Abstract:
A blowout preventer (BOP) protector is adapted to support a tubing string in a well bore so that the tubing string is directly accessible during a well treatment to stimulate production. The BOP protector includes a mandrel having an annular sealing body bonded to its bottom end for sealing engagement with a bit guide that protects a top of a casing of a well to be stimulated. The mandrel is connected at its top end to a fracturing head, including a central passage and radial passages in fluid communication with the central passage. The mandrel is locked in a fixed position by a lockdown mechanism that prevents upward movement induced by fluid pressures in the wellbore and downward movement induced by the weight of a tubing string supported at a top of the fracturing head by a tubing adapter. The advantages are that the BOP protector permits access to the tubing string during well treatment and enables an operator to move the tubing string up and down or run coil tubing into or out of the wellbore without removing the tool. This reduces operation costs, saves time and enables many new procedures that were previously impossible or impractical.

Description:
TECHNICAL FIELD 
     The present invention relates to equipment for servicing oil and gas wells and, in particular, to an apparatus and method for protecting blowout preventers from exposure to high pressure and abrasive or corrosive fluids during well fracturing and stimulation procedures while providing direct access to production tubing in the well and permitting production tubing or downhole tools to be run in or out of the well. 
     BACKGROUND OF THE INVENTION 
     Most oil and gas wells eventually require some form of stimulation to enhance hydrocarbon flow to make or keep them economically viable. The servicing of oil and gas wells to stimulate production requires the pumping of fluids under high pressure. The fluids are generally corrosive and abrasive because they are frequently laden with corrosive acids and abrasive proppants such as sharp sand. 
     The components which make up the wellhead such as the valves, tubing hanger, casing hanger, casing head and the blowout preventer equipment are generally selected for the characteristics of the well and not capable of withstanding the fluid pressures required for well fracturing and stimulation procedures. Wellhead components are available that are able to withstand high pressures but it is not economical to equip every well with them. 
     There are many wellhead isolation tools used in the field that conduct corrosive and abrasive high pressure fluids and gases through the wellhead components to prevent damage thereto. 
     The wellhead isolation tools in the prior art generally insert a mandrel through the various valves and spools of the wellhead to isolate those components from the elevated pressures and the corrosive and abrasive fluids used in the well treatment to stimulate production. A top end of the mandrel is connected to one or more high pressure valves, through which the stimulation fluids are pumped. In some applications, a pack-off assembly is provided at a bottom end of the mandrel for achieving a fluid seal against an inside of the production tubing or casing so that the wellhead is completely isolated from the stimulation fluids. One such tool is described in Applicant&#39;s U.S. Pat. No. 4,867,243, which issued Sep. 19, 1989 and is entitled WELLHEAD ISOLATION TOOL AND SETTING TOOL AND METHOD OF USING SAME. The length of the mandrel need not be precise because the location of the pack-off assembly in the production tubing or casing is immaterial, so long as the pack-off assembly is sealed against the inner wall of the production tubing or casing. Consequently, variations in the length of the wellhead of different oil or gas wells are of no consequence. 
     In an improved wellhead isolation tool configuration, the mandrel in an operative position, requires fixed-point pack-off in the well, as described in Applicant&#39;s U.S. Pat. No. 5,819,851, which issued Oct. 13, 1998 and is entitled BLOWOUT PREVENTER PROTECTOR FOR USE DURING HIGH-PRESSURE OIL/GAS WELL STIMULATION. A further improvement of that tool is described in Applicant&#39;s co-pending U.S. patent application Ser. No. 09/299,551 which was filed on Apr. 26, 1999, now U.S. Pat. No. 6,247,537, and is entitled HIGH PRESSURE FLUID SEAL FOR SEALING AGAINST A BIT GUIDE IN A WELLHEAD AND METHOD OF USING SAME. The mandrel described in this patent and patent application includes an annular sealing body attached to the bottom end of the mandrel for sealing against a bit guide which is mounted on the top of a casing in the wellhead. 
     This type of isolation tool advantageously provides full access to a well casing and permits use of downhole tools during a well stimulation treatment. A mechanical lockdown mechanism for securing a mandrel requiring fixed-point pack-off in the well is described in Applicant&#39;s U.S. patent application Ser. No. 09/338,752 which was filed on Jun. 23, 1999 and is entitled BLOWOUT PREVENTER PROTECTOR AND SETTING TOOL. The mechanical lockdown mechanism has an axial adjusting length adequate to compensate for variations in a distance between a top of the blowout preventer and the top of the casing of the different wellheads to permit the mandrel to be secured in the operative position even if a length of a mandrel is not precisely matched with a particular wellhead. The mechanical lockdown mechanism secures the mandrel against the bit guide to maintain a fluid seal but does not restrain the mandrel from downwards movement. The force exerted on the annular sealing body between the bottom end of the mandrel and the bit guide results from a combination of the weight of the isolation tool and attached valves and fittings, a force applied by the lockdown mechanism and an upward force exerted by fluid pressures acting on the mandrel. 
