Others types of release mechanisms are feasible and could include grapples that pull upward on a portion of the drill lock assembly rather than being a downward acting tool. 
   A retrieval tool latch or gripper  91  is mounted to retrieval tool  73  for gripping or latching to drill lock assembly  49 . In this embodiment, retrieval tool gripper  91  comprises a collet type member with an annular base at its upper end and a plurality of fingers. Each finger has a gripping surface on its exterior for gripping the inner diameter of the housing of drill lock assembly  49 . The fingers of gripper  91  are backed up by a ramp surface  93  located at the lower end of body  80  within gripper  91 . Gripper  91  is able to slide down and out a portion of ramp surface  93  to tightly engage drill lock assembly  49 . Retrieval tool  73  thus supports the weight of drill lock assembly  49  when drill lock assembly  49  is suspended below. 
   A friction type member  95 , referred to herein as “slips” for convenience, is mounted to body  80  of retrieval tool  73 . Slips  95  comprise a gripping or clutch device that moves between a retracted position, shown in  FIG. 3  and an engaged position shown in  FIG. 6 . As shown in  FIG. 4 , slips  95  comprise in this example a collet type member having an annular base  97  and a plurality of upward extending fingers  99 . Each finger  99  has a gripping surface  101  on its outer surface. Fingers  99  slide upward and outward on ramp surface  93  when moving to the gripping position. A coil spring  103  urges fingers  99  upward to the gripping position. When retrieval tool  73  moves upward, gripping surfaces  101  slide on the inner diameter of casing string  13 . When retrieval tool  73  starts to move downward, fingers  99  wedge between ramp surface  93  and the casing string  13  inner diameter to suspend retrieval tool  73 . Other arrangements for a friction mechanism that allows upward movement but suspends the retrieval tool when moving downward are feasible. 
   A retainer mechanism initially will hold slips  95  in the retracted position. In this example, the retainer mechanism comprises a plurality of pins  105  (only one shown). Each pin  105  extends laterally through an opening in body  80  and is able to slide radially inward and outward relative to body  80 . Each pin  105  has an outer end that engages an annular recess in the inner diameter of base  97 . The inner end of each pin  105  is backed up or prevented from moving radially inward by plug  85  when plug  85  is in the blocking position shown in  FIG. 3 . When plug  85  moves to the open position shown in  FIG. 6 , pins  105  are released to slide inward, which frees slips  95  to be pushed upward by spring  103 . Other mechanisms are feasible for retaining slips  95  in the retracted position while retrieval tool  73  is being pumped down casing string  13  ( FIG. 1 ). 
   In operation of the embodiment of  FIGS. 1-10 , when it is desired to retrieve bottom hole assembly  47 , the operator drops retrieval tool  73  down casing string  13 , as shown in  FIG. 2 , followed by less dense fluid  67 . Less dense fluid  67 , typically water, flows into pump inlet  68  and is pumped by mud pump  37  through hose  35  down casing string  13 . Valves  46 ,  61 ,  74  and  76  will be closed and valve  39  open. Retrieval tool  73  will be configured as in  FIG. 3  while being pumped in, with slips  95  retracted and plug  85  in the upper blocking position. 
   Referring to  FIG. 6 , release member  89  contacts drill lock mandrel  78  and pushes it downward, which allows dogs  51  to retract from locking engagement with casing string  13 . Continued downward fluid pressure from mud pump  37  causes plug  85  to shear pins  88  and move from the position in  FIG. 3  to the position in  FIG. 6 . The downward movement of plug  85  frees slips  95 , which are pushed by spring  103  outward into engagement with casing string  13 . Gripper  91  will be in engagement with the inner diameter of the housing of drill lock assembly  49 , which secures retrieval tool  73  to drill lock assembly  49 , making the assembly a retrievable unit. The operator then ceases to pump less dense fluid  67 , but will initially block back flow through choke  71 . 
   The heavier weight of drilling fluid  43  in annulus  15  exerts an upward acting force against seals  77  on drill lock assembly  49  ( FIG. 6 ) because drill lock assembly check valve  79  prevents upward flow through drill lock assembly  49 . The more dense drilling fluid  43  in annulus  15  tends to “U-tube”, pushing less dense fluid  67  up and out casing string  13  until reaching an equilibrium. To enable U-tubing to occur, at the surface the operator closes valves  39 .  70  and  61 , as shown in  FIG. 9 . Valves  74  and  76  are opened. The operator begins to open the orifice of choke  71 , which allows less dense fluid  67  from casing  13  to flow upward through hose  35 , through flow meter  69  and choke  71  and into less dense fluid tank  65 , as shown in  FIG. 9 . 
