Abstract:
A method of drilling a wellbore using a cryogenic fluid to prevent sloughing of the wellbore during drilling operations is provided. The method includes providing non-rotatable tubing constructed of a material capable of withstanding temperatures of at least about −320° F. and back pressure exerted on the tubing during the drilling operations, the tubing having an internally disposed electric power cable extending the length thereof. A drill bit constructed of materials capable of withstanding cryogenic temperatures of at least about −320° F., is operably connected to a distal end potion of the tubing. A power source capable of rotating the drill bit, the power source operable connected to the drill bit via the electric power cable such that upon activation of the power source the bit is caused to rotate. A liquid cryogenic material is injected into the tubing for cooling the drill bit of about −320° F. during drilling operations and to freezing the formation and thereby preventing sloughing of shale and infiltration of water into the wellbore.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 60/899,712, filed on Feb. 6, 2007, the disclosure of which is hereby incorporated into this disclosure in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a new, revolutionary method of drilling oil and gas wells. In one aspect, the present invention relates to a method of drilling a wellbore using a cryogenic fluid to prevent sloughing of the wellbore during drilling operations, while increasing the rate of penetration (ROP) and permit drilling of deep hot zones in a formation. 
       BACKGROUND 
       [0003]    Various types of techniques have hereto have been proposed for the drilling of oil and gas wells. For example, drilling muds have been circulated during the drilling of a well to remove chips from the wellbore which are produced during the drilling operation. 
         [0004]    Air or gas drilling has also been employed in the drilling of oil and/or gas wells. In air or gas drilling techniques, air or gas is substituted for drilling muds for removing chips from the wellbore. Air or gas drilling (hereafter referred to as gas drilling) has a significantly greater penetration rate than mud drilling for many well known reasons. Since penetration rate is one of the most significant factors in determining the cost of drilling a well, gas drilling should have become widely used; however, gas drilling has some rather serious drawbacks that have prevented acceptance of the air drilling method except where certain strict conditions are met. Where, for example, excessive water influx into the low pressure wellbore occurs, large amounts of gas are required to lift the water from the bore hole. Also where certain shales swell or hydrate in the presence of an influx of water into the borehole, the shale will become loosened and slough into the borehole necessitating large quantities of shale and cuttings to be lifted out of the wellbore. If enough shale is loosed, the drill pipe may even become stuck. 
         [0005]    Another technique for drilling oil and gas wells is disclosed in U.S. Pat. No. 3,612,192 wherein air drilling of a formation is enhanced by cooling the air to a cryogenic temperature of approximately −200° F. by compressing the air to a pressure from about 50 to 100 pounds per square inch absolute. The cooled air is then injected through a pipe to the drill pipe where the cooled air circulates around the drill bit and up an annulus formed between the walls of the wellbore and the drill pipe. The cooled air from the annulus of the wellbore passes to a cutting trap and filter. The outlet of the cutting trap and filter is then passed through a heat exchanger and thereafter discharged into the air. 
         [0006]    While the prior cut is replete with various methods and techniques for drilling oil and gas wells, each of the prior art methods and techniques suffer from numerous disadvantages such as, for example, drill bit life, rate of penetration (ROP), problems associated with drilling into and through deep hot zones and the likes. Therefore, new and improved methods and techniques for drilling oil and gas wells are constantly being sought. It is to such a new and revolutionary method of drilling oil and gas wells that the present invention is directed. 
       SUMMARY OF THE INVENTION 
       [0007]    Broadly, the method of the present invention relates to drilling of a wellbore without the use of drilling muds to lift chips out of a wellbore while at the same time preventing sloughing of the formation into an annulus of the wellbore. More specifically the method involves providing a coil of tubing wherein the tubing is constructed of a material capable of withstanding cryogenic temperatures and back pressure exerted on the tubing during drilling operations. The tubing, which is maintained in a non-rotating condition during drilling, is provided with an internally disposed electric power cable extending the length thereof. The electric power cable, which is at approximately 75% super-compressibility because of the cryogenic temperatures inside the tubing, is connected at one end to a drill bit which is operably connected to a distal end potion of the tubing. The electric power cable is connected at its other end to a power source so that upon activation of the power source the drill bit is caused to rotate. A cryogenic fluid, such as liquid nitrogen, is passed through the tubing to cool the drill bit and freeze the formation so that sloughing of shale and infiltration of water into the wellbore is prevented. 