     The wellhead isolation tools described in the above patents and patent applications work well and are in significant demand. However, it is also desirable from a cost and safety standpoint, to be able to leave the tubing string, or as it is sometimes called the “kill string”, in the well during a well stimulation treatment. The above-described wellhead isolation tool is not adapted to support a tubing string left in the well because the weight of a long tubing string may damage the seal between the bottom of the mandrel and the bit guide. 
     Some prior art wellhead isolation tools are adapted for well stimulation treatment with a tubing string left in the well. For example, Canadian Patent No. 1,281,280 which is entitled ANNULAR AND CONCENTRIC FLOW WELLHEAD ISOLATION TOOL AND METHOD OF USE THEREOF, which issued to McLeod on Mar. 12, 1991, describes an apparatus for isolating the wellhead equipment from the high pressure fluids pumped down to the production formation during the procedures of fracturing and acidizing oil and gas wells. The apparatus utilizes a central mandrel inside an outer mandrel and an expandable sealing nipple to seal the outer mandrel against the casing. The bottom end of the central mandrel is connected to a top of the tubing string and a sealing nipple is provided with passageways to permit fluids to be pumped down the tubing and/or the annulus between the tubing and the casing in an oil or gas well. One disadvantage of this apparatus is that the fluid flow rate is restricted by the diameter of the outer mandrel which must be smaller than the diameter of the casing of the well and further restricted by the passageways in the sealing nipple between the central and outer mandrels. The sealing nipple also blocks the annulus, preventing tools from being run down the wellbore. The passageways in the sealing nipple are also susceptible to damage by the abrasive particle-laden fluids and are easily washed-out during a well stimulation treatment. A further disadvantage of the isolation tool is that the tool has to be removed and re-installed every time the tubing string is to be moved up or down in the well. 
     Therefore, there exists a need for an improved isolation tool which is adapted for use with a tubing string to be left in the well, or run into or out of the well during a well stimulation treatment. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a BOP protector which is adapted to support a tubing string in a wellbore so that the tubing string is accessible during a well treatment to stimulate production. 
     It is a further object of the invention to provide a BOP protector that permits a tubing string to be moved up and down in the wellbore without removing the BOP protector from the wellhead. 
     It is another object of the present invention to provide a BOP protector that permits a tubing string to be run into or out of the wellbore without removing the BOP protector from the wellhead. 
     In accordance with one aspect of the invention, there is provided an apparatus for protecting a blowout preventer from exposure to fluid pressures, abrasives and corrosive fluids used in a well treatment to stimulate production. The apparatus is adapted to support a tubing string in a wellbore so that the tubing string is accessible during the well treatment. The apparatus includes a mandrel adapted to be inserted down through the blowout preventer to an operative position. The mandrel has a mandrel top end and a mandrel bottom end. The mandrel bottom end includes an annular sealing body for sealing engagement with a bit guide at a top of a casing of the well when the mandrel is in the operative position. A base member is adapted for connection to the wellhead and includes fluid seals through which the mandrel is reciprocally moveable. The apparatus further comprises a fracturing head, a tubing adapter and a lock mechanism. The fracturing head includes a central passage in fluid communication with the mandrel and at least one radial passage in fluid communication with the central passage. The tubing adapter is mounted to a top end of the fracturing head and supports the tubing string while permitting fluid communication with the tubing string. The lock mechanism for locking the apparatus in a fixed position to inhibit upward movement of the mandrel induced by fluid pressures in the wellbore and downward movement of the mandrel induced by a weight of the tubing string supported by the tubing adapter. 
     The apparatus preferably includes a mandrel head affixed to the mandrel top end and the fracturing head is mounted to the mandrel head. The lock mechanism preferably includes a mechanical lockdown mechanism which is adapted to inhibit upward movement of the mandrel head induced by fluid pressures when the mandrel is in the operative position and a load transferring mechanism for transferring a substantial part of the weight of the tubing string from the mandrel head to the wellhead to protect the sealing body from the entire weight of the tubing string when the tubing string is supported by the tubing adapter. 