   The level of drilling fluid  43  in annulus  15  would drop as it begins to U-tube, and to prevent it from dropping, the operator should continue to add a heavier fluid, such as drilling fluid  43 , to annulus  15  to maintain annulus  15  full. In this example, the operator will cause fill-up pump  72  to flow drilling fluid  43  through annulus inlet  23  into annulus  15 , as shown in  FIG. 9 . The flow rate should be only sufficient to keep the level of fluid  43  in annulus  15  from dropping. 
   The operator may monitor the flow rate of the returning less dense fluid  67  with flow meter  69  as well as the flow rate of the drilling fluid  43  flowing into annulus  15 . Unless there is some overflow of drilling fluid  43  at the surface, these flow rates should be equal. The quantity of drilling fluid  43  flowing into annulus  15  should substantially equal the quantity of displaced less dense fluid  67  flowing through choke  71 . If more drilling fluid  43  has been added to annulus  15  at any given point than the less dense fluid  67  bled back through choke  71 , it is likely that some of the drilling fluid  43  is flowing into an earth formation in borehole  11 . If less drilling fluid  43  has been added at any given point than the less dense fluid  67  bled back through choke  71 , it is likely that some of the earth formation fluid is flowing into the annulus  15 . Neither is desirable. 
   Bottom hole assembly  47  and retrieval tool  73  will move upward as a retrievable unit during the U-tubing occurrence. The operator controls choke  71  to a desired flow rate as indicated by meter  69 , which also is proportional to the velocity of bottom hole assembly  47 . This velocity should be controlled to avoid the downward flow in annulus  15  being sufficiently high so as to damage any of the open formation in borehole  11 . Eventually, the operator will open the flow area of choke  71  completely. 
   As the drilling fluid  43  in casing annulus  15  flows into casing string  13 , the pressure acting upward on bottom hole assembly  47  will eventually drop to a level that is inadequate to further push bottom hole assembly  47  upward, and it will stop at an intermediate position in casing string  13 , as shown in  FIG. 10 . When it stops, slips  95  ( FIG. 3 ) will prevent downward movement of the bottom hole assembly  47 . Slips  95  will be engaging casing string  13  as bottom hole assembly  47  moves upward, thus once it ceases upward movement, slips  95  will immediately prevent downward movement. The operator will detect the cessation of movement by flow meter  69 , which will show substantially zero flow rate at that point. 
   Referring to  FIG. 10 , while bottom hole assembly  47  is held by slips  95  in the intermediate position, the operator then pumps more of the less dense fluid  67  down casing string  13 . The less dense fluid  67  flows through bottom hole assembly  47  and preferably down to substantially the lower end of casing. The operator will control the amount of fluid pumped in so as to avoid pumping large amounts of less dense fluid  67  up casing annulus  15 , although some overfill is feasible. The operator pumps the less dense fluid  67  downward with mud pump  37  through hose  35 . Valve  70  will be open for drawing less dense fluid  67  from tank  65  into the intake line  68  of pump  37 . Valves  46 ,  61 ,  74  and  76  will be closed. The downward pumping of less dense fluid  67  pushes the drilling fluid  43  that had previously U-tubed up into casing string  13  back up casing annulus  15 . The displaced drilling fluid  43  flows out annulus return  22  into mud tank  41 . 
   Once casing string  13  is again substantially filled with less dense fluid  67 , the cumulative weight of drilling fluid  43  in annulus  15  will again exceed the cumulative weight of less dense fluid  67  in casing  15  plus the weight of bottom hole assembly  47 . The operator then repeats the steps in  FIG. 9  to again create a U-tube flow, which causes the bottom hole assembly  47  to move upward again as less dense fluid  67  is displaced out the upper end of casing string  13 . The operator will repeat these U-tube steps until bottom hole reaches casing gripper  27 . 