         [0008]    The liquid cryogenic material, e.g. liquid nitrogen, is injected into the tubing in an amount sufficient to maintain the temperature of the drill bit at approximately −320° F. and the temperature in the annulus of the wellbore at the position of the formation surrounding the wellbore at a temperature of at least about −50° F. 
         [0009]    The coil of tubing, which is non-rotatable is provided with at least one, and more desirably a plurality, of electrically operated orifice mandrels which are connected to the electric power cable and in fluid communication with the cryogenic fluid in the tubing such that, upon energizing the orifice mandrels, controlled amount of the cryogenic fluid are injected into the annulus of the wellbore at predetermined locations to maintain portions of the formation adjacent the wellbore at the desired temperature. 
         [0010]    In order to maintain the wellbore at the desired cryogenic temperature, at least one turbo-expander is operably connected to and in fluid communication with the annulus of the wellbore. Thus, vapors, and/or cryogenic fluid, exiting the annulus of the wellbore are compressed and cooled to cryogenic temperatures, e.g. −320° F. prior to recirculating the cryogenic liquid to the drill bit via the tubing. 
         [0011]    The method of the present invention will increase the rate of penetration (ROP) many times over, eliminate all wellbore problems and permit the drilling of deep hot zones. 
         [0012]    When employing such drilling techniques the rate of penetration (ROP) will greatly exceed present methods and the method will provide the following benefits:
       1. Thermal shock will significantly lower the strength of rock.   2. Because of bit cooling much better bit life will occur.   3. Bit rotation can be greatly increased resulting in very significant ROP.   4. Use of a hammer bit will be particularly effective in view of thermal shock and expansion pressure of the vaporizing nitrogen.   5. Superconductivity will permit the installation of a very large electric motor.   6. Because of rapid expansion of vaporizing nitrogen, fractures should occur ahead of the drilling surface and increase the effect of thermal shock.   7. The high pressures resulting from the vaporization of nitrogen used in cooling the electric motor can be used to increase the impact force of the hammer bit which will also increase weight on the bit.   8. The “air drilling” effect will greatly increase ROP compared to normal mud drilling.   9. The effect on hole conditions will be dramatic. With the hole frozen, no slumping will occur and the hole will stay completely to gauge.   10. No lost circulation will occur because of the “air drilling” effect.   11. The volume of nitrogen to be used will more than remove cuttings in a timely manner.   12. Except for deep drilling below the formation freezing depth (perhaps 20,000 feet) there will be no need to run intermediate casing. This will be a significant time and cost saving.   13. The method, by freezing the walls of the hole, will eliminate water incursion and eliminate temperature problems.   14. Also, the cryogenic temperatures will significantly increase the strength of the coil tubing, increase bit life and strength.       
 
         [0027]    The use of coil tubing will significantly decrease the cost of the drilling operation; however, the greatest saving will be in reduced drilling time. It is difficult to estimate the reduced drilling time but theoretically a 15,000 foot well can be drilled in 3 days. 
         [0028]    The method of the present invention is also unique in that it eliminates much of the equipment and costs associated with rotary drilling with mud and normal air drilling. Instead of mud pumps, rotary drilling motors, circulation equipment and air compressors required in air drilling, the method of the present invention maintains a 700 psi back pressure on the casing vent, thus eliminating the need for any air compressors. The only energy requirements are for generating electricity and a considerable amount of the electricity can be generated by compressors operated off the turbo expanders. 
         [0029]    The coil tubing unit eliminates the need for large rotary rigs, except for very deep drilling, and also increases the rate of penetration (ROP) by eliminating drill pipe connections. 