     More especially, according to an embodiment of the invention, the base member has a central passage to permit the insertion and removal of the mandrel. The passage is surrounded by an integral sleeve having an elongated spiral thread for engaging a lockdown nut that is adapted to secure the mandrel in the operative position. A passage from the mandrel head top end to the mandrel head bottom end is provided for fluid communication with the mandrel and permits the tubing string to extend therethrough. The mandrel head includes a spiral thread for operatively engaging a load transfer nut that is adapted to be rotated down so that a head of the load transfer nut rests against a top of the lockdown nut to transfer the weight of the tubing string from the mandrel head to the base member. 
     The tubing adapter is configured to meet the requirements of a job. It may be a flange for mounting a BOP to the top of the apparatus so that tubing can be run into or out of the well. Alternatively, the tubing adapter may include a threaded connector to permit the connection of a tubing string that is already in the well. 
     A blast joint may be connected to the tubing adapter if coil tubing is run into the well. The blast joint protects the coil tubing from erosion when abrasive fluids are pumped through the fracturing head. 
     In accordance with another aspect of the invention, a method is described for providing access to a tubing string while protecting a blowout preventer on a wellhead from exposure to fluid pressure as well as to abrasive and corrosive fluids during a well treatment to stimulate production. The method comprises: 
     a) suspending the apparatus above the wellhead; 
     b) aligning the apparatus with a tubing string supported on the wellhead and lowering the apparatus until a top end of the tubing string extends through the axial passage above the fracturing head; 
     c) connecting the top end of the tubing string to a top end of the fracturing head, lowering the tubing string and the apparatus until the apparatus rests on the wellhead, and mounting the base member to the wellhead; 
     d) opening the blowout preventer; 
     e) lowering the tubing string and the fracturing head to stroke the mandrel bottom end down through the blowout preventer, and adjusting a lock mechanism until the mandrel is in an operative position in which the annular sealing body is in fluid sealing engagement with a bit guide mounted to a top of the casing of the well; 
     f) adjusting the lock mechanism to lock the mandrel in the operative position and to transfer weight of the tubing string and the apparatus to the wellhead so that the sealing body is not compressed against the bit guide by a full weight of the tubing string. 
     In accordance with a further aspect of the invention, a method is described for running a tubing string into or out of a well while protecting a first blowout preventer on a wellhead of the well from exposure to fluid pressure as well as to abrasive and corrosive fluids during a well treatment to stimulate production. The method related to the use of the above-described apparatus comprises: 
     a) mounting the base member of the apparatus to the wellhead; 
     b) closing at least one second blowout preventer which is mounted to an adapter flange a top the fracturing head; 
     c) opening the first blowout preventer; 
     d) lowering the fracturing head to stroke the mandrel bottom end down through the blowout preventer, and adjusting a lock mechanism until the mandrel is in an operative position in which the annular sealing body is in fluid sealing engagement with a bit guide mounted to a top of the casing of the well; 
     e) adjusting the lock mechanism to lock the mandrel in the operative position and to transfer weight of the tubing string and the apparatus to the wellhead so that the sealing body will not be compressed against the bit guide by a full weight of the tubing string; and 
     f) running the tubing string into or out of the well through the at least one second blowout preventer. 
     A primary advantage of the invention is the capability to support a tubing string in a wellbore during the well stimulation treatment. This provides direct access to both the tubing string and the well casing so that the use of the apparatus is extended to a wide range of well service applications. 
