Balanced isolation tool enabling clean fluid in tubing perforated operations

In tubing conveyed perforating well completion operations, shaped charge detonation is achieved by dropping a detonating bar in the tubing string. Velocity of the detonating bar is controlled by placing a column of standing fluid above the tubing conveyed perforating assembly. This apparatus and method isolate a column of standing fluid. Moreover, invasion by a well fluid is prevented to assure that the retardation characteristics of the standing fluid are not changed by invading well fluids. The apparatus includes a tubing string pressure isolation piston assembly slidable within a sleeve and a frangible closure disk broken by the detonating bar; the isolation tool further includes check valve means controllably venting fluid pressure across the valve means.

BACKGROUND OF THE DISCLOSURE 
In completion of a well, one process presently in favor is the use of a 
tubing conveyed perforating assembly suspended on a tubing string to form 
perforations at a specified depth in the well. The TCP process typically 
involves suspending a set of perforating guns (ranging from a few to 
several hundred) at the lower end of the tubing string. The tubing string 
is assembled at the well head and lowered into the well. The TCP assembly 
is guided by a packer to register the TCP assembly opposite the formation 
of interest prior to forming the perforations. Ordinarily, a detonating 
bar is dropped free fall in the tubing string. The bar strikes the top end 
of the apparatus with the TCP assembly thereby triggering detonation. The 
detonating bar normally weighs quite a bit. Moreover, the tubing string 
can be quite long, easily more than 10,000 feet, and the bar may well 
reach significant velocity as it falls into the well. If the bar falls 
freely without impediment, it will travel with sufficient kinetic energy 
that it may do damage to the equipment at the top end of the TCP assembly. 
Because of this, it is desirable to retard the rate of fall of the 
detonating bar. One way to do this is to place a standing column of liquid 
above the TCP assembly so that the detonating bar is retarded by the 
liquid. This regulates detonating bar velocity to assure that the kinetic 
energy in the impact is in an acceptable range. 
One problem which makes detonating bar velocity variable is a change in 
viscosity of the fluid in the tubing string. Assume as an easy example 
that the tubing string is filled with certain depth with clean water. This 
will provide a known retardation to the velocity of the drop bar. On the 
other hand, if the water mixes with drilling fluids or formation fluids or 
both, it can easily become quite different in physical characteristics and 
thereby provide significantly different retardation to the velocity of the 
detonating bar. It is therefore desirable to limit commingling of the 
fluids so that drilling fluids or formation fluids on the exterior of the 
tubing string do not invade the string and thereby change the viscosity of 
the standing column of liquid. It is particularly possible to mix drilling 
fluid in the water and thereby significantly change the retardation of the 
water to the dropped detonating bar. 
The present apparatus enables isolation of the standing column of water 
above the TCP assembly. Moreover, there maybe variations in downhole 
pressure. The present apparatus accommodates pressure differentials 
between the column of standing fluid above the TCP assembly and the 
exterior in the annulus of the well. Briefly, this apparatus includes a 
floating piston assembly which is enclosed in a suitable sub. The piston 
assembly can ride up and down to achieve a pressure balance. The floating 
piston assembly is sealed over by glass disk. When the detonating bar is 
dropped, it shatters the glass disk and passes through it. The sacrificial 
glass disk isolates fluid therebelow to assure that that fluid is clean. 
Thus, the floating piston assembly rises and falls for clean fluid 
isolation. Moreover, should pressure increase below the glass disk, an 
O-ring valve assembly vents fluid in one direction only, thereby 
accomplishing controllable pressure relief, all as will be set forth in 
detail herein after.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Attention is first directed to FIG. 1 of the drawings. There, a well has 
been shown where production steps are being undertaken including the 
detonation of shaped charges to form perforations. The well is cased at 10 
and a packer or other landing nipple is located at 12 to support a TCP 
assembly 14. The assembly 14 is of any suitable length, and includes 
detonating apparatus as well as a specified number of shaped charges. They 
point radially outwardly at selected spacing and angular positions to 
perforate the casing 10. They will also perforate through the surrounding 
cement 16 which anchors the casing in location. The perforations are 
formed into adjacent formations including a sand of interest indicated at 
18. It is intended that production be obtained from the sand, and to this 
end, the TCP assembly 14 is positioned so that the perforations are formed 
at the proper depth in a required number. The TCP assembly 14 is thus 
placed in registry with a packer or other landing device which assures 
that the perforations are formed at the proper depth. 
The TCP assembly supports an internally located detonator mechanism (at the 
upper end) which is actuated by a dropped detonating bar. The numeral 20 
represents such a detonating bar traveling free fall in the tubing string. 