         [0030]    Further, the method of the pressure does not require any new equipment but only variations to existing equipment. For example, metals used in the fabrication of the equipment will have to be altered to allow for cryogenic service but equipment such as coil tubing, turbo expanders, electric driven bits and hammer bits are all widely known and commercially available equipment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a pictorial representation, partially in cross-section, of a drilling system utilizing a cryogenic drilling method in accordance with the present invention. 
           [0032]      FIG. 2  is a pictorial representation of the drilling system utilizing the cryogenic drilling method of the present invention. 
           [0033]      FIG. 3  is a schematic diagram of the drilling system for drilling oil and gas wells utilizing the cryogenic drilling method of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    Referring now to the drawings, shown therein is a pictorial representation of a drilling system  10  utilizing a cryogenic drilling method of the present invention. A drilling rig  12  is set on a desired location  14  where a wellbore  16  is to be drilled. The drilling equipment of the drilling rig  12  includes coil tubing  18 , such as a three and one-half inch coil tubing, having an electric power cable  20  extending throughout the length of the coil tubing  18 . The electric power cable  20  will be at approximately seventy-five percent super-compressibility because of the cryogenic fluid, i.e., the liquid nitrogen, injected down the coil tubing  18  will be at approximately −320° F. Thus, an electric motor  22  operably connected to a distal end portion  24  of the coil tubing  18 , can be operated at a higher horse power than under normal conditions where power is supplied to electric motor  22 . 
         [0035]    A drill bit  26 , such as a rotary cutting bit or a hammer drill bit, is operably connected to the distal end portion  24  of the coil tubing  18  is that upon providing power to the motor  22  via the electric power cable  20 , the drill bit  26  is caused to rotate and commence the desired drilling of the borehole  16  in a subterranean formation. 
         [0036]    It may be desired to provide at least one, and desirably a plurality of electrically operated orifice mandrels  28  for permitting controlled amounts of the cryogenic fluid to be injected into an annulus  30  of the wellbore  16  to maintain the annulus  30  of the wellbore  16  and the portion of the formation adjacent the wellbore  16  at the desired cryogenic temperature. The electrically operated orifice mandrels  28  are connected to the coil tubing  18  so that fluid communication is maintained between the electrically operated orifice mandrels  28  and the coil tubing  18 . In addition, the electrically operated orifice mandrels  28  are operably connected to the electric power cable  20  so that selective or all of the electrically operated orifice mandrels  28  can be activated from the surface by supplying power to the electrically orifice mandrels  28  via the electric power cable  20 . The presence, as well as the number of electrically operated orifice mandrels  28  employed will vary widely depending upon the depth to which the wellbore  16  is drilled. For example, when using the coil tubing  18  to drill depths greater than about 1,000 feet, one of the electrical operated orifice mandrels  28  is positioned on the coil tubing  18  at approximately every 1,000 feet of the coil tubing  18 . As previously stated, the use of the electrically operated orifice mandrels  28  permit given amounts of cryogenic fluid, i.e., liquid nitrogen, to be injected into the annulus  30  of the wellbore  16  to maintain the annulus  30  of the wellbore  16  and thus the portion of the formation surrounding the wellbore  16  at desired temperatures. 
         [0037]    Electrically operated orifice mandrels are commercially available and their installation and operation are well known in the art. Thus, no further discussion of the electrically operated orifice mandrels  28  is believed necessary to enable a person to fully understand and practice the method of the present invention. 
         [0038]    A cryogenic liquid, herein after referred to as liquid nitrogen, is injected down the coil tubing  18  at various rates depending on the depth of drilling. Every 1,000 feet one of the electrically operated orifice mandrels  28  is activated so that liquid nitrogen can be injected into the annulus  30  of the wellbore  16 . This will permit the introduction of fresh liquid nitrogen into the annulus  30  of the wellbore  16  in addition to the volumes of cryogenic vaporous nitrogen coming from formation drilled downhole. This refrigeration will keep the walls of the annulus  30  of the wellbore  16  frozen. 
