Abstract:
Methods of drilling open hole and apparatus for achieving same are described. One apparatus comprises a pressure control component strategically positionable in or above a wellbore to isolate downhole pressure while drilling open hole, the pressure control component comprising a rotational drive member effective to rotate a tubular member while drilling open hole. This abstract allows a searcher or other reader to quickly ascertain the subject matter of the disclosure. It will not be used to interpret or limit the scope or meaning of the claims.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The present invention involves use of a snubbing apparatus to drill open hole. More particularly, the present invention involves use of a snubbing apparatus to drill open hole in underbalanced and managed pressure situations.  
         [0003]     2. Related Art  
         [0004]     Heretofore, various configurations of drilling rigs have been proposed. The basic purpose of all such rigs has been the same: to drill a hole (a well or wellbore) that taps into an oil or gas reservoir. Subsequently, crude oil and/or natural gas are extracted from the tapped reservoir and processed into usable forms, such as gasoline and heating oil.  
         [0005]     In oil and gas operations, pipes or tubular members are usually run in or pulled out of a well using conventional drilling or workover rigs or a snubbing apparatus. Workover rigs are effectively small drilling rigs having a derrick and drawworks. Although less costly and easier to employ than full sized drilling rigs, workover units can still be quite costly. Snubbing units are generally smaller, easier to transport and less expensive than workover or drilling rigs. The term “snubbing unit” is an industry expression designating equipment used to raise or lower pipe and bottom hole assembly components in a wellbore through some form of packoff assembly, with wellbore pressure. Thus, snubbing units are often employed when working a pressurized well which requires that tubular members be forced into the wellbore.  
         [0006]     Conventional snubbing units require some means for forcing a pipe or tools into a well until the imposed force is sufficient to overcome any lifting or ejecting force of the well pressure on the pipe. The forcing can be accomplished with wireline blocks and wire rope. Typically, the pipe or tools are forced through a stripper head or blowout preventer. The term “stripper head” includes rotating control heads or rotating blowout preventers or annular blow out preventer.  
         [0007]     It is often desirable during snubbing operations to rotate the pipe or tubular member. Most snubbing units, however, do not provide a method for imposing torque on a held pipe or tubular member (e.g., the snubbing unit described in U.S. Pat. No. 4,085,796); other snubbing units do (e.g., the snubbing apparatus described in U.S. Pat. No. 5,746,276).  
         [0008]     Generally, rigs can rotate a bit in one of three different ways. One way uses a rotary table and kelly. Alternatively, a topdrive device can be employed. A third possible way involves use of a downhole motor.  
         [0009]     A rotary table and kelly system involves use of a rotating turntable that mates with a special flattened sided pipe (a kelly). A top drive, on the other hand, uses a “power swivel,” not a kelly, to rotate the drill stem and bit. The third alternative, downhole motors, does not rotate the pipe; instead, it rotates the drill bit, and is powered by a drilling fluid, most often.  
         [0010]     A “packoff assembly” has previously been mentioned. To “pack off” is to place a packer or other elastomeric equipment in or above the wellbore and to activate it so that it forms a seal between the drillstring or tubing and the casing. Often, the packoff assembly comprises a blowout preventer (BOP). A BOP is equipment that prevents the escape of pressure either in the annular space between the casing and drill pipe, or from the drill pipe itself when used in conjunction with some form of internal drillstring valve, during drilling and completion operations.  
         [0011]     “Casing” is steel pipe placed (and then generally cemented) in a well as drilling progresses to prevent the walls of the wellbore from caving in during drilling, and to provide a conduit to the surface. An open hole, on the other hand, is a wellbore having no casing.  
         [0012]     Conventionally, drilling is carried out in an overbalanced condition, a condition in which fluid pressure in a wellbore is greater than fluid pressure in the formation surrounding the wellbore. Mud (a fluid circulated in a well) is used in conventional drilling to ensure that hydrocarbons are not produced during drilling, which can be unsafe. In underbalanced drilling (UBD), on the other hand, fluid pressure in a wellbore is less than fluid pressure in the surrounding formation. A primary value of underbalanced drilling is to reduce formation damage. Negative or differential pressure between the formation and a wellbore also enhances fluid and gas flow from the reservoir by reducing the damaging effects encountered when using fluids. Increased penetration rates are also often observed in wells drilled underbalanced. In UBD, hydrocarbon production is assumed, and planned for. Underbalanced drilling is, however, initially more costly than overbalanced drilling, and to maximize benefits extreme care must be taken to keep drilling and completion operations underbalanced at all times.  
         [0013]     “Managed pressure drilling” involves precise control of the annular fluid pressure within a wellbore. Managed pressure drilling (MPD) is a technique or tool which will, in comparison with conventional drilling, mitigate some of the risks and costs associated with drilling wells with narrow downhole environmental limits by proactively managing (or “walking the line” with regard to) the annular hydraulic pressure vs. formation or reservoir pressure profile. The purpose of MPD is both to ascertain the downhole pressure environmental limits and to manage the annular hydraulic pressure profile to fill that window. In MPD, hydrocarbon production is not planned for or is limited in volume.  