     FIG. 11  illustrates the same equipment as in  FIGS. 1-10 , however rather than filling annulus  15  while BOP  21  is open, BOP  21  is closed and mud pump  37  is used to pump drilling fluid  43  into annulus  15 . Valve  61  is open and valves  39 ,  70 ,  74  and  76  are closed. Therefore, some surface pressure will exist at the upper end of annulus  15 . This surface pressure will be monitored by the existing pressure gauge of mud pump  37  and also metered by flow rate meter  63 . The more dense fluid  43  plus the surface pressure creates U-tube flow, with less dense fluid  67  flowing back through choke  71 . The embodiment of  FIG. 11  operates in the same manner as described in connection with the embodiments of  FIGS. 1-10 , other than applying a positive surface pressure to annulus  15 . 
     FIGS. 7 and 8  are graphs illustrating the advantage of lightening the density of fluid in casing string  13  ( FIG. 1 ) when retrieving bottom hole assembly  47  ( FIG. 1 ). Referring also to  FIGS. 2 and 9 ,  FIG. 7  shows schematically the surface pressure that exists at the surface, such as at choke  71 , due to heavier fluid  43  in annulus  15  than in casing string  13 .  FIG. 7  designates the density of the heavier fluid  43  in pounds per gallon as being P 1  and the density of the less dense fluid  67  in pounds per gallon as being P 2 . The pressure force is equal to the depth times 0.052 times the difference between the two densities P 1  and P 2 . The heavier fluid is generally the drilling fluid or mud being used to drill the well. 
   Once the less dense fluid  67  has filled casing string  13 , as shown in  FIG. 2 , the heavier fluid  43  in annulus  15  will exert an upward force tending to push more dense fluid  43  back out of casing string  13 . When this occurs, drill lock assembly  49  will move upward with the less dense fluid  67  flowing out of casing string  13 . The amount of pressure available for pushing bottom hole assembly  47  upward is due to the difference in the densities of less dense fluid  67  and more dense fluid  43 . As indicated by the curve in  FIG. 7 , the greatest pressure exists when casing string  13  is completely filled with less dense fluid and the annulus  15  completely filled. At this point, which is designated by the numeral  1  under the legend “Casing ID Volume Pumped”, the greatest surface pressure, such as at choke  71  ( FIG. 2 ), will exist. As bottom hole assembly  47  moves upward, the available energy to keep it moving upward decreases proportional to the distance it is moved. When all of the less dense fluid has been bled back (or U-tubed), the surface pressure at choke  71  would be zero, and the portion of casing string  13  below bottom hole assembly  47  would be filled with the heavier fluid  43 . 
   One problem with this technique is that if only the fluid in the inner diameter of casing string  13  is displaced with less dense fluid  67 , the energy available to overcome the weight of bottom hole assembly  47  plus the mechanical friction in the casing string  13  is insufficient to transport the bottom hole  47  from the bottom of casing string  13  all the way to the surface. This problem can be overcome by “over-displacing” the casing string  13  with the less dense fluid  67 , as shown in  FIG. 7 . The term “over-displaced” means that more of the less dense fluid is pumped into the casing string than casing string  13  can hold, causing some of the less dense fluid  67  to flow up the casing annulus  15 . For example, if the inner diameter of casing string  13  is over-displaced by 20% (shown by the numeral  1 . 2  on the graph of  FIG. 7 ), the maximum available surface pressure for transporting bottom hole assembly  47  occurs after it has moved 20% up casing string  13 . The maximum pressure occurs once all of the overfilled less dense fluid  67  has moved from annulus  15  back into casing string  13 . If the amount of over displacement is proportional to the weight of bottom hole assembly  47 , a single U-tube occurrence may be sufficient to transport bottom hole assembly  47  from the bottom of casing string  13  all the way to the surface.  FIG. 7  shows some surface pressure in existence when an amount equal to the volume of the casing string has been bled back. If that surface pressure is sufficient to support the weight of bottom hole assembly  47  while it is at the surface, the U-tube flow would be able to transport bottom hole assembly  47  from the bottom to the surface in one occurrence. This assumes that casing annulus  15  is continually filled or topped up with higher density fluid  43  as the less dense fluid  67  is bled from casing string  13 . 