         [0039]    Because of heat of vaporization, (approximately 47% of total refrigeration), the liquid nitrogen will engage the drill bit  26  at a temperature of about −320° F. The −320° F. liquid nitrogen will cool the drill bit  26  and partially vaporize thus removing the cuttings from the wellbore  16  and drastically lowering the temperature of the drilling surface. The drastic reduction of the drilling surface causes “thermal shock” which reduces the strength of the rock and permits a significant increase in the rate of penetration of the drill bit  26  into the formation. 
         [0040]    The thermal shock will also cause fractures to occur ahead of the bit  26 , permitting liquid nitrogen to enter the drilling surface ahead of the drill bit  26  and increase the effect of thermal shock. Additionally, the rate of penetration will occur because the cryogenic temperature will permit the drill bit to be rotated at much higher speeds than normal. 
         [0041]    In addition, one may desire to use a hammer bit as the drill bit  26 . When the hammer bit rises it permits the bottom of the hammer bit to be cooled so that when it strikes the drilling surface the hammer bit drastically increases the effect of thermal shock. The hammer bit can be constructed so that after striking the drilling surface it will be held in a raised position for a brief period of time (i.e. some fractions of a second) to permit a lower surface of the hammer bit to be cooled. 
         [0042]    The electric motor  22  will drive the drill bit  26  operably connected to the distal end portion  24  of the coil tubing  18  in response to power provided to the electric motor  22  via the electric power cable  20  disposed within and extending the length of the coil tubing  18 . As previously stated, since the liquid nitrogen will be at approximately −320° F., the electric power cable  20  Will be at approximately 75% of super-compressibility which will substantially reduce the electric friction in the electric cable  20 . As a result, the electric motor  22  can be a much larger electric motor and as such the electric motor  22  permits even faster rotation of the drill bit  26  and a higher rate of penetration. 
         [0043]    As the vaporized liquid nitrogen leaves the drilling surface and starts up the annulus  30  of the wellbore  16 , it will be reinforced by the introduction of liquid nitrogen from at least one of the electrically operated orifice mandrels  28  which injects approximately −320° F. liquid nitrogen at a position approximately 50 feet from the drilling surface. The cryogenic temperatures of the liquid nitrogen immediately freeze the hot formation face and prevent any fluids from entering the wellbore  16 . By preventing any water from entering the annulus  30  of the wellbore  16  “air” drilling is practical. 
         [0044]    When employing the method of the present invention wellbore  16  stability will be excellent. All the wellbore surface is frozen solid to at least about −50° F. and the electrically operated orifice mandrels  28  located at about every 1,000 feet of the coil tubing  18  are designed to inject additional liquid nitrogen into areas far up the wellbore  16  to insure that the area of the formation surrounding the wellbore  16  remains frozen. Thus, in production operations, formation damage will be eliminated by drilling with liquid nitrogen. 
       Drilling In Unfrozen Wellbore 
       [0045]    It is believed that the method of drilling the wellbore  16  using the method of the present invention will be feasible to a depth of approximately 20,000 feet, the depth where the formation temperatures reach about +300° F. After this temperature, the wellbore  16  normally cannot be frozen nor can reservoir fluids be controlled under normal practice. However, even at formation temperatures of +400° F. to +500° F. the area 1 to 2 inches from the wellbore may remain frozen because of the continuous injection of fresh liquid nitrogen in accordance with the method of the present invention. 
         [0046]    To extend the method of the present invention to depths hotter than +300° F. and in the event the formation at from 1 to 2 inches of the wellbore  16  does not remain frozen, one may set a string of intermediate casing near the +300° F. area of the formation. After WOC, drilling will continue as before with liquid nitrogen at −320° F. being injected down the coil tubing  18  and through the drill bit  26 . The cryogenic liquid nitrogen will still cool the drill bit  26  as before, permit very fast bit rotation and permit thermal shock to occur. The result will be very fast rate of penetration. Thus, a change will occur on the effect of “air” drilling. 
         [0047]    To control the formation pressure of high pressure, oil, gas or water zones, an exit valve  31 on an intermediate wellbore annulus  32  needs to be closed so that the pressure of the vaporized nitrogen will build up to control these pressures. The exit valve  31  can then be cracked to keep the intermediate wellbore annulus pressures just above the total depth pressure. Using this method, depths as deep as 50,000 feet can be reached. The controlling factor is providing steel strong enough to reach 50,000 feet and also to control the hot temperature in the coil tubing  18  and casing during production operations. 
