Downhole drilling tool cooling system

A dual walled pipe string structure defines an annular cavity between the walls into which hard granules may be packed to oppose wall collapse when inserted to high pressure well depths. Segments of pipe thus constructed will reduce the heat transfer across the pipe string allowing for the conveyance of drilling mud from a surface cooler to a downhole tool. The rate of cooled drilling mud transfer to the tool may be controlled to match the heat transfer rates and the heat generated at the tool. To further oppose wall pressures inert gas at opposing pressure may be introduced into the interstices between the granules.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to well bore fluid cooling systems, and more 
particularly to heat exchange systems useful in cooling downhole tools in 
the course of drilling into geothermally heated strata. 
2. Description of the Prior Art 
The drilling of boreholes into the Earth's crust is a process in which 
cutting energy is expended at the downhole tool, the energy dissipation 
invariably raising the tool temperature. Along with the energy dissipation 
associated with drilling the local temperature of the borehole increases 
with depth, and the combination of the cutting tool temperature limit, the 
heat generated in the course of cutting, together with the increasing 
temperature profile in the borehole with depth become the limiting factor 
on the effectiveness of the drilling process. Simply, the effective 
temperature difference at the tool end becomes insufficient for successful 
heat exchange of the locally generated cutting heat and the cutting 
process eventually reaches impractical rates, setting the limit of 
effective penetration. 
In the past drilling fluids were circulated through the drill pipe down to 
the tool, bringing back to the surface the cutting debris together with 
the entrained heat generated by cutting. As the depth increased the heat 
exchange directly across the drill pipe wall required further temperature 
chilling of the drilling fluid achieved in one example by a chilling 
system described in my prior U.S. Pat. No. 4,215,753. Without limitation, 
the foregoing chilling system circulated the drilling fluids through heat 
exchangers at the surface and then circulated the chillied fluid down the 
wellbore. While commercially successful, the circulation path nonetheless 
entails long drill pipe dimensions and the consequent heat transfer from 
the returning fluid to the chilled fluid, across the pipe wall, becomes 
significant with increased bore depths. 
Associated with increasing depth is also the pressure within the wellbore. 
In fact, the drilling fluid, sometimes referred to as the drilling mud, is 
often augmented with high mass particulate matter in suspension in order 
to equlise the local static pressure in the bore. Thus the drill pipe is 
exposed to substantial pressures in a fluid entrained with cutting debris 
and density augmentation matter. These combined conditions limit any 
usefulness of known insulation coatings, and the heat transfer across the 
drill pipe wall is thus determined by the pipe material. 
In the past various techniques have been devised which in one way or 
another insulate the well casing. Examples of such techniques may be found 
in the teachings of U.S. Pat. No. 3,820,605 to Barber et al; U.S. Pat. No. 
4,693,313 to Stephenson et al; and U.S. Pat. No. 4,276,936 to McKinzie. 
While suitable for the purposes intended, each of the foregoing techniques 
fails to attend to the heat transfer rates across the drill pipe wall. The 
exchange between the chilled fluid pumped down the pipe and the returning 
fluid in the surrounding annulus of the bore is thus unattended. 
Of those techniques directed at controlling the heat transfer to a downhole 
tool it has been the typical practice to provide tool shielding, as in the 
U.S. Pat. No. 5,016,716 to Donovan et al, or to enclose the tool in a 
jacket as in U.S. Pat. No. 4,926,949 to Forrest. While these examples are 
effective for the purposes intended, the heat transfer across the drill 
pipe has again had little attention. It is the technique for increasing 
the effective R factor of the drill pipe itself that is addressed herein, 
particularly attending the severe environments of deep well bores and 
those directionally angled, as in slant drilling. 
SUMMARY OF THE INVENTION 
Accordingly, it is the general purpose and object of the present invention 
to form a drill pipe structure for reduced heat transfer rates. 
Other objects of the present invention are to provide insulated drill pipe 
conformed for exposure in a well bore. 
Yet further objects of the present invention are to provide an insulated 
conveyance for chilled drilling fluids to a downhole tool. 
Briefly, these and other objects are accomplished within the present 
invention by providing a surface deployed heat exchanger, generally of the 
type earlier described in my U.S. Pat. No. 4,215,753, including a fan 
cooled radiator exposed to a water misting array. The radiator is pump fed 
a flow of drilling mud from a local storage tank or pond, conveying its 
chilled output into the drill pipe string extending into a well bore. This 
flow of chilled drilling fluid is then useful in cooling the downhole 
tool. 