     A further advantage of the invention is to permit a maximum flow rate into the well during a stimulation treatment because the mandrel has a diameter at least as large as that of the casing of the well. Furthermore, the apparatus permits the tubing string to be moved up and down, or run in or out of the well without removing the apparatus from the wellhead. The tubing string can even be moved up or down in the well while well treatment fluids are being pumped into the well. Labour and the associated costs are thus reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be further described by way of illustration only and with reference to the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view of a preferred embodiment of the BOP protector in accordance with the invention, showing the mandrel in an exploded view; 
     FIG. 2 is a cross-sectional view of the embodiment shown in FIG. 1 illustrating the BOP protector in a condition ready to be mounted to a wellhead; 
     FIG. 3 is a cross-sectional view of the BOP protector shown in FIG. 2 suspended over the wellhead prior to installation on the wellhead; 
     FIG. 4 is a cross-sectional view of the BOP protector shown in FIG. 3 illustrating a further step in the installation procedure, in which the tubing string is connected to a tubing adapter; 
     FIG. 5 is a cross-sectional view of the BOP protector shown in FIG. 4 illustrating a further step in the installation procedure, in which the mandrel of the BOP protector is inserted through the wellhead and locked in an operative position; 
     FIG. 6 is a cross-sectional view of the BOP protector shown in FIG. 5 illustrating a final step in the installation procedure, in which a load transfer nut is tightened to complete the installation; 
     FIG. 7 shows an alternate embodiment of the lockdown mechanism for the BOP protector shown in FIG. 1; 
     FIG. 8 shows another alternate embodiment of the lockdown mechanism for the BOP protector shown in FIG. 1; 
     FIG. 9 is a partial cross-sectional view of a first embodiment of an annular sealing body fused to the bottom end of the mandrel of the BOP protector (shown in FIG. 1) for sealing against a bit guide in a wellhead; 
     FIG. 10 is a partial cross-sectional view of an alternate embodiment of an annular sealing body for sealing against a bit guide in a wellhead; and 
     FIG. 11 is a partial cross-sectional view of a BOP protector in accordance with the invention, showing a tubing adapter flange used for mounting a BOP to permit tubing to be run into or out of the well without removing the BOP protector from the wellhead. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a cross-sectional view of the apparatus for protecting the blowout preventers (hereinafter referred to as a BOP protector) in accordance with the invention, generally indicated by reference numeral  10 . The apparatus includes a lockdown mechanism  12  which includes a base member  14 , a mandrel head  16  and a lockdown nut  18  that detachably interconnects the base member  14  and the mandrel head  16 . The base member  14  includes a flange and an integral sleeve  20  that is perpendicular to the base member  14 . A spiral thread  22  is provided on an exterior of the integral sleeve  20 . The spiral thread  22  is engageable with a complimentary spiral thread  24  on an interior surface of the lockdown nut  18 . The flange of the base member  14  with the integral sleeve  20  form a passage  26  that permits a mandrel  28  to pass therethrough. The mandrel head  16  includes an annular flange, having a central passage  30  defined by an interior wall  32 . A top flange  34  is adapted for connection to a fracturing head  35 . A lower flange  36  retains a top flange  38  of the lockdown nut  18 . The lockdown nut  18  secures the mandrel head  16  from upward movement with respect to the base member  14  when the lockdown nut  18  engages the spiral thread  22  on the integral sleeve  20 . 
     The mandrel  28  has a mandrel top end  40  and a mandrel bottom end  42 . Complimentary spiral threads  43  are provided on the exterior of the mandrel top end  40  and on a lower end of the interior wall  32  of the mandrel head  16  so that the mandrel top end  40  may be securely attached to the mandrel head  16 . One or more O-rings (not shown) provide a fluid-tight seal between the mandrel head  34  and the mandrel  28 . The passage  26  through the base member  14  has a recessed region at the lower end for receiving a steel spacer  44  and packing rings  46  preferably constructed of brass, rubber and fabric. The steel spacer  44  and packing rings  46  define a passage of the same diameter as the periphery of the mandrel  28 . The packing rings  46  are removable and may be interchanged to accommodate different sizes of mandrel  28 . The steel spacer  44  and packing rings  46  are retained in the passage  26  by a retainer nut  48 . The combination of the steel spacer  44 , packing rings  46  and the retainer nut, provide a fluid seal to prevent passage to the atmosphere of well fluids from an exterior of the mandrel  28  and the interior of the BOP when the mandrel  28  is inserted into the BOP, as will be described below with reference to FIGS. 5 and 6. 
     An internal threaded connector  50  on the mandrel bottom end  42  is adapted for the connection of mandrel extension sections of the same diameter. The extension sections permit the mandrel  28  to be lengthened, as required by different wellhead configurations. An optional mandrel extension  52 , has a threaded connector  54  at a top end  56  adapted to be threadedly connected to the mandrel bottom end  42 . An extension bottom end  58 , includes a threaded connector  60  that is used to connect a mandrel pack-off assembly  62 , which will be described below in more detail. High pressure O-ring seals  64 , well known in the art, provide a high pressure fluid seal in the threaded connectors between the mandrel  28 , the optional mandrel extension(s)  52  and the mandrel pack-off assembly  62 . 
     The mandrel  28 , the mandrel extension  52  and the mandrel pack-off assembly  62  are preferably each made from 4140 steel, a high-strength steel which is commercially available. 4140 steel has a high tensile strength and a Burnell hardness of about 300. Consequently, the assembled mandrel  28  is adequately robust to contain extremely high fluid pressures of up to 15,000 psi, which approaches the burst pressure of the well casing. In order to support an annular sealing body  66 , however, the walls of the mandrel pack-off assembly  62  are preferably about 1.75″ (4.45 cm) thick. 