         [0014]     With regard to both UBD and MPD, the associated wellbore is under pressure (putting aside hydrocarbon production), and therefore pressure in the wellbore must be handled appropriately.  
         [0015]     U.S. Pat. Nos. 5,029,642 and 5,180,012 disclose a method and apparatus for the running of various tools and devices used to service oil and gas wells in combination with coiled tubing apparatus that permits the application of a sudden downward force of predetermined magnitude. These patents teach away from use of a snubbing apparatus to retrieve stuck valves, as coil tubing apparatus can (according to these patents) provide a much greater pulling capacity than a wireline saving the use of the much more expensive standby work-over rig snubbing apparatus necessary in the event the wireline apparatus cannot retrieve a stuck valve.  
         [0016]     U.S. Pat. Nos. 5,662,170 and 5,842,528 (assigned to Baker Hughes) disclose a method and assembly for completing a well. These patents do mention open hole drilling and completion, and that snubbing units are less expensive; however, they do not teach or suggest use of snubbing units for drilling open hole: “In order to minimize cost [of drilling and completion], several techniques have been employed with varying degrees of success. One technique has been to drill and case the well, and then immobilize the drilling rig. A replacement rig is then utilized to complete the well. The replacement rig may vary from a snubbing apparatus, coiled tubing apparatus, work over rig using smaller inner diameter pipe, and in some cases wire line. Thus, rather than completing the well with the more expensive rig, a less expensive rig is utilized.” (column  2 , lines  22 - 30 ) These passages are not enough to teach or suggest using a snubbing apparatus for drilling open hole.  
         [0017]     U.S. Pat. No. 6,065,550 discloses a method and system of drilling multiple radial wells using underbalanced drilling.  FIG. 11  of this patent illustrates a flow diagram for underbalanced drilling or completing of multilateral wells off of a principal wellbore utilizing the two string technique where there is a completed multilateral well that is producing and a multilateral well that is producing while drilling is ongoing utilizing drill pipe and a snubbing apparatus as part of the system. However, from the disclosure it appears that the snubbing apparatus is not being used to drill, but only being used in the traditional sense to control pressure in an underbalanced situation.  
         [0018]     U.S. Pat. No. 6,209,663 discloses apparatus and methods for a deployment valve used with an underbalanced drilling system to enhance the advantages of underbalanced drilling. The underbalanced drilling system may typically comprise elements such as a rotating blow out preventer and drilling recovery system. The deployment valve is positioned in a tubular string, such as casing, at a well bore depth at or preferably substantially below the string light point of the drilling string. This patent teaches away from using a snubbing apparatus for removing drill string, and does not disclose or suggest using a snubbing apparatus for drilling open hole. After discussing some of the problems of UBD, the patent states “Another very effective and safe practice is that of providing a snubbing apparatus for removing the drilling string. However, the snubbing apparatus takes considerable time to rig up, requires considerable additional time while tripping the well, and then requires considerable additional time to rig down. Thus, the cost of tripping the drill string can be quite considerable due to the rig time costs and snubbing apparatus costs. Additional tripping of the well may also be necessary and again require the snubbing apparatus. This procedure then, while effective and safe, increases drilling costs considerably.” 
         [0019]     U.S. Pat. No. 6,367,566 discloses a system and method that permits control of down hole fluid pressures during underbalanced drilling, tripping of the drill string, and well completion to substantially avoid “killing” of the well and thereby damaging the producing formations in the well bore. The system and method utilizes separate and interconnected fluid pathways for introducing a downwardly flowing hydrodynamic control fluid through one fluid pathway and for removing through the other fluid pathway a commingled fluid formed by mixing of the hydrodynamic control fluid and the well bore fluids flowing upwardly in the well bore. This patent teaches away from using a snubbing apparatus, and does not teach or suggest using a snubbing apparatus to drill open hole. The patent states “As an alternative to killing the well with a heavy well-bore fluid, the drill pipe, production tubing or other equipment may be stripped in or out of the well under high well-head pressure through a snubbing apparatus. This procedure is expensive and complicated. Furthermore, if the well is shut-in under high pressure while stripping pipe or equipment in or out of the well through a snubbing apparatus, the liquids at the bottom of the well bore may be injected into the formation adjacent to the well bore and adjacent to the hydraulic fractures. In the zones invaded by these bottom-hole well-bore liquids while stripping through the snubbing apparatus, the rock permeability to the formation fluids may be severely damaged as described above.”  