   Additional pressure for bottom hole assembly  47  transport can also be generated by filling casing annulus  15  with a fluid having a density greater than P 1  or by closing blowout preventer  21  and adding surface pressure with mud pump  37 , as in  FIG. 11 . In either case, the open portion of borehole  11  may be exposed to a higher pressure than it is desirable. In the embodiment of  FIGS. 1-10 , bottom hole assembly  47  is transported to the surface in a plurality of stages or steps, wherein lesser dense fluid  67  is replaced in casing string  13  after it flows back from casing string  13  sufficiently so that the transport energy is dissipated. 
   When the flow path is open for less density fluid  67  to flow out of the top of casing string  13 , the fluid will accelerate to a velocity that creates a zero net force balance. Assuming that annulus  15  is kept full of high density fluid  43 , the major forces involved are the hydraulic friction of the fluid flowing downward in the annulus  15 , the pressure force required to support the weight of bottom hole assembly  47  and the mechanical friction of moving bottom hole assembly  47  of casing  13 . Also, hydraulic friction pressure exists in the circulation system at the surface. The sum of these pressures is equal to the potential pressure shown in  FIG. 7  for any position of bottom hole assembly  47  in casing string  13 . If the surface equipment pressure losses were negligible, bottom hole assembly  47  would accelerate upwards until the frictional pressure loss in casing annulus  15  plus the bottom hole assembly support pressure is equal to the pressure shown in  FIG. 1 . 
   The frictional pressure in annulus  15  acts in a direction to oppose the fluid flow, thus it tends to reduce well bore pressure in annulus  15 . The maximum reduction in pressure occurs at the bottom of casing string  13 . The reduction in pressure below the hydrostatic head of the fluid used to drill the well may create borehole instability or induce an influx of formation fluid into casing string  13 . Neither occurrence is desirable. The undesirable effect can be negated by incorporating a device to regulate the flow of fluid from casing string  13  so that the velocity of the downward flowing fluid in annulus  15  is controlled to a desirable range. In the preferred embodiment, this regulation is handled by gradually opening adjustable choke valve  71  ( FIG. 2 ). As bottom hole assembly  47  is transported to the surface, the bottom hole assembly  47  velocity can be maintained constant. 
     FIG. 8  shows an example of the effective pressure exerted on the open hole portion of borehole  11  while U-tubing a bottom hole assembly in a 7″ diameter casing string. The simulation is for a flow rate of 300 gallons per minute and mud weight of 10 lbs. per gallon at 8,000 ft. depth, as indicated by curve C. While drilling and flowing 300 gallons per minute, the pressure exerted on the open hole portion of borehole  11  is relatively constant at 10.6 lbs. per gallon, as indicated by curve D. The annular pressure loss is 246 psi. Two separate U-tubing cases are evaluated. In both cases, the complete casing string  13  is displaced with water, which would provide a 695 psi potential to start the reversing process. This pressure is equivalent to an upward force of 22,000 lbs on bottom hole assembly  47 . Referring also to  FIG. 2 , curve A assumes that annulus  15  is kept full of 10 lbs. per gallon drilling fluid, but there is no additional pressure at the surface applied to annulus  15 . The return fluid flows through choke  71 , which is used to throttle the flow initially significantly, but is continuously opened as the well U-tubes to maintain approximately 300 gallons per minute flow measured by flow meter  69 . 
   At some point near the surface, it will not be possible to maintain this flow rate as the potential energy of the differential density is dissipated. The wellbore pressure is generally about 9.4 lbs. per gallon or about 1.2 lbs. per gallon less than when drilling and 0.6 lbs. per gallon less than when the well is static. By comparison, if casing string  13  were to be abruptly open to atmosphere as the U-tube process is started, the bottom hole pressure would fall to the equivalent of 8.3 lbs. per gallon, or even less if the dynamic forces are considered. 
   Curve B simulates closing well annulus  15  in at the surface, such as with blowout preventer  21  as illustrated in  FIG. 11 . Curve B simulates pumping into the well at a constant flow rate of 300 gallons per minute. Choke  71  is operated to maintain a constant pressure of 246 psi on casing annulus  13  at the surface. For this case, the bottom hole pressure is exactly the same as the hydrostatic well pressure of curve A, but the formation of borehole  11  near the lower end of casing  17  is exposed to substantially higher pressure. In some cases, it may be desirable to add a slight surface pressure to annulus  15  by pumping into the annulus as in  FIG. 11  to overcome any reduction and effective hydraulic pressure due to friction. 