     The fracturing head  35  includes a sidewall  74  surrounding a central passage  76  that has a diameter not smaller than the internal diameter of the mandrel  28 . A bottom flange  78  is provided for connection in a fluid tight seal to the mandrel head  16 . Two or more radial passages  80 ,  82  with threaded connectors  84 ,  86  are provided to permit well stimulation fluids to be pumped through the wellhead. 
     The radial passages  80 ,  82  are preferably oriented at an acute upward angle with respect to the sidewall  74 . At the top end  88  of the sidewall  74 , a threaded connector  90  removably engages the threaded connector  92  of one embodiment of a tubing adaptor  94 , in accordance with the invention. The tubing adapter  94  includes a flange  96 , a threaded connector  92  and a sleeve  98 . The tubing adapter  94  also includes a central passage  100  with the threads  102  thereon for detachably connecting a tubing joint of a tubing string. A spiral thread  104  is provided on the exterior of the sleeve  98  and adapted for connecting other equipment, for example, a high pressure valve. 
     A spiral thread  106  is provided on the periphery of the top flange  34  of the mandrel head  16 . The spiral thread  106  engages a complimentary spiral thread  108  of a load transfer nut  110 . The load transfer nut  110  includes a bottom flange  112  that rests on the top flange  38  of the lockdown nut  18  to transfer a weight of a tubing string from the fracturing head  35  to the base member  14  when the load transfer nut  110  is rotated downwardly. Rotating the load transfer nut  110  upwards, releases the lockdown nut  18  to permit free rotation of the lockdown nut  18 . A plurality of handles  114 , only two of which are shown, are preferably attached to a periphery of the load transfer nut  110 . The handles  114  facilitate rotation of the load transfer nut  110 . 
     The mandrel head  16  with its upper and lower flanges  34 ,  36 , the lockdown nut  18  with its top flange  38  and the load transfer nut  110  with its bottom flange  112  are illustrated in FIG. 1 respectively as an integral unit assembled, for example, by welding or the like. However, persons skilled in the art will understand that any one of the mandrel head  16 , the lockdown nut  18  or the load transfer nut  110  may be constructed to permit the mandrel head  16 , the lockdown nut  18  or the load transfer nut  110  to be independently replaced. 
     FIG. 2 illustrates the BOP protector  10  shown in FIG. 1, prior to being mounted to a BOP for a well stimulation treatment. The mandrel head  16  is connected to the top end of the mandrel  28 , which includes any required extension section(s)  52  and the pack-off assembly  62  to provide a total length of the mandrel  16  required for a particular wellhead. The load transfer nut  110  is rotated upwardly and the lockdown nut is disengaged from the integral sleeve  20  of the base member  14  because the mandrel  28  is to be inserted into the wellhead while the base member is mounted to the top end of the BOP. 
     FIGS. 3 through 6, illustrate the installation procedure of the BOP protector  10  to a wellhead  120  with a tubing string  122  supported, for example, by slips  124  or some other supporting device, at the top of the wellhead  120 . Several components may be included in a wellhead. For purposes of illustration, the wellhead  120  is simplified and includes only a BOP  126  and a tubing head spool  128 . The BOP  126  is a piece of wellhead equipment that is well known in the art and its construction and function do not form a part of this invention. The BOP  126 , the tubing head spool  128  and the slips  124  are, therefore, not described. The tubing string  122  is usually supported by a tubing hanger, not shown, in the tubing head spool  128 . The tubing string  122  is therefore pulled out of the well to an extent that a length of the tubing string  122  extending above the wellhead  120  is greater than a length of the BOP protector  10 . The tubing string  122  is then supported at the top of the BOP  126  using slips, for example, before the installation procedure begins. Two high pressure valves  130  and  132  are mounted to the threaded connectors  84 ,  86 , preferably before the BOP protector  10  is installed. 
     As illustrated in FIG. 3, the BOP protector  10  is suspended over the wellhead  122  by a crane or other lift equipment (not shown). The BOP protector  10  is aligned with the tubing string  122  and lowered over the tubing until the top end  134  of the tubing string  122  extends above the top end  88  of the sidewall  74 . 