         [0020]     U.S. Pat. Nos.6,394,184; 6,520,255; and 6,957,701 (assigned to ExxonMobil Upstream Research Company) disclose methods of, as well as apparatus and systems for, perforating and treating multiple intervals of one or more subterranean formations intersected by a wellbore by deploying within said wellbore a bottom-hole assembly (“BHA”) having a perforating device and a sealing mechanisms, wherein pressure communication is established between the portions of the wellbore above and below the sealing mechanism. These patents discuss snubbing, but not in terms of drilling using a snubbing apparatus.  FIG. 10  of the patents illustrates in detail an embodiment of the invention where a jetting tool is used as the perforating device and jointed tubing is used to suspend the BHA in the wellbore. As the patent states, in this embodiment, jointed tubing is preferably used with a mechanical compression-set, re-settable packer since it can be readily actuated and de-actuated by vertical movement and/or rotation applied via the jointed tubing. Vertical movement and/or rotation is applied via the jointed tubing using a completion rig-assisted snubbing apparatus with the aid of a power swivel apparatus as the surface means for connection, installation, and removal of the jointed tubing into and out of the wellbore.  
         [0021]     U.S. Pat. Nos. 6,457,540 and 6,745,855 disclose a method and system of drilling straight directional and multilateral wells utilizing hydraulic frictional controlled drilling, by providing concentric casing strings to define a plurality of annuli therebetween; injecting fluid down some of the annuli; returning the fluid up at least one annulus so that the return flow creates adequate hydraulic friction within the return annulus to control the return flow within the well. A snubbing unit is used for well control during trips out of the hole and to keep the well under control during the process. The snubbing units is considered a key component for being able to safely trip in and out of the wellbore during rotary drilling operations. The snubbing unit allows one to retrieve the drill bit out of the hole and yet maintain the pressure of the underbalanced well to keep the well as a live well. However, the snubbing unit is used only when the drilling or completion assembly is being tripped in and out of the hole. These passages do not teach or suggest using a snubbing unit for drilling open hole.  
         [0022]     U.S. Pat. No. 6,755,261 (assigned to Varco I/P, Inc.) discloses methods and systems for varying fluid pressure in a circulation system while circulating a “kick” out of a wellbore drilled through a subterranean formation. While the patent does discuss open hole, the only mention of snubbing is in a boilerplate paragraph near the end, where it is stated that equipment used may also include conventional and known non-conventional equipment, including coiled tubing units or snubbing units.  
         [0023]     Published US patent application 20040238177 discloses an arrangement and a method to control and regulate the bottom hole pressure in a well during subsea drilling at deep waters, involving adjustment of a liquid/gas interface level in a drilling riser up or down. The only mention of snubbing is in discussing the gas pressure in the riser being  
       DETAILED DESCRIPTION  
       [0024]     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.  
         [0025]     All phrases, derivations, collocations and multiword expressions used herein, in particular in the claims that follow, are expressly not limited to nouns and verbs. It is apparent that meanings are not just expressed by nouns and verbs or single words. Languages use a variety of ways to express content. The existence of inventive concepts and the ways in which these are expressed varies in language-cultures. For example, many lexicalized compounds in Germanic languages are often expressed as adjective-noun combinations, noun-preposition-noun combinations or derivations in Romanic languages. The possibility to include phrases, derivations and collocations in the claims is essential for high-quality patents, making it possible to reduce expressions to their conceptual content, and all possible conceptual combinations of words that are compatible with such content (either within a language or across languages) are intended to be included in the used phrases.  
         [0026]     The invention describes apparatus and methods of using same in drilling open hole in wellbores. The terms “well” and “wellbore” are used interchangeably herein, and may be any type of well, including, but not limited to, a producing well, a non-producing well, an experimental well, and exploratory well, and the like. Wellbores may be vertical, horizontal, some angle between vertical and horizontal, diverted or non-diverted, and combinations thereof, for example a vertical well with a non-vertical component. Although methods for drilling open hole have been improved over the years, there remains a need for improved designs, especially those that may operate in a variety of wellbore pressure situations without killing a well.  
         [0027]     The methods and apparatus of the present invention may be employed in an underbalanced application or in a managed pressure application.  
         [0028]     There are many reasons for using a snubbing unit instead of a conventional drilling rig, some of those reasons including: very low, causing the drill string to be “pipe heavy” at all times, excluding the need for snubbing equipment or “pipe light” inverted slips in the drilling operation.  
         [0029]     Published US patent application 20050006098 generally relates to a method and an apparatus for stimulating the production of an existing well. The patent teaches away from the use of a snubbing apparatus for pressure control, and there is no teaching or suggestion of drilling with a snubbing apparatus. As a selective treatment assembly is urged toward the surface of the wellbore, a releasable mechanical connection fails, thereby allowing a plug assembly to separate from the selective treatment assembly and remain downhole in a polished bore receptacle (PBR) while the selective treatment assembly continues to be moved toward the surface of the wellbore. In this respect, the plug assembly separates and seals a treated portion of the wellbore below the PBR from an untreated portion of the wellbore above the PBR. Thereafter, the pressure in the untreated portion of the wellbore is bled down to 0 Psi, thereby allowing the jointed pipe connected to the selective treatment assembly to be removed without the use of a snubbing unit.  