   In a particular situation, knowledge of the formation sensitivities may be used to determine the most critical point in the well bore for preventing an inflow of drilling fluid into an earth formation or well bore instability due to changes in pressure in annulus  15 . If the annulus  15  frictional loss is calculated from the surface to the most critical point using the flow rate that provides the most desirable bottom hole assembly  47  transport rate, fluid can be injected into annulus  15  at this flow rate. Choke  71  is adjusted to maintain a pump  37  pressure equal to calculated annulus  15  loss. These steps will cause the annulus pressure at the bottom of borehole  11  to be maintained at the hydrostatic pressure of the annulus fluid. 
   It is desirable to keep annulus  15  full of drilling fluid when circulating out bottom hole assembly  47 . This can be done by an open system or with a closed system. An example of an open system is by using fill-up pump  72  ( FIG. 9 ) to return drilling fluid into the top of annulus  15 . The pump rate would not be critical as long as it achieved the rate needed to replace the fluid in casing annulus  15  that would normally drop as fluid  67  flows out of casing  13 . An example of a closed system is shown in  FIG. 11 , wherein BOP  21  is closed to allow surface pressure to be applied by mud pump  37 . In  FIG. 11 , mud pump  37  is operating, valves  61  and  76  are open and valves  39 ,  70  and  74  are closed. 
   In  FIG. 12 , rather than rely solely on the U-tubing effect to push bottom hole assembly  47  to the surface in stages, a cable or wireline  115  will be employed to assist the upward force due to the heavier fluid flowing down casing annulus  15 . Wireline  115  passes through a wireline entry sub  113  that will be mounted at the upper end of casing string  13  below casing gripper  27 . Wireline  115  has a retrieval unit  116  on its end that may be pumped and latched into engagement with bottom hole assembly  47 . Wireline  115  extends over a sheave to a drum  117  that pulls upward on bottom hole assembly  47 . Alternately, the wireline entry can be made between top drive  31  and casing string gripper  27  or above top drive  31 . 
   In the operation of the embodiment of  FIG. 12 , retrieval unit  116  is pumped down and latched into engagement with bottom hole assembly  47  while it is attached to wireline  115  and wireline  115  fed out. Retrieval unit  116  releases the locking member of bottom hole assembly  47 . Preferably, the operator pumps retrieval unit  116  downward or follows it with less dense fluid  67  so that casing string  13  will now be filled with less dense fluid  67 . The more dense fluid  43  in casing annulus  15  will exert an upward force on the seals on bottom hole assembly  47 . As indicated in  FIG. 12 , U-tubing occurs when valves  74  and  76  are open, fill-up pump  72  is operating, and valves  39 ,  70 ,  46  and  61  are closed. This upward force will be assisted by pulling upward on wireline  115 . As wireline unit  116  and bottom hole assembly  47  start moving upward, the operator may control the rate of ascent by gradually opening choke  71 . The operator maintains annulus  15  full of drilling fluid  43 , preferably with fill-up pump  72 . When the force due to the heavier drilling fluid  43  in annulus  15  is inadequate to lift bottom hole assembly  47 , the operator may continue pulling bottom hole assembly  47  upward with wireline  115 . 
   Slips  95  ( FIG. 3 ) may be used on retrieval tool  116  and the incremental U-tubing steps previously described used in conjunction with wireline  115 . The arrangement of  FIG. 12  avoids wireline  115  from having to supply all of the force to lift bottom hole assembly  47  when it is located at the bottom of casing string  13 ; while at the bottom, a greater force is required than at any other points because of the additional weight of wireline  115  in casing string  13 . Also, bottom hole assembly  47  may tend to stick while at the bottom of casing string  13 . In addition, the greatest weight of fluid acting downward on the seals of bottom hole assembly  47  exists when bottom hole assembly  47  is at the lower end of casing string  13 . In addition, combining wireline  115  with incremental U-tubing steps allows the operator to use commercially available line of less strength than would otherwise be required. 