Once significant depths are reached, however, the heat in the returning 
fluid surrounding the drill pipe exchanges with the chilled fluid conveyed 
down the pipe, and the fluid temperatures at the tool are too close to the 
local working temperature for effective cooling. This loss of cooling 
temperature difference is dominantly a function of the heat transfer rates 
across the drill pipe walls, as the length of the pipe string is a 
canonical object. Thus in deep well drilling the extremely high length to 
diameter ratios of the pipe string preclude effective compensation by 
increased pumping rate. 
Those skilled in the art will note that the downhole environment of the 
pipe string is extremely severe. The temperatures increase with depth, as 
does the pressure, and the well fluid is laden with cutting particulate 
and density enhancing matter in suspension. Any coating of the pipe 
segments to increase insulation coefficients is thus quickly abraded away, 
or collapsed by the well pressures. This destructive environment is 
further exacerbated by the common preference for slant drilling, now 
dictated by environmental and aesthetic concerns. Thus pipe insulation by 
coating is a difficult object, limited by the material properties of the 
known insulative materials. 
To provide insulative properties to the pipe string in these severe 
conditions, I have devised a pipe segment arrangement in which each 
segment is formed as a concentric dual wall cylinder. The gap between the 
inner cylinder and the outer cylinder is then filled with generally 
spherical silica beads, with the bead sizes determined by the well 
pressure to which the particular segment is exposed. The interstitial gaps 
between the silica granules can then receive an inert gas, at some 
pressure, to further oppose the collapse of the annular cavity between the 
walls. Accordingly, those pipe segments closer to the well surface that 
are exposed to lower pressures may receive a coarser granular fill, as the 
wall bridging span between the granules can be greater. In this manner a 
selection of the segment filler granule size can be made for fewer 
wall-to-granule contacts, thereby increasing the insulative qualities of 
the segment. As the segments extend into deeper parts of the well bore, 
where the well pressure is greater, smaller granulation of the silica fill 
offers smaller bridging spans to accomodate the higher pressure wall 
loadings, thus increasing crush resistance. 
The segments are each terminated in the conventional threaded end 
connectors which also close the annular cavity. Thus an assortment of 
segments is provided which meets the increasing pressure loads with bore 
depth. In each instance, the packed granular filler and the dual wall 
structure combine to icrease the segment structural strength and some wall 
thickness reduction is therefore possible to reduce the string mass. The 
increased sectional rigidity of the segment, moreover, reduces the 
incidence of torsional buckling, an advantage of particular interest in 
slant drilling. Thus the insulation factor of each segment is increased, 
by a multiple of 4 or more, while the structural strength is also 
increased. 
The doubled wall construction, additionally, allows for higher internal 
drilling fluid pressures, thus accomodating the use of downhole tools 
powered by the pumped drilling mud. The present inventive combination, 
therefore, accomodates the most rigorous concerns of deep well drilling.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIGS. 1-5, inclusive, the inventive deep well drilling system, 
generally designated by the numeral 10, includes a drilling rig DR of 
conventional form provided with a derrick D, a hoist H, and a drive unit U 
to turn the drill pipe string PS within a well bore WB. Of course, in mud 
driven down hole motors torsional power at the surface may be omitted. By 
common practice drilling fluid or mud is conveyed to the exposed end of 
the string PS, by way of a mud pump 11, the fluid F passing to the cutting 
tool 12 at the lower end of the string, and from there returning upwardly 
through the well bore to the well head 14 to be then directed into 
settling tanks 21, 22 and 23 in which the cutting particulate is 
separated. A mobile chilling unit C, of the type earlier described by me 
in U.S. Pat. No. 4,215,753 then draws the separated drilling mud from the 
upper layers of tank 22, cooling same through misted heat exchangers and 
returning the cooled flow to tank 21. Tanks 21, 22, and 23 are provided 
with weirs 21a, 22a, and 23a, respectively, each conformed to collect the 
cuttings brought up from the well bore. The last stage of the settling and 
separating process, in tank 23, is then fed by pump 11 to the upper end of 
the pipe string PS. 
This arrangement, therefore, both chills the drilling mud inchiller C and 
feeds it under pressure to the tool 12. Mud pressure operated down hole 
tools are therefore accomodated in the present arrangement. 