         [0030]     Published U.S. patent application 20050115713 discloses a wellbore junction including a first passage extending from a first opposite end to a second opposite end of the wellbore junction. A window is formed through a sidewall of the wellbore junction and provides fluid communication between the first passage and an exterior of the wellbore junction. Snubbing is mentioned discussing UBD as a possible implementation of the invention. A fluid loss control device such as a valve may be used in these embodiments, which permits a drill string to be tripped in and out of the branch wellbore while the wellbore is in an underbalanced condition, and without a need for killing the well or snubbing the drill string out of the well under pressure. There is no teaching or suggestion of using a snubbing apparatus to drill open hole.  
         [0031]     U.S. Pat. No. 4,085,796 (assigned to Otis Engineering Corporation, Dallas, Tex.) describes snubbing units as well tubing handling systems employed in running and pulling well tubing under pressure, particularly, for well workover operations. The patent notes that one of the most vital factors in the operation of well snubbing units is the cost of servicing a well with the apparatus. Such cost is directly related to the time factor which is primarily affected by the speed of operation of the apparatus. In addition to the number of cycles required for the apparatus to run or pull a given length of well tubing, the actual weight which the apparatus can handle is an important factor. Usually, such apparatus is rented or leased on a time basis and the cost of operation is additionally affected by the cost of paying the wages of the operators. A still further element in the total expense of servicing a well is the length of time the well is shut down and, thus, not producing an income.  
         [0032]     U.S. Pat. No. 4,649,777 discloses back-up power tongs for holding a tubular member, such as a drill pipe, against rotation of a connected tubular member, the back-up tongs comprising a body, having a center opening of sufficient size for the tubular member to pass therethrough, a slot communicating between the edge of the body and the center opening, and a cavity disposed within the body. A plurality of jaw members are disposed within the body around the perimeter of the center opening, each jaw member having a concave surface generally conforming to the curvature of the tubular member and facing the interior of the opening so as to be grippingly engageable with the tubular member. At least one cylinder assembly is disposed within the cavity and fixedly connected to one jaw member so as to extend or retract the jaw member on a radial path centered at the center of the tubular member.  
         [0033]     U.S. Pat. Nos. 5,746,276 and 5,566,769 (assigned to Eckel Manufacturing, Odessa, Tex.), disclose a tool for rotating a tubular member passing through a slip assembly powered by a rotary table of a drilling rig. The patents state: “While snubbing operations have significant advantages compared to workover operations where the well is killed, a recognized disadvantage of a snubbing operation is that the workover string may become stuck in a wellbore. When this occurs, remedial operations to “unstick” the string can be very expensive and time consuming. In some instances, it is also desirable to rotate a work string during a conventional workover operation even when the well is killed. The patent describes improved techniques and equipment which allow rotation of a workover string during the snubbing or workover operations. More particularly, the tool allows for the rotation of the entire work string when making up or breaking apart a threaded joint at the well. While this patent appears to disclose means to rotate a workover string during snubbing, there is no teaching or suggestion of using a snubbing apparatus to drill open hole.  
         [0034]     U.S. Pat. No. 6,640,939 discloses an improved snubbing unit including a snubbing unit base, a lifting assembly having a platform positioned above the base, a rotary table positioned on the snubbing unit, and a first slip assembly positioned on the rotary table. While this patent appears to disclose means for rotating a workover string during snubbing, there is no teaching or suggestion of drilling open hole with the snubbing apparatus.  
         [0035]     Notwithstanding all of the advances made in the drilling art, some of which are discussed briefly above, it is a shortcoming and deficiency of the prior art that there has not heretofore been a quick and easy way to drill, especially in certain unconventional situations.  
       SUMMARY OF THE INVENTION  
       [0036]     In accordance with the present invention, methods and apparatus are described that reduce or overcome problems in previously known methods and apparatus.  
         [0037]     A first aspect of the invention are methods, one method comprising: 
        a) isolating or controlling downhole pressure in a wellbore; and     b) drilling open hole while isolating or controlling the downhole pressure.        
 
         [0040]     Methods of the invention include those methods wherein the drilling open hole while isolating or controlling of downhole pressure comprises using a push-pull component selected from a deployment valve, a snubbing unit, a well workover rig, a wireline push-pull unit, and combinations thereof. Certain methods of the invention include those wherein the drilling open hole comprises drilling in an underbalanced condition. Other methods of the invention are those wherein the drilling open hole comprises drilling in a managed pressure condition. Methods of the invention may include producing one or more materials from the well, and may further comprise processing the material or materials, wherein the materials may be any materials producible from a wellbore, including an open hole, and may comprise materials selected from hydrocarbons (including but not limited to hydrocarbon oils and hydrocarbon gases), dirt, water, brine, and the like, and combinations and mixtures thereof. Where the material comprises hydrocarbon oil and gas, and water, processing may include separating oil, gas and water, and may further include reinjecting one or more processed fluids downhole.  
         [0041]     A second aspect of the invention are apparatus, one apparatus comprising: 
        a) a pressure control component strategically positionable in a wellbore to isolate or control downhole pressure while drilling open hole;     b) the pressure control component comprising a rotational drive member effective to rotate a tubular member while drilling open hole.        