The tubing string is assembled above the TCP assembly 14 by placing a 
first tubing section 22 thereabove. It has a desired length. The isolation 
tool of the present disclosure is located thereabove at 24 in the tubing 
string. Additional joints of tubing are added at 26 to obtain the 
necessary length such that the TCP assembly is located at the proper 
depth. The tubing 26 can be several thousand feet in length. By contrast, 
the tubing 22 (above the TCP assembly and below the isolation tool) is a 
desired length and is filled with a clean fluid. As a representative 
example, it might be 60 feet from the top of the TCP assembly at 14 to the 
isolation tool 24 of the present disclosure. The requisite tubing joints 
are placed below the isolation means 24 and are filled with clean fluid to 
obtain a standing column of fluid retarding the velocity of the dropped 
detonating bar to a desired velocity. The fluid is clean and is also 
isolated, thereby assuring that invading well fluids do not change the 
viscosity or makeup of the standing column of fluids. In a typical 
situation, the tubing will range from about 23/8 inches in diameter and 
up. The present apparatus is installed in the tubing string at a desired 
location above the TCP assembly 14. Connections are made with conventional 
threaded pin and box connections well known for tubing strings. 
Going now to the detailed view of the isolation tool 24 better shown in 
FIG. 2, its construction will be described from the top to the bottom. The 
numeral 30 identifies an upper end sub which has a mating threaded box 32 
for makeup in the tubing string. The sub 30 has a specified length and 
terminates at a shoulder 34. Moreover, it threads to a sleeve 36 which is 
fixedly attached by means of a suitable set screw 38. The components not 
only thread together as illustrated, but they are also held against 
unthreading by positioning the set screw at the illustrated location. The 
sleeve 36 is of any suitable length and terminates at a pin connection 40 
at the lower end to enable continuation of the tubing string. The pin 
connection is immediately adjacent to an enlarged or thickened wall 
portion 42 which defines a shoulder 44. This shoulder limits travel of 
components in the isolation tool which will be described. 
The numeral 46 identifies the cylindrical interior wall of the sleeve. This 
serves as a guide and seal surface for a traveling assembly. This assembly 
is generally described as a floating piston assembly at 50. This assembly 
is formed by several components which move together. One of the components 
is a sleeve 52. It is axially hollow and is formed with a fluid drainage 
port at 54. Any fluid introduced into the tubing string thereabove will 
drain to the exterior through the port 54. The sleeve 52 is captured on 
the interior of the sleeve 36. The sleeve 36 has several long slots formed 
at 56. The sleeve 52 is threaded to a valve sleeve 58. The two components 
thread together capturing a frangible glass disk 60. The disk 60 is sized 
so that it is easily broken by the falling detonating bar 20. The disk 60 
is sufficiently thick that is supports a standing of a column of fluid 
thereabove that in the event that fluid accumulates in the tubing string 
above the isolation tool 24. Leakage past the disk is prevented by 
securing the disk with suitable O-ring seals in facing grooves as 
illustrated in FIG. 2. 
The valve sleeve 58 threads to another sleeve at 62. The sleeve 62 extends 
the length of the floating piston assembly, and cooperates with the valve 
sleeve 58 in a special fashion as will be described. The valve sleeve 58 
terminates at a shoulder 64. The shoulder 75 supports on O-ring 66 riding 
on the shoulder. The O-ring is captured by its own resiliency and tends to 
shrink against the shoulder 75. It is confined by the abutting shoulder, 
thus fitting in a V-shaped groove. A fluid flow path is defined through 
ports or openings 70 below the O-ring 66, thereby forcing the O-ring 66 to 
expand. Fluid flows past the O-ring and along the shoulder 75 to escape 
through lots 76 from between the components 58 and 62. This fluid is then 
on the exterior of the floating piston assembly 50. It is voided through 
the slots 56 as shown in FIG. 2. 
The piston assembly 50 travels upwardly and downwardly. It is guided by the 
elongate construction shown in FIG. 2. Suitable O-rings 72 and 74 prevent 
leakage below the piston assembly 50. The floating piston assembly 50 thus 
defines two flow paths from the interior of the balanced isolation 
assembly to the exterior. The large port 54 is located above the glass 
disk at 60. Any fluid which is in the tubing string above the isolation 
tool 24 drains to the port 54 and out through the slots 56. A second 
drainage path is included for the tubing string below the glass disk. This 
flow path is controlled by a check valve mechanism. The flow path includes 
the several holes 70. The check valve mechanism includes the O-ring 66 on 
the tapered surface 75. The flow path is from the interior to the exterior 
under control of the check valve. Flow in the opposite direction is not 
permitted by operation of the check valve O-ring 66. 
