Patent Publication Number: US-9422784-B2

Title: Snubbing stack

Description:
TECHNICAL FIELD 
     This relates to a snubbing stack used to snub into or out of a pressurized wellhead. 
     BACKGROUND 
     When it is necessary to inject or remove a tubing string or other elongate string from a pressurized wellhead, a snubber is used. The wellhead will generally be sealed, such as by a blow-out preventer, and the snubber will either push or pull the tubing through the seal. As such, the snubber allows the pressure to be contained within the wellhead. 
     SUMMARY 
     There is provided a snubbing apparatus for a wellbore having a wellhead, an elongate string being inserted into the wellbore through the wellhead. The snubbing apparatus comprises a snubbing stack having a upper sealing element and a lower sealing element, wherein, in operation, the upper and lower sealing elements seal between the elongate string and the snubbing stack. A string driver is carried by the snubbing stack, the string driver manipulating the elongate string through the upper sealing element and the lower sealing element. A source of inert liquid is connected to the snubbing stack between the upper sealing element and the lower sealing element. A pressure sensor is connected to sense the pressure below the lower sealing element and connected to provide a pressure signal to the source of inert liquid, the source of inert liquid being configured to pressurize the snubbing stack between the upper sealing element and the lower sealing element in response to the pressure signal. 
     According to an aspect, the source of inert liquid may be configured to equalize the pressure between the upper sealing element and the lower sealing element to the pressure below the lower sealing element. 
     According to an aspect, the snubbing apparatus may further comprise a bleed off valve connected to the snubbing stack between the upper sealing element and the lower sealing element. 
     According to an aspect, the snubbing stack may comprise a tubing spool that extends below the lower sealing element, and the pressure sensor may be connected to the tubing spool. 
     According to an aspect, the tubing driver may be snubbing slips. 
     According to an aspect, the upper sealing element may be an annular blowout preventer. 
     According to an aspect, the lower sealing element may be a resilient sealing element. 
     According to an aspect, the lower sealing element may be within a tubing spool. 
     According to an aspect, there may be a bleed off valve connected below the lower sealing element. 
     According to an aspect, the source of inert liquid is a liquid pump. 
     According to an aspect, the inert liquid may be glycol. 
     According to another aspect, there is provided a method of snubbing an elongate string into a wellbore having a wellhead, the method comprising the steps of: providing a snubbing stack having a upper sealing element and a lower sealing element; inserting the elongate string into the snubbing stack such that the top and lower sealing elements seal between the elongate string and the snubbing stack; driving the elongate string to move through the upper sealing element and the lower sealing element; sensing a wellhead pressure below the lower sealing element; injecting an inert fluid into the snubbing stack; and pressurizing the inert fluid within the snubbing stack based on the wellhead pressure. 
     According to an aspect, pressurizing the inert fluid may comprise equalizing the pressure to the wellhead pressure. 
     According to an aspect, injecting an inert fluid into the snubbing jack may further comprise bleeding off fluid within the snubbing stack as the inert fluid is injected. 
     According to an aspect, the snubbing stack may comprise a spool that extends below the lower sealing element and the pressure is measured by a pressure sensor connected to the spool. 
     According to an aspect, driving the elongate string may comprise using snubbing slips. 
     According to an aspect, the upper sealing element may be an annular blowout preventer. 
     According to an aspect, the lower sealing element may be a resilient sealing element. 
     According to an aspect, the lower sealing element may be within a spool. 
     According to an aspect, the inert liquid may be pressurized by a liquid pump connected to a source of inert liquid. 
     According to an aspect, the inert liquid may be glycol. 
     Other aspects will be apparent from the description and drawings below. Each of the described aspects may be combined with other aspects except when mutually exclusive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein: 
         FIG. 1  is a side elevation view of a snubbing stack for a pressurized wellhead. 
     
    
    
     DETAILED DESCRIPTION 
     A snubbing apparatus generally identified by reference numeral  10  will now be described with reference to  FIG. 1 . 
     Structure and Relationship of Parts: 
     The snubbing apparatus  10  is primarily intended for use when servicing high temperature and/or high pressure wells and is designed to help reduce the risk of fire or explosions. The snubbing apparatus  10  as shown in  FIG. 1  is mounted to a wellhead  12  generally is mounted above a casing string  14  that extends into a wellbore below the wellhead  12 . An elongate string  16  is inserted into wellhead  12  and casing string  14 . As shown, the elongate string  16  is a tubing string. The following will describe the snubbing apparatus in terms of a tubing string  16 , although it will be understood that elongate string  16  may also be a rod or other elongate string that may be snubbed as known in the art. Snubbing apparatus  10  has a snubbing stack  18  with an upper sealing element  20  and a lower sealing element  22 . The upper sealing element  20  and the lower sealing element  22  seals between the tubing string  16  and the snubbing stack  18 . As shown, upper sealing element  20  is an annular blowout preventer, and lower sealing element  22  is a resilient sealing element. An example of a suitable resilient sealing element is a RS-100 Seal produced by Mitey Titan Industries Inc. of Edmonton, Alberta, although other seals may be produced. It will be understood that other seals as are known in the art may be used for either the top or bottom seals. 
     The snubbing stack  18  carries a string driver  24 , which is used to manipulate tubing string  16  through the upper sealing element  20  and the lower sealing element  22 . As shown, tubing driver  24  is snubbing slips  34  used to drive tubing string  16 , although it will be understood that different drivers  24  may be used as are known in the art. Snubbing slips  34  as shown include three types of slips  34 , specifically, travelling slips  36 , stationary slips  38 , and heavy slips  40 . Travelling slips  36  grip tubing string  16  and move it upwards or downwards. Stationary slips  38  grip the tubing string  16  to prevent movement while travelling slips  36  release tubing string  16  while they are repositioned to grip tubing string  16  to again apply force to move tubing string  16 . Heavy slips  40  are generally optional, and are used to bear the weight of tubing string  16  when desirable. 
     Snubbing stack  18  is designed to be filled with an inert liquid between upper sealing element  20  and lower sealing element  22 . Inert fluid is provided by a source of inert liquid  26  connected by a supply line  27  to a port  29 . The source of inert liquid  26  is configured to pressurize the liquid within snubbing stack  18  between upper sealing element  20  and lower sealing element  22  relative to the wellbore pressure, which as shown is measured by a pressure sensor below lower sealing element  22 . Preferably, the pressure of the liquid is equalized with the pressure in casing string  14 . However, sealing element  22  is designed to withstand some pressure, and the pressure within stack  18  may be slightly lower. On the other hand, to ensure no leakage into stack  18  from casing  14 , the pressure within stack  18  may be equal to or greater than the pressure in casing  14 . The source of inert liquid  26  is, for example, a liquid pump  25 . The inert liquid is pressurized by a liquid pump  25  connected to a source of inert liquid  26 . The inert liquid provided by the source of inert liquid  