         [0048]    When drilling to depths much below 15,000 feet, coil tubing  18  cannot be used in its present design parameters. Improvements can be made but probably not enough to go below 25,000 feet. To drill below a depth of 25,000 feet a special automatic drilling rig maybe required wherein 3 joints of pipe can be stacked in the rig and run like one joint is presently run. The rig will also need to be enclosed so that cryogenic temperature can be maintained. Connections every 90 feet are much slower than the coil tubing  18  but much faster than drilling at very deep depths today. However, when utilizing the joints of pipe as described above, the joints of pipe, like the coil tubing  18 , is not rotated. That is only the drill bit  26  rotates when using the cryogenic method of drilling disclosed herein. 
       Method To Run Casing 
       [0049]    To run intermediate casing or production casing, special measures must be taken. Prior to reaching casing depth a mouse hole of 100 feet depth should be drilled and 3 joints of casing should be made up. Then the 3 joint stand should be stacked until the weight accumulation of the rig is reached. This will permit rapid running of casing particularly with an automatic hands free rig. 
         [0050]    After reaching casing total depth, the wellbore  16  should be circulated with liquid nitrogen through the drill bit  26  and all the electrically operated orifice mandrel  28  for a period of about one hour. After the circulating period, the coil tubing  18  is pulled from the wellbore  16 . When the coil tubing  18  has been pulled, 3-joint strands of casing are run. Because of cryogenic temperatures, the casing, like the coil tubing  18  of the drill but  26  should contain 9% nickel. 
         [0051]    When the complete string is run to T.D., the wall of the wellbore  16  will still be frozen. To prepare the well for cementing, hot vaporous nitrogen is circulated down the casing and around into the annulus  30  of the wellbore  16 . The hot vaporous nitrogen need to be circulated only long enough to heat up the outer  2  inches of the wellbore  16  and keep the rest of the wellbore  16  frozen to prevent fluids from entering the wellbore  16 . After heating, arctic type cement is circulated into the annulus  30  of the wellbore  16  and allowed to set. 
         [0052]    To speed up cement healing, hot vaporous nitrogen is circulated down the coil tubing  18  inside the casing. After WOC, complete well if at T.D. or if intermediate casing, resume drilling with liquid nitrogen as before. 
         [0053]    Another option compensate for expansion or contraction of the casing string while the casing is at cryogenic temperatures or heats up during cementing or production operations, a sliding mandrel should be included in the casing string. When all drilling operations are completed and production operations need to begin, the sliding mandrel can be enclosed with cement when the casing string is cemented. 
       Method to Shutoff High Pressure Gas Zone 
       [0054]    To combat a high pressure gas zone, drill into the top of the zone then set a packer  33  on the string of coil tubing  18  and begin injecting liquid nitrogen into the top of the zone. This will result in liquid nitrogen penetrating deep into the zone which will assure that any moisture or water contained in the gas will freeze into hydrates inside the zone rather than in the wellbore  16 . 
         [0055]    After drilling through the complete zone, continue to inject liquid nitrogen deep into the zone. Then begin injecting warm vaporous nitrogen down the coil tubing  18  followed by very hot steam. This will warm up the previous cryogenic temperature coil tubing  18  and permit liquid water to reach the gas zone. 
         [0056]    After injecting all the water/steam into the gas zone follow it with additional volumes of liquid nitrogen. This will cause the previously injected water to freeze all void spaces and shut off the flow of any gas. 
         [0057]    An alternative to this is the probability that high pressure gas flows, because they contain some small amounts of water and at least moisture, will immediately form gas hydrates and freeze over in the formation. 