Operation of this isolation tool 24 should be considered. Assume that it is 
installed in the tubing string and located in the well. Assume further 
that a measured standing column of clean fluid is located therebelow. The 
standing column of clean fluid is protected by this apparatus. Assume 
further that there is fluid in the tubing string above the isolation tool 
24. In that instance, when the detonating bar 20 is dropped, it simply 
travels along the tubing string and ultimately arrives at the fluid above 
the isolation tool 24. The fluid above the isolation tool 24 will slow the 
bar 20 to cushion impact on the isolation tool 24. The detonating bar will 
strike and break the glass disk 60. Then, it falls through the standing 
column of clean fluid, having the desired retardation. 
Consider the column of liquid above the disk 60. Assume that the fluid is 
quite different from the isolated clean fluid below the isolation tool. 
Excess fluid above the isolation tool is free to drain out through the 
port 54 assuming there is a pressure differential acting across the port 
54. Whether some fluid drains or not, the dropped detonating bar will fall 
along the tubing string through fluid above to tool 24, strike the disk 60 
and then fall in the column of clean fluid. Even when the bar 20 falls 
through heavy or viscous drilling fluid above the glass disk 60, the 
velocity of the dropped detonating bar is still sufficient to break the 
disk 60 and then fall at a desired velocity in the clean fluid such that 
the TCP assembly therebelow is properly operated. 
The isolation tool 24 isolates two separate columns of fluid. The fluid 
below the isolation tool 24 is clean to obtain controlled bar velocity, 
and also has a fixed length to assure a desired terminal velocity. The 
isolation tool 24 separates the upper fluid column thereabove. The upper 
fluid column is included to slow down the bar 20 to limit impact damage at 
the tool 24. To illustrate, assume that the packer is located at a depth 
of 10,000 feet. Assume further that the isolation tool 24 is at 10,050 
feet. Assume further that 60 feet of clean water is isolated between the 
tool 24 and the TCP detonating apparatus. If the bar 20 is dropped in open 
tubing, the velocity may well be in excess of 100 miles per hour at the 
impact with the glass disk; such a high velocity impact will destroy the 
disk and may well damage the bar 20. Therefore a column of standing fluid 
is placed above the disk to slow the projectile bar. As an example, the 
velocity can be slowed by 50 feet of relatively thick mud. 
The dropped detonating bar 20 will impact the first fluid column (above the 
tool 24) and be slowed to some speed; in fact, any speed sufficient to 
break the glass disk will suffice. The velocity is retarded to limit 
impact damage. Then the bar 20 falls through the controlled viscosity 
fluid at a velocity regulated by the isolated column of fluid. This rate 
of fall is controlled or limited to a desired range. By contrast, the 
column of fluid above the isolation tool 24 can vary over a wide range in 
viscosity and fluid column height. Even though the upper column of fluid 
may vary widely, the isolated column does not vary (by virtue of its 
isolation) so much and hence the bar 20 velocity is regulated. This limits 
impact damage and yet assures adequate impact and detonation. 
As a further possibility, the pressure on the outside of the tubing string 
may increase. The floating piston assembly is free to travel downwardly 
through a specified stroke, the stroke being determined by the spacing 
between the downwardly facing shoulder 77 and the upwardly facing shoulder 
44 near the bottom of the isolation tool 24. The glass disk 60 is 
sufficiently thick to withstand some pressure differentials acting 
thereacross. 
Assume however, that the pressure differential acting on the floating 
piston assembly 50 forces it upwardly. It is free to travel upwardly, but 
travel is limited by the shoulder 34. Pressure relief from below the 
floating piston assembly is obtained by the valve means incorporating the 
O-ring 66. This function as a check valve. When a suitable pressure 
differential acts across the device, fluid flows past the O-ring 66. The 
escape path for the fluid extends to the slots 56 formed in the 
surrounding sleeve. Because of this arrangement, the piston assembly 50 
can travel downwardly to equalize pressure. Additionally, it can travel 
upwardly to equalize pressure. If travel upward to the shoulder at 34 
limits further movement, the O-ring 66 functions as a check valve thereby 
venting pressure fluid to obtain pressure equalization. 
In a typical installation, the travel of the traveling piston assembly is 
quite small compared to the height of the column of standing clean fluid 
therebelow. Thus, when the detonating bar is dropped, it can be known with 
certainty that the detonating bar velocity through the standing column of 
fluid is regulated. This assures proper operation of the detonating bar, 
particularly controlling the velocity and impact of the detonating bar on 
the TCP assembly. Moreover, the clean fluid is protected because it is 
isolated to avoid invasion by well fluids which might change of the nature 
of the column of fluid. 
While the foregoing is directed to the preferred embodiment, the scope is 
determined by the claims which follow.