     FIG. 4 illustrates the next step of the installation procedure. A tubing adapter  94  is first connected to the top end  134  of the tubing string  122 . The tubing adapter  134  is then connected to the top of the fracturing head. A high pressure valve  136  is mounted to the tubing adapter  94  via the thread  104  on the sleeve  98 . The tubing string  122  and the BOP protector  10  are then lifted using a rig, for example, so that the slips  124  can be removed. The rig lowers the tubing string  122  and the BOP protector  10  onto the top of the BOP so that the base member  14  rests on the BOP  126 . The mandrel  28  is inserted from the top into to the BOP  126  but remains above the BOP rams (not shown) Persons skilled in the art will understand that in a high pressure wellbore, the tubing string  122  is plugged and the rams of the BOP are closed around the tubing string  122  before the installation procedure begins, so that the fluids under pressure in the wellbore are not permitted to escape from the tubing string or the annulus between the tubing string and the wellhead  120 . 
     To open the rams of the BOP  126  and further insert the mandrel  28  down through the wellhead, the high pressure valves  130 ,  132  and  136  must be closed and the base member  14  mounted to the top of the BOP  126 . The packing rings  46  and all other seals between interfaces of the connected parts, seal the central passage of the BOP protector  10  against pressure leaks. The BOP rams are now opened after the pressure is balanced across the BOP rams. This procedure is well known in the art and is not described. After the BOP rams are opened, the rig further lowers the BOP protector  10  to move the mandrel bottom end down through the BOP. When the BOP protector  10  is in an operative position in which the bottom end of the pack-off assembly  62  is in sealing contact with a bit guide  140  attached to a top of a casing  142  (FIG.  5 ). The bit guide  140  caps the casing  142  to protect the top end of the casing  142  and provides a seal between the casing  142  and the tubing head spool  128 , in a manner well-known in the art. As noted above, the extension section(s) is optional and of variable length so that the assembled mandrel  28 , including the pack-off assembly  62 , has adequate length to ensure that the top end of the mandrel  28  extends above the top of the BOP  74 , just enough to enable the mandrel to be secured by the lockdown assembly  12 , described above, when the pack-off assembly  62  is seated against the bit guide  142 . However, the distance from the top of the bit guide  140  to the top of the BOP  126 , may vary to some extent in different wellheads. 
     In accordance with the invention, the mechanical lockdown mechanism  12  is configured to provide a broad range of adjustment to compensate for variations in the distance from the top of the BOP  126  to the top end  40  of the mandrel  28 , which is described with reference to FIGS. 7 and 8. The complimentary spiral threads  22 ,  24  on the respective integral sleeve  20  and lockdown nut  18 , have a length adequate to provide the required compensation. Preferably, the respective threads  22 ,  24  are at least about 9″ (22.86 cm) in axial length. A minimum engagement for safely containing the elevated fluid pressures acting on the BOP protector  10  during a well treatment to stimulate production is represented by a section labelled “A”. Sections “B” represent the adjustment available to compensate for variations in the distance from the top of the BOP  126  to the top end  40  of the mandrel  28 . A spiral thread with about 9″ of axial length provides about 5″ of adjustment while ensuring that a minimum engagement of the lockdown nut  18  is maintained. 
     The lockdown nut  18  shown in FIG. 5, secures the mandrel  28  in the operative position only against an upward fluid pressure and, therefore, does not stop the mandrel from moving downwardly under a downward force, such as the weight of the tubing string  122  which is transferred to the mandrel  28  through the fracturing head  35  and the mandrel head  16  when the tubing string is unhooked from the rig. As illustrated in FIG. 6, the load transfer nut  110  is rotated down until the bottom flange  112  firmly rests on the top flange  38  of the lockdown nut  18 . Therefore, the tubing adapter  94 , fracturing head  35 , the mandrel head  16  and the base member  14 , cooperate to support the weight of the tubing string  122  and transfer the load to the wellhead  120 , so that the mandrel  28 , the pack-off assembly  62  and the bit guide  140  do not bear the weight of the tubing string  122 . The installation procedure of the BOP protector  10  is thereby completed and the installed apparatus, as shown in FIG. 6, is ready for various types of well treatment to stimulate production. As described in Applicant&#39;s co-pending U.S. patent application Ser. No. 09/338,752, which is incorporated herein by reference, the base member  14  includes at least two connection points  150  for attaching an insertion tool used when a rig is not required to mount the BOP protector  10  to a wellhead. 