       Drilling Below Freezing Point 
       [0058]    The before-discussed cryogenic drilling method will not be able to keep the wellbore  16  frozen when the temperature increases to about +300° F. unless the first 1 or 2 inches can be kept frozen. To drill below this point it will be necessary before reaching it to run intermediate 9% nickel casing and cement it in or control it with a packer only. Then, resume drilling with liquid nitrogen as before. The same rate of penetration should occur as thermal shock will still weaken the rock and at an increased rate, the drill bit  26  will still be cooled therefore a high rotation rate will continue and the hammer bit effect will continue to probably greater effect. What will be missing is the “air” drilling effect previously employed. 
         [0059]    Fluids can still be prevented from entering the wellbore  16  by closing the intermediate casing surface vent valve  3 land allowing the pressure from the injected liquid nitrogen to build up to a value exceeding the BHP. When this point is reached, maintain it by venting enough nitrogen to remain constant. 
         [0060]    By employing the method of the present invention, drilling can still be maintained at very high rates of penetration to depths of very high temperatures perhaps to 50,000 feet. To alleviate excessive pressure on zones above the drilling point, controlled volumes of gas could be vented in a limited under-balanced situation. A smaller volume of the liquid nitrogen included in the coil tubing will be directed into a smaller conduit to cool the electric motor. This nitrogen will vaporize and will greatly increase its pressure. Part of this high pressure can be used in the hammer bit to increase the force of impact. The remainder will increase the force emitting from the bit. 
       Production of Liquid Nitrogen 
       [0061]    To drill a well employing liquid nitrogen may require 50,000 gallons of liquid nitrogen per day to drill to a depth of about 5,000 feet. To drill to a depth of about 30,000 feet 400,000 to 500,000 gallons of liquid nitrogen per day may be required. These volumes would be difficult or impossible to use because of costs and even more, so transportation problems. 
         [0062]    Referring now to  FIGS. 2 and 3 , to generate the volume of liquid nitrogen necessary, it is believed desirable that one produce the liquid nitrogen on site by employing one or more turbo expanders  40  and affiliated equipment. This can be accomplished by first injecting liquid nitrogen from a trailer mounted cryogenic tank  42  into the coil tubing  18  via a cryogenic pump  44 . The liquid nitrogen from the cryogenic pump  44  is then passed into a coil tubing assembly  50  which is an insulated enclosure containing a reel or spool of the coil tubing  18 . The coil tubing is removed from the coil tubing assembly  50  and directed to the drilling rig  12  for drilling the wellbore  16 . It should be noted that the coil tubing  18  is maintained in a non-rotating condition during drilling, the only portion which is rotated, is the drill bit  26  which is rotatably connected to a distal end portion  24  of the coil tubing  18 . Once the liquid nitrogen has been disposed within the wellbore  16  the liquid nitrogen is returned to the top of the surface casing. If the casing discharge pressure is held to about 700 PSI and the temperature is held at about −50° F., the gaseous nitrogen recovered is capable of operating the turbo expanders  40 . Thus, the gaseous nitrogen and any entrained liquid nitrogen is injected two two-stage turbo expanders  40  and a liquid nitrogen exiting the turbo expanders  40  is delivered to and stored in the cryogenic trailer tank  42 . The liquid nitrogen is then injected by the high pressure cryogenic pump  44  into the coil tubing  18  for further drilling. By recycling the liquid/gaseous nitrogen, the system becomes a substantially closed system. 
         [0063]    The turbo expanders  40  produce an effective amount of energy to run the electric motors  46  which, in turn, are coupled to the electric motor  48  of the coil tubing assembly  50 . The coil tubing assembly  50  includes, the spool or coil of tubing  18  and the electric motor  48 . The electric motor  48  serves as a source of power for the electric motor  22  mounted on the distal end portion  24  of the coil tubing  18  which, upon activation, rotates or activates the drill bit  26 . 
         [0064]    As more clearly shown in  FIG. 3 , the cryogenic pump  44  pumps the cryogenic liquid nitrogen into the wellbore  16  being drilled by the drilling rig  12  via the coil tubing  18 . The liquid nitrogen which is returned to the surface casing is then pumped to a centrifugal cutting trap  52  wherein cuttings are separated and forwarded to a pit  54 . The liquid nitrogen recovered from the wellbore  16  is passed through a filter  56  and into one of the turbo expander  40 . The liquid nitrogen recovered from the turbo expander  40  is then passed through a separator  58  and into the second turbo expander  40  for further treatment. Upon leaving the second turbo expander  40  the liquid nitrogen is passed through a distillation tower  60  before being stored in the trailer mounted cryogenic tank  42 . When additional nitrogen is required in the drilling operation the recycled liquid nitrogen is pumped from the trailer mounted cryogenic tank  42  into the coil tubing via the cryogenic pump  44 . 
         [0065]    The only transported liquid nitrogen required in the practice of the present invention is the initial volume required to activate the turbo expanders  40 . After this the turbo expanders  40  recycle the necessary nitrogen except for that required for the continuing volume expansion due to drilling. For a 5,000 foot well the volume of nitrogen required would be about 2700 gallons to permit recycling of nitrogen to the turbo expanders the nitrogen must be cleaned and to accomplish this a central cuttings trap and air filter will be used to remove cuttings and dust. 
         [0066]    The method of the present invention not only solves the nitrogen problem but permits the use of very large volumes of liquid nitrogen so that the wellbore  16  can be kept frozen to very low temperatures and will be helpful in deep drilling. 
         [0067]    It is believed that up to 15,000 cubic feet or equivalent liquid nitrogen be used in the first 5,000 feet of drilling and up to 15,000,000 cubic feet in very deep drilling. Substantially all of the nitrogen will be recycled through the turbo expanders. 
       Equipment Illustration 
       [0068]    To illustrate the equipment used an its configuration, consider the liquid nitrogen generated by the turbo expanders  40  entering the cryogenic liquid nitrogen trailer tank  42 . Using the cryogenic pump  44  the liquid nitrogen is pumped into trailer mounted coil tubing  18 , which is enclosed in an air free insulated enclosure, and down to the drill bit  26 . For deep drilling a cryogenic insulated enclosure may be installed around the drilling derrick rig  12 . 
         [0069]    All equipment would be trailer mounted for quick transportation  FIGS. 1 ,  2  and  3  show the configuration of equipment. Further, all equipment in contact with the cryogenic liquid must be fabricated of materials capable of handling the cryogenic temperatures. An example of such of a material is stainless steel containing about 9% nickle. 
       SUMMARY 
       [0070]    The advantage of using liquid nitrogen are as follows:
       1. The (−320° F.) temperature will eliminate bit heat thus greatly increase bit life.   2. The (−320° F.) temperature will permit the bit rotation to be greatly increased thus greatly increasing ROP.   3. The (−320° F.) temperature will result in significant thermal shock thus lowering the strength of the rock and result in increased ROP, and also cause thermal fractures to occur ahead of the drilling surface.   4. A hammer bit can be used to help cool the cutting surface as the bit rises vertically and would be particularly efficient because of thermal shock.   5. The stability of the borehole will be solid. Because the wall temperature will vary between −320° F. at the bottom of the hole to −50° F. at the top, the frozen wall will prevent borehole enlargement.   6. The method will freeze any water entering the borehole thus permitting “air” drilling (liquid nitrogen).   7. The method will eliminate the flow of high pressure gas into the borehole.   8. By using an electric motor to drive the bit, the power cable will be at about 75% of super conductivity (−320° F.), therefore reducing the electric resistance and permitting a larger horsepower motor to drive the bit even faster.   9. Drilling with liquid nitrogen will also alter the need for multiple casing strings. In liquid nitrogen drilling, one drills the same size borehole to total depth, eliminate the influx of all fluids into the wellbore and end up with a frozen wellbore.   10. Drilling with liquid nitrogen will greatly affect the problems relating to heat in deep drilling on associated equipment.   11. Cryogenic temperatures will strengthen coil tubing and permit deeper drilling.   12. The method permits drilling to 50,000 feet even beyond the ability to freeze the wellbore.   13. Thermal shock should eliminate crooked holes as the formation directly below the bit should immediately lose strength, while the area sideways to the bit will remain strong and thus resist being drilled.