In typical practice well bore WB is of a diameter greater than the pipe 
string PS, thus forming an annular cavity AC around the string. The 
drilling mud or fluid F emmitted at the tool 12 therefore passes to the 
surface in intimate contact with the pipe string PS. Heretofore, this 
intimate contact has resulted in substantial heat transfer from the 
returning flow to the chilled flow forced down the string. As result the 
benefit of cooling has been limited as the drilling depths increased. To 
increase the insulative qualities of the pipe string PS, an annular 
structure has been devised, generally exemplified by pipe segments 40-1, 
40-2, and 40-3. Each of the segments 40-1, 40-2, and 40-3 is similar in 
its form, and like numbered parts describe like functioning structures. 
More precisely, each segment includes a tubular exterior shell 41 provided 
with a coaxial interior liner 42, shell 41 being separated from liner 42 
by an annular space or cavity 43 within which a quantity of generally 
spherical silica, aluminum oxide, or other granules 45 is filled. The 
spherical diameter of granules 45 may be graded, segment 40-1 receiving 
the largest granules 45-1, segment 40-2 receiving smaller granules 45-2, 
and so on. 
Both the upper and lower ends of the annular cavity 43 are closed by 
pressed insertion of a circular projection 51 at each interior threaded 
couplings 50. This pressed engagement may be controlled to a selected 
pressure, thus prestressing the annulus against collapse. Once thus 
compressed each threaded coupling 50 may be fixed in place by parallel 
expanded method, and inert gas, under pressure, may be further introduced 
between the interstices of the granules 45 through threaded ports 85. This 
sealed and pressurised cavity then increases in its internal pressure with 
increasing gas temperature, opposing the well bore pressures in the 
surrounding fluid. As illustrated in detail in FIG. 5 the typical well 
temperature profile T increases with depth, as does the pressure P. The 
inclusion of a confined quantity of gas in the annulus will therefore 
counteract wall collapsing exterior pressures. 
Further optimization may be effected by the selection of the granule size. 
As shown in FIGS. 3a, 3b, and 3c granules 45-1, 45-2, and 45-3 are each of 
a smaller dimension. The number of direct contacts 49 per unit surface 
area of the inner and outer concentric tubes 42 and 41, respectively, thus 
decreases in direct proportion to the granule size. The direct conductive 
heat exchange paths HP can thus be controlled by the selection of the 
granule size, the wall thickness of the inner and outer tubes, and their 
material selection. Of course, the lower well pressures closer to the 
surface accomodate better the larger granule 45-1, and segment 40-1 can 
thus be installed in that part of the pipe string PS. As deeper insertions 
are contemplated segments 40-2 or 40-3 can be installed. In each instance 
it is the intent to reduce the number of the direct contacts 49 where 
temperature differences between the downward flow and the return flow are 
the highest, avoiding the exponential paradox of conductive heat transfer. 
In this manner a pipe string may be formed by connecting segments 40-1, 
40-2, and 40-3, into a string, the combination of the selected segment 
types being determined by the cooling requirements of the tool, the 
downhole temperature, and the thermal profile of the well. 
As shown in FIG. 4 these same input parameters may be sensed by an array of 
temperature sensors 111-1 to 111-n each connected to an ultrasonic 
encoding unit 112-1 10 112-n sending discretely coded ultrasonic signals 
U-1 to U-n up the well bore, through the drilling mud in the annulus 
surrounding the string. Signals U-1 to U-n are then received in a receiver 
stage 115 at the surface and through a demultiplexer 116 are then fed to a 
data processing stage 120. It is this data processor that then controls, 
through a servo loop 121 tied to a valve 122, the flow rate delivered by 
pump 11. 
The data processing unit 120 may then be coded for the particular 
combination of segments 40-1, 40-2, and 40-3 and may then determine a 
weighted function set as follows 
EQU PR=.SIGMA.(U-1*K1)+(U-2*K2)+(U-3*K3)+(U-n*Kn)+. . . 
where the sum PR is the pump rate signal and K1 to Kn are the weighting 
coefficients determined by the ambient temperature in the well and the 
heat transfer rate of the segment. In this manner the particular well 
temperature profile can be matched with the pumping rate, and the transfer 
rates at each level. 
Thus as shown in FIG. 5 the temperature TT with depth DD is set out against 
the heat transfer slopes SS-1, SS-2, and SS-3 of the corresponding 
segments. Of course, those skilled in the art will know that the heat 
transfer rate is an exponential function of the temperature difference, 
and by selecting a correct segment combination the temperature difference 
at the tool 12 may be optimised. The foregoing arrangement, therefore, 
allows for a convenient matching of the array with the local heat 
gradient. 
Obviously, many modifications and variations can be effected without 
departing from the spirit of the present teachings. It is therefore 
intended that the scope of the instant invention be determined solely by 
the claims appended hereto.