     As noted above, FIGS. 7 and 8 illustrate two alternate embodiments of the mechanical lockdown mechanism  12  in accordance with the invention. In FIG. 7, the spiral thread  24  on the lockdown  18  has an axial extent “A” to ensure the minimum engagement required for safety and the thread  22  on the integral sleeve  20  of the base member  14  has a full length spiral thread which includes the “A” section for the minimum engagement and the “B” for adjustment. The mechanical lockdown mechanism  12 , illustrated in FIG. 8, provides a similar adjustable lockdown with length “A” for minimum safe threaded engagement on the integral sleeve  20  and length “B” for adjustment on the lockdown nut  18 . 
     A second mechanical locking mechanism may be added to advantageously improve the range of adjustment of the lockdown mechanism, so that the length of a mandrel may be less precisely matched to the distance from the top of the well to the fixed-point pack-off position in the well. The embodiment with the second mechanical lock-down mechanism is described in Applicant&#39;s co-pending U.S. patent application No. 09/373,418, now U.S. Pat. No. 6,179,053, which is entitled MECHANISH FOR WELL TOOLS REQUIRING FIXED-POINT PACKOFF and was filed on Aug. 12, 1999, the specification which is also incorporated herein by reference. 
     FIGS. 9 and 10 illustrate the pack-off assembly  62  in accordance with alternate embodiments of the invention. The pack-off assembly  62 , illustrated in FIGS. 9 and 10, may be used for the BOP protector  10  to improve performance, as described in Applicant&#39;s U.S. Pat. No. 6,247,537, which is likewise incorporated herein by reference. In FIG. 9, a high pressure seal  198  is an elastomeric material, preferably a plastic material such as polyethylene or a rubber compound such as nitryl rubber. The elastomeric material preferably has a hardness of about 80 to about 100 durometer. The high pressure fluid seal  198  is bonded directly to the bottom end of the pack-off assembly  62 . The bottom end of the pack-off assembly  62  includes at least one downwardly protruding annular ridge  200 , which provides an area of increased compression of the high pressure fluid seal  198  in an area preferably adjacent to an outer wall  202  of the pack-off assembly  62 . The annular ridge  200  not only provides an area of increased compression, it also inhibits extrusion of the high pressure fluid seal  198  from a space between the pack-off assembly  62  and the bit guide  142  when the mandrel  28  is exposed to extreme fluid pressures. The annular ridge  200  likewise helps to ensure that the high pressure fluid seal  198  securely seats against the bit guide  142  even if the bit guide  142  is worn due to impact and abrasion resulting from the movement of the production tubing or well tools into or out of the casing  140 . A pair of O-rings  204  are preferably provided as backup seals to further ensure wellhead components are isolated from pressurized stimulation fluids. 
     The pack-off assembly  62 , illustrated in FIG. 10, has an inner wall  206  which extends downwardly past the bit guide  142  and a top edge of the casing  140  into an annulus of the casing  140 . High pressure fluid seal  208  is particularly designed for use in wellheads where the bit guide  142  does not closely conform to the top edge of the casing  140 , leaving a gap  210  in at least one area of a circumference of a joint between the casing  140  and the bit guide  142 . The gap makes the top edge of the casing  140  susceptible to erosion called “wash-out” if large volumes of abrasives are injected into the well during a well stimulation process. The pack-off assembly  62 , in accordance with this embodiment of the invention, covers any gaps at the top of the casing  140  to prevent wash-out. The length of the inner wall  206  is a matter of design choice. 
     As noted above, the high pressure fluid seal  208  is bonded directly to the end  212  of the pack-off assembly  62 , using techniques well-known in the art. The high pressure fluid seal  208  covers an outer wall portion  220  of the inner wall  206 . It also covers a portion of an outer wall  222  located above the end  212 . A bottom end of the outer wall  222  of the pack-off assembly  62  protrudes downwardly in an annular ridge  224 , as described above, to provide extra compression of the high pressure fluid seal  208  to ensure that the high pressure fluid seal  208  is not extruded from a space between the pack-off assembly  62  and the bit guide  142  when the high pressure fluid seal  208  is securely seated against the top surface of the bit guide  142 . 
     The BOP protector  10 , in accordance with the above-described embodiments of the invention, has extensive applications in well treatments to stimulate production. After the BOP protector  10  is installed to the wellhead as illustrated in FIG. 6, a pressure test is usually done by opening the tubing head spool side valve to ensure that the BOP and the wellhead are properly sealed. The high pressure lines (not shown) can be hooked up to high pressure valves  130 ,  132  and  136  to begin a wellhead stimulation treatment. A high pressure well stimulation fluids can be pumped down through any one or more of the three valves into the well. The tubing string can also be used to pump a different fluid or gas down into the well while other materials are pumped down the casing annulus so that the fluids only commingle downhole at the perforations area and are only mixed in the well. 