   Referring to  FIG. 13 , in this embodiment, hose  35  is not used for returning displaced fluid from casing string  13 . Instead, when the operator wishes to commence retrieval, the operator will support casing string  13  in slips (not shown) at rig floor  25 . The operator then disconnects casing string gripper  27  from casing string  13  and attaches casing string gripper  27  to a circulation sub  119 . In the example of  FIG. 13 , circulation sub  119  is connected by an adapter  121  to the upper end of casing string  13 . Circulation sub  119  has one or more outlet ports  123  in its sidewall. A swivel housing  125  preferably mounts around circulation sub  119 . Swivel housing  125  is mounted on bearings  127  so as to allow circulation sub  119  to rotate relative to swivel housing  125 , if desired. A tether (not shown) may attach swivel housing  125  to the rig to prevent its rotation. Swivel housing  125  is connected to an outlet flow line  129  that leads from its sidewall and which is in communication with outlet ports  123 . Seals  131  are located above and below outlet ports  123  for sealing swivel housing  125  to circulation sub  119 . 
   Outlet flowline  129  preferably leads to less dense tank  65  for discharging less dense fluid  67 . Preferably flow meter  69  and choke  71 , as well as valve  76  are mounted in outlet flowline  129 . A bypass loop  133  may extend around flow meter  69  and choke  71  in order to protect meter  69  if a well control situation develops. 
   Circulation sub  119  may also have a latch pin  135  for latching into engagement with retrieval tool  73 , shown by dotted lines. Latch pin  135  will hold retrieval tool  73  in circulation sub  119  until it is released. Circulation sub  119  may also contain a tool catcher  137  mounted therein. Catcher  137  has a grapple  139  on its lower end for engaging the upper end of retrieval tool  73  when it returns to the surface. Flow ports  141  extend through its mounting portion to allow downward flow through circulation sub  119 . 
   In this example, casing string gripper  27  is shown as an external type that has gripping members  143  that grip the exterior of sub  119 . Alternately, it could have a gripper that grips the inner diameter of sub  119 . A spear  145  extends downward from casing gripper  27  into the upper end of circulation sub  119 . Spear  145  has a seal  147  that seals against the inner diameter of circulation sub  119 . 
   In operation,  FIG. 13  illustrates the operator beginning to pump retrieval tool  73  down for engagement with bottom hole assembly, which is not shown in  FIG. 13 , but which would be similar to bottom hole assembly  47  in  FIG. 2 . Latch pin  135  has just been released. Mud pump  37  is pumping less dense fluid; valves  39  and  70  are open and valves  46 ,  61  and  74  are closed. The fluid flows downward through hose  35  and acts against the seal  75  ( FIG. 2 ) on retrieval tool  73 . Alternately, if desired, light weight fluid  67  can be pumped into casing string  13  behind retrieval tool  73  through line  129 . This would be desired if the less dense fluid was not compatible with the pumping system of the rig or if the rig operator preferred not to pump this fluid with mud pump  37 . Also, pumping through line  129  may save rig time by not having to reroute the system components to the retrieval configuration once retrieval tool  73  reaches the bottom hole assembly. 
   The operator then follows one or more of the methods of  FIGS. 1-11 . When retrieval tool  73  is returning to the surface, as shown in  FIG. 14 , fill-up pump  72  will be topping up casing annulus  15  with drilling fluid  43 . The displaced less dense fluid  67  will flow out flowline  129  into less dense fluid tank  65 . Valves  74  and  76  are open and valves  39 ,  61  and  70  are closed. The operator controls the velocity of the upward movement of retrieval tool  73  by varying the flow area of choke  71 . When retrieval tool  73  reaches grapple  139 , it will be caught and held in place along with bottom hole assembly  47  ( FIG. 2 ). Preferably seal  75  ( FIG. 3 ) on retrieval tool  73  will pass and locate above outlet ports  123  when engaged by grapple  139 . As seals  75  pass outlet ports  123 , a pressure differential will be observed because no additional fluid will be flowing out of outlet ports  123 . 
   While the invention has been shown in several of its forms, it should be apparent to those skilled in the art that it is not so limited but it is susceptible to various changes without departing from the scope of the invention. For example, rather than flowing less dense fluid back into a tank, the operator could simply dispose of the fluid. Other ways exist to reduce the density of the fluid in the casing above the bottom hole assembly, such as injecting air into the casing while it is still filled with drilling fluid. The slips on the retrieving tool could be mounted on the drill lock assembly.