 
         [0044]     Apparatus of the invention include those wherein the pressure control component is a push-pull unit selected from a snubbing unit, a workover rig, a wireline push-pull unit, and combinations thereof. Apparatus of the invention include those apparatus further comprising means for processing any material extracted from the wellbore, and apparatus wherein the pressure control component achieves an underbalanced situation in the wellbore. Certain apparatus of the invention are those apparatus in which the pressure control component achieves a managed pressure situation in the wellbore, or in an annulus. Apparatus of the invention include those apparatus comprising processing components, which may be selected from gas extracting components, dirt extracting components, and the like, and combinations thereof. Certain apparatus of the invention comprise separation units capable of separating oil, gas and water, and may further comprise processing units capable of reinjecting wellbore fluids downhole.  
         [0045]     Another apparatus of the invention comprises: 
        a) a snubbing unit for drilling a well open hole in a surrounding geologic formation having a formation fluid pressure, the well having a well fluid pressure; and     b) means for maintaining the well fluid pressure less than the formation fluid pressure.        
 
         [0048]     Apparatus within this aspect of the invention include a surface system for processing any material extracted from the well, wherein system for processing may comprise any one or more of a gas extractor, an oil extractor, and a water extractor. The processing system may comprise a fluid reinjector for reinjecting fluid downhole. The apparatus may comprise means for maintaining the well in an underbalanced situation or in a managed pressure situation.  
         [0049]     These and other features of the apparatus and methods of the invention will become more apparent upon review of the brief description of the drawings, the detailed description of the invention, and the claims that follow.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0050]     The manner in which the objectives of the invention and other desirable characteristics can be obtained is explained in the following description and attached drawings in which:  
         [0051]      FIG. 1  is a schematic, partial block and partial longitudinal cross section view illustrating an embodiment of the present invention;  
         [0052]      FIG. 2  is a schematic side elevation view, partially in cross section, with some parts broken away, illustrating a snubbing unit which may be used according to the teachings of the present invention;  
         [0053]      FIG. 3  is a schematic side elevation view illustrating a snubbing stack incorporating some of the features of the present invention;  
         [0054]      FIG. 4  is a graph depicting depth versus pressure profiles in underbalanced drilling (UBD); and  
         [0055]      FIG. 5  is a schematic process flow diagram illustrating a surface processing system that may be employed in practicing the present invention. 
     
    
       [0056]     It is to be noted, however, that the appended drawings are not to scale and illustrate only typical embodiments of this invention, and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. Identical reference numerals are used throughout the several views for like or similar elements.  
         [0057]     a. Reduced rig/operational modifications for drilling programs requiring flammable drilling fluids/gases. Electrical/ignition sources are easily removed from the areas of operations during well planning operations.  
         [0058]     b. Apparatus of the present invention are designed for pressure operations and allows tripping of pipe without killing the well. This is especially important on IADC level  4  and level  5  wells where flow rates and pressures can be extremely important.  
         [0059]     c. Rig crews are for the most part trained and ready for operating in pressured conditions. Use of apparatus and methods of the present invention reduces or eliminates the cost of crew training and the fear factor for conventional rig crews.  
         [0060]     d. A stand alone hydraulic snubbing unit may allow drilling operation without the cost and availability of a rig.  
         [0061]     e. The footprint of systems incorporating apparatus of the invention is much smaller than that of a conventional rig modified for underbalanced drilling. This should prove effective on small platforms and on small land locations where space is critical.  
         [0062]     f. Stack configurations incorporating apparatus of the invention may accommodate items such as core barrels and other bottom hole assembly systems.  
         [0063]     g. Apparatus and methods of the invention provide an excellent system for underbalanced drilling and managed pressure drilling or coring.  
         [0064]     h. When apparatus of the invention are used in conjunction with a full opening valve which is placed on the wellhead, the stack out becomes a lubricator for the bottom hole assembly. The valve functions as a shallow set deployment valve, an emergency blind ram, and it supplies a barrier upon rig up or rig down operations.  
         [0065]     The methods of using apparatus of the invention and systems incorporating same provide for drilling a well open hole, including isolating the pressure downhole. Such isolation may be provided by a deployment valve.  
         [0066]     Still further, the methods and apparatus of the invention and systems incorporating same may provide means for processing material removed from a well, including separating oil, gas, and other material (such as water, or dirt) and means for reinjecting fluid back into the well, however, the invention is not so limited; the ability to separate fluids at the surface is not required.  
         [0067]     Referring now to the drawings, and more particularly to  FIG. 1 , which is a schematic side elevation view, partially using blocks, and in partial cross section, of the major components of apparatus and methods of the present invention. In broad terms, the present invention comprises apparatus and methods for isolating pressure downhole or beneath some type of pressure control equipment, whether this is a stripping head, rotating control head or other type of pressure control component. The pressure control component of the inventive apparatus may be a snubbing unit. Alternatively, or in conjunction with a snubbing unit, the pressure control component may be (or include) a deployment valve placed in the wellbore. In the latter case, a deployment valve may be pulled or run into the wellbore to the point below where the drillpipe is still “heavy” in the well; there, pressure would be bled off above the valve so that pipe may be pulled out without formally “snubbing” the well.  
         [0068]     Referring specifically to snubbing units, any type of snubbing unit—hydraulic, mechanical, combination thereof, or any other equivalent type, for that matter—may be used in embodiments of the present invention. “Snubbing”, a verb, is well known in the art, and generally means forcing a pipe into a well against pressure. As previously mentioned, snubbing units (i.e., apparatus that perform snubbing) are well known and readily available; apparatus of the invention may comprise any type of snubbing unit, and methods of using apparatus of the invention may use any type of snubbing unit to practice the present invention, although the invention is not limited to use of snubbing units for pressure control, as previously discussed.  
         [0069]     Referring again to  FIG. 1 , illustrated schematically therein is a pipe  4  being forced by a snubbing unit  2  into a well  6 . At the farthest end (distal end) of pipe  4  into well  6  is a bit  8 . Bit  8  is a cutting or boring element that drills well  6  into and through a formation  10  surrounding well  6 . Well  6  may also be called a wellbore. Ideally, hydrocarbon oil and/or gas will be tapped into by well  6 , and the hydrocarbon oil and/or gas can then be brought to the surface  12  for processing.  
         [0070]     The instant invention involves the use of snubbing unit  2  to drill open hole. While snubbing units have previously been used in the art for their intended use (snubbing), the inventor herein is aware of no instance where a snubbing unit has been used to drill open hole in the underbalanced condition. Using a snubbing unit to drill open hole is an aspect of the present invention. An “open hole” is a well in which casing or a protective liner has not been set. Casing, illustrated in  FIG. 1  by reference numeral  14 , is pipe placed (and generally cemented) in a hydrocarbon oil or gas-bearing well as drilling progresses to prevent the wall of well  6  from caving in. A portion of well  6  designated with reference numeral  16  in  FIG. 1  has no casing; portion  16  is an open hole. Open hole drilling has no casing; the walls of the borehole are the substrate.  
         [0071]     Referring still to  FIG. 1 , snubbing unit  2  is capable of raising and lowering pipe  4  and bottom hole assembly (i.e., bit  8 ) into well  6  through some form of packoff assembly  18 . Packoff assemblies insure that a well is drilled correctly (i.e., having a proper flow course). A blowout preventer (BOP) is equipment installed at the surface at the top of well  6  (or possibly on the seafloor), which equipment prevents the escape of pressure during drilling and completion operations. A rotating control head (RCH), an annular BOP or a similar such packoff assembly structure  18  could also be employed at this location. Any and all of which manner of packoff assemblies  18  through which pipe could be manipulated by a snubbing unit according to the teachings of the present invention will readily occur to those skilled in the art. By way of illustration, a BOP is illustrated in block form at  18  in  FIG. 1 .  
         [0072]     For convenience in further description, well  6  will be said to contain hydrocarbon oil or gas or water having certain fluid pressure, while the surrounding formation  10  also has an associated fluid pressure. Underbalanced drilling involves the pressure of the fluid in well  6  being less than the pressure of formation  10 . Underbalanced drilling is well known to increase safety and productivity. See “Underbalanced Completions Improve Well Safety and Productivity,” by Tim Walker and Mark Hopman (World Oil, November 1995), which is hereby incorporated by reference herein. Apparatus and methods of the invention are especially ideal for drilling open hole in an underbalanced condition.  
         [0073]     Yet another term frequently used in the art is “managed pressure drilling.” Managed pressure drilling, or MPD, as it is called, encompasses adaptive drilling techniques (e.g., use of backpressure, variable fluid density, fluid rheology, circulating friction, and/or annular fluid pressure profile) within a well. Further information regarding the techniques and advantages of MPD may be found in materials generated for the Managed Pressure Drilling Forum, Jan. 27-29, 2004, at the Moody Gardens Hotel and Conference Center in Galveston, Tex., which is hereby incorporated by reference herein. Apparatus and methods of the present invention are also especially ideal for open hole drilling in managed pressure situations.  
         [0074]      FIG. 2  is a schematic side elevation view, partially in cross section, with some parts broken away, illustrating a snubbing unit which may be used according to the teachings of the present invention. Snubbing unit  2  may be any conventional snubbing unit, except that it is used to drill open hole, particularly in underbalanced and managed pressure situations in accordance with the teachings herein.  
         [0075]     Snubbing unit  2  incorporates slip assemblies  24 . Snubbing unit  2  in  FIG. 2  is illustrated positioned on a blowout preventer  26  and generally comprises a base  28 , basket support columns  30 , a basket  32  (with a basket railing  34 ), and a lifting assembly  36 . Lifting assembly  36  includes a lifting platform  38  supported by hydraulic cylinders  40  which raise and lower lifting platform  38 . Positioned atop lifting platform  38  is a rotary table  42  with a first (upper) slip assembly  44  connected thereto. Rotary table  42  may be any conventional torque generating device which may be positioned atop lifting assembly  36 . Many different types of rotary tables are well known in the drilling industry and could be applied in snubbing unit  2 , although the rotary table illustrated in  FIG. 2  is hydraulically driven. Hydraulic fluid may be supplied to rotary table  42  through hydraulic hoses  46  and to slip assembly  44  through a conventional hydraulic swivel. Hydraulic swivel assemblies are well known in the art and one such hydraulic swivel assembly is available from Superior Manufacturing, Inc., located at 4225 Highway 90 East, Broussard, La., under the tradename Clincher Hydraulic Rotary Table, Model No. HRT-20B (although the model number may vary based upon the rotary table&#39;s size). Another suitable rotary table is available from Hydra Rig, located at 6000 Berry Street, Fort Worth, Tex. 76119.  
         [0076]     The detailed insert of hydraulic swivel assembly  48  illustrated in  FIG. 2  schematically illustrates how a conventional hydraulic swivel supplies fluid to slip assembly  44 . Hydraulic swivel assembly  48  allows a fixed hydraulic fluid line  50  to transfer fluid through a rotating hub  52 . While the main  FIG. 2  illustration only illustrates a single fluid line  50 , the detailed insert more precisely illustrates line  50  divided into dual internal fluid lines  50   a  and  50   b.  Swivel assembly  48  includes a hydraulic swivel ring  54  which encircles rotating hub  52 , but is held stationary (by a structure hidden from view in  FIG. 2 ) while rotating hub  52  is attached to rotary table  42 . Hydraulic swivel assembly  48  will further have two annular passages  56  and  58  formed at the junction of swivel ring  54  and rotating hub  52 . It should be understood that passages  56  and  58  are annular in the sense that they form a space completely encircling the circumference of rotating hub  52 . Because passage  56  is annular, passage  56  may maintain fluid communication between internal fluid lines  50   a  and  60  throughout rotating hub  52 &#39;s entire range of rotation. Likewise, it can be seen that annular passage  58  maintains communication between internal hydraulic lines  50   b  and  62  in the same manner. Seals  64  will ensure fluid does not escape from the point where swivel ring  54  mates with rotating hub  52 . Internal line  60  will typically be attached to an external line (not illustrated) as internal line  60  exits rotating hub  52  and that external line will connect to an inlet (not illustrated) of cylinders (not illustrated). As is well known in the art, line  50   a  may direct fluid to the upper inlet on a cylinder (thus retracting the cylinder) while fluid line  50   b  may direct fluid to the lower inlet on the cylinder (thus extending the cylinder). A second (lower) slip assembly  66  is illustrated positioned upon base  28 . It will be understood that all elements positioned along the center line of snubbing apparatus  2  will have a central aperture allowing a pipe or other tubular member  68  to pass therethrough. A cut-away section illustrates a tubular joint  70  connecting two successive tubular members  68 . A fuller description of snubbing units and their operation may be seen in references such as U.S. Pat. No. 4,085,796 to Council, which is incorporated herein by reference.  
         [0077]     Conventional snubbing units generally include a power and a backup power tong. Additionally, an upper slip assembly may be positioned upon a swivel base which allows the slips to rotate when the tubular string rotates. In operation, the upper slip assembly will grip the tubular string and a lower slip assembly will release the tubular. of the snubbing unit could be designed to possibly impose torque on a tubular  68 . Many other variations of snubbing units are possible, and any and all of those variations are considered within the scope of the invention.  
         [0078]     The inventor herein has found power tongs, a crane or gin pole package (which assists in picking up and lowering pipe), slip rams (in a BOP section) and a long riser or lubricator package, to be especially useful.  
         [0079]     Power tongs have previously been described. A crane is a machine for manipulating heavy pieces of equipment, especially offshore. A gin-pole truck also lifts heavy equipment with hoisting equipment and a pole or an arrangement of poles. Slip rams have also been described. A riser is a pipe through which liquid may travel upward. A lubricator is a specially fabricated length of casing or tubing usually placed temporarily above a valve on top of the casing or tubing head. A lubricator allows a device such as a wireline to pass into the well. All of the terms used herein and the equipment associated with them are well known in the art and should be readily understood and appreciated by those skilled in the art.  
         [0080]     Notwithstanding the particular makeup of a snubbing unit, the instant invention contemplates and covers using a snubbing unit to drill open hole in underbalanced situations, as well as in managed pressure situations.  
         [0081]     Referring now to  FIG. 3 , there is illustrated a snubbing stack  2  that may employ an apparatus of the invention.  FIG. 3  illustrates a conventional 7- 1/16″ 5M CIW “U” Snubbing Stack. The depicted stack  2  includes a Shaffer Annular BOP  80 . Recognizing that the term ram denotes a closing or sealing component, stack  2  also comprises a blind ram  82  that when closed, forms a seal that has no pipe through it. Stack  2  also comprises stripper rams  84  that can perform sealing functions. Stack  2  also comprises safety rams  86  and “VARI VARI”  88 . Please note that elements in  FIG. 3  are not illustrated to scale: for example, the mid placed “snubbing stack” in  FIG. 3  is designated to be 48 feet long; the immediately adjacent elements to it ( 84 / 86  and  86 / 82 ) are each designated to be 44 inches long, although each is depicted as longer than the 48 foot section, mentioned above. Further, it should also be realized that the elements depicted in  FIG. 3  are exemplary only; those skilled in the art should well recognize those elements, their configuration, function Lifting devices such as hydraulic cylinders will lift the upper slip assembly in order to position the tubular joint between the power tong and the backup power tong. The power tong will apply torque to a tubular above a joint while the backup tong holds the tubular against rotation below the joint. As is well known in the art, alternative gripping and releasing of the slip assemblies in conjunction with raising and lowering of the upper slip assembly allows successive joint sections to be positioned between the power tong and backup tongs. In this manner, successive sections of tubulars in the string may be made-up or broken out.  
         [0082]     Prior art snubbing units generally require the use of power tongs to rotate the pipe because prior art slip assemblies are intended to only resist the weight of the tubular string and such slip assemblies cannot effectively apply torque (or resist torque applied) to a tubular member. However, in the snubbing unit illustrated in  FIG. 2 , first slip assembly  44  may be the slip assembly  24  described in detail in U.S. Pat. No. 6,640,939, which is incorporated herein by reference. Further, first slip assembly  44  is fixed to rotary table  42  such that torque may be applied to slip assembly  44 . The slip assemblies  24  are well adapted to applying torque (or resisting torque applied) to the tubular being gripped. Thus, when slip assembly  44  grips tubular  68  as discussed in U. S. Pat. No. 6,640,939, slip assembly  44  may apply torque to the tubular  68  in the same manner as done by power tongs in some prior art snubbing units.  
         [0083]     Certain embodiments of snubbing unit  2  may include a backup tong  72  positioned on snubbing unit  2  and may be connected underneath lifting assembly  38 . In  FIG. 2 , brackets  74  will be fixed to lifting assembly  38  and backup tong  72 , when present, is slid between brackets  74 . In this manner, backup tong  72  will be removably positioned on snubbing apparatus  2 . Backup tong  72  may be any conventional backup tong such as that disclosed in U.S. Pat. No. 4,649,777 to Buck which is incorporated herein by reference. Backup tong  72  will hold lower tubular  68  against rotation while first slip assembly  44  applies torque to upper tubular  68 . In this embodiment of snubbing unit  2 , slip assembly  66  may be any conventional slip assembly.  
         [0084]     Other variations are possible and are considered within the invention. For example, a snubbing unit may not include backup tongs; however, the second slip assembly and use; likewise, the configuration depicted in  FIG. 3  has been successfully employed in practice of the present invention.  
         [0085]     Referring now to  FIG. 4 , there is a graph showing further details about drilling pressure. In the  FIG. 4  graph, the x axis designates pressure whereas the y axis designates well depth. “Pressure” and “depth” are used as labels in  FIG. 4 . Marked on the graph are three lines, each denoting that pressure increases as depth decreases. One of those three lines (marked with dashes) denotes fracture pressure  92 . The area marked  94  then, denotes breaking pressure of a formation. A second line in the  FIG. 4  graph marked with alternating dots and dashes) denotes pore pressure  96 . The area marked  98  then, denotes an area in which underbalanced drilling (UBD) will not produce anything out of the well. The goal then, is to “manage” well pressure in drilling so that the well pressure (indicated by the solid line in  FIG. 4 )  100 , stays between the pore pressure  96  and the fracture pressure  92 .  
         [0086]     Referring now to  FIG. 5 , there is illustrated a processing system that has been successfully employed in an embodiment of the present invention. In  FIG. 5 , various elements are depicted that comprise a drilling apparatus (that is, elements  2  and  16  in  FIG. 1 ). On line  102  the aforementioned components hopefully produce a hydrocarbon gas and fluid mixture. At the gas buster  104  the gas is extracted. Line  106 , then, transmits liquids and solids only. In frac tanks  108  any dirt is extracted from the liquid, leaving only usable oil. The elements depicted in  FIG. 5  are exemplary only. Having a processing system is important but not necessary to practice drilling open hole using snubbing unit. Likewise, it is important but not necessary to practice drilling open hole using a snubbing unit to be able to separate gas and oil and to render any separated oil usable (e.g., by removing dirt from the oil). In a particularly useful processing system, gas, oil and water may be separated, and fluid may be reinjected downhole. Various details of the processing system are not important to practice embodiments of the present invention.  
         [0087]     The apparatus of the invention may connect in any number of ways to their equipment counterparts. Each end of apparatus of the invention may be adapted to be attached in a tubular string. This can be through threaded connections, friction fits, expandable sealing means, and the like, all in a manner well known in the oil tool arts. Although the term tubular string is used, this can include jointed or coiled tubing, casing or any other equivalent structure. The materials used can be suitable for use with fluids typically used in the art.  
         [0088]     Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, no clauses are intended to be in the means-plus-function format allowed by 35 U.S.C. § 112, paragraph 6 unless “means for” is explicitly recited together with an associated function. “Means for” clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.