     In the event of a “screen-out”, the high pressure valve  136  which controls the tubing string, may be opened and hooked to the pit. This permits the tubing string  122  to be used as a well evacuation string, so that the fluids can be pumped down the annulus of the casing and up the tubing string to clean and circulate proppants out of the wellbore. In other applications for well stimulation treatment, the tubing string  122  can be used as a dead string to measure downhole pressure during a well fracturing process. 
     The tubing also can be used to spot acid in the well. To prepare for a spot acid treatment, a lower limit of the area to be acidized is blocked off with a plug set in the well below a lower end of the tubing string, if required. A predetermined quantity of acid is then pumped down the tubing string to treat a portion of the wellbore above the plug. The area to be acidized may be further confined by a second plug set in the well above the first plug. Acid may then be pumped under pressure through the tubing string into the area between the two plugs. 
     As will be understood by those skilled in the art, coil tubing can be used for any of the stimulation treatments described above. If coil tubing is used, it is preferably run through a blast joint so that the coil tubing is protected from abrasive proppants. 
     FIG. 11 illustrates a configuration of the BOP protector  10  in accordance with the invention, that is adapted to permit tubing to be run into or out of the well. Coil tubing, which is well known in the art, is particularly well adapted for this purpose. Coil tubing is a jointless, flexible tubing available in variable lengths. If tubing is to be run into or out of the well, pressure containment is required. Accordingly, the tubing adapter  394 , in this embodiment, is different from the tubing adapter  94  shown in FIGS. 1-6. The tubing adapter  394  has a flange  396  with a threaded connector  392  for engaging the thread  90  on the top of the fracturing head  35 . The flange  396  is adapted to permit a second BOP  326  to be mounted to a top of the fracturing head  35 . A blast joint  300 , having a threaded top end  301  engages a thread  302  so that the blast joint  300  is suspended from the tubing adapter  394 . The blast joint has an inner diameter large enough to permit the coil tubing  322  to be run up and down therethrough. The blast joint  300  protects the coil tubing  322  from erosion when abrasive fluids are pumped through the radial passages  80 ,  82  in the fracturing head  35 . The coil tubing  322  is supported, for example, by slips  324  or other supporting mechanisms to the top of the BOP  326 . As is understood by those skilled in the art, a “stripper” for removing hydrocarbons from coil tubing pulled out of the well may also be associated with the second BOP  326 . 
     If tubing is to be run in and out of the well during a stimulation treatment, a third BOP, not shown, may be required, as is also well known in the art. As is well understood, the BOPs are operated in sequence whenever the tubing is pulled from or inserted into the well. 
     The method of installing the BOP protector  10  shown in FIG. 11, to permit tubing to be run into or out of a well while protecting the BOP  126  on the wellhead during a well stimulation treatment is described below. The base member  14  is first mounted to the top of the BOP  126  while the bottom end of the mandrel is inserted from the top into the BOP  126 . The BOP  326  is closed and the BOP  126  is opened after the pressure across the BOP  126  is equalized. The fracturing head  35  and attached BOP  326  are lowered to stroke the mandrel bottom end down through the BOP  126 . The lockdown nut  18  is screwed down until the mandrel  28  is in the operative position and the annular sealing body is sealed against the bit guide (not shown). The load transfer nut  110  is then rotated down to firmly rest on the lockdown nut  18  so that the weight of the coil tubing is run into the well. 
     The apparatus in accordance with the invention does not restrict fluid flow along the annulus of the casing or include components susceptible to wash-out. More advantageously, the apparatus in accordance with the invention enables an operator to move the tubing string up and down or run coil tubing into and out of a well without removing the apparatus from the wellhead. A tubing string can also be moved up or down in the well while stimulation fluids are being pumped into the well, as will be understood by those skilled in the art. The apparatus is especially well adapted for use with coil tubing which provides a safer operation in which there are no joints, no leaking connections and no snubbing unit needed if it is run in under pressure. Running coil tubing is also a faster operation that can be used easier and less expensively in remote areas, such as off-shore. 
     Modifications and improvements to the above-described embodiments of the invention, may become apparent to those skilled in the art. For example, although the mandrel head and the fracturing head are shown and described as separate units, they may be constructed as an integral unit. Many other modifications may also be made. 
     The foregoing description is intended to exemplary rather than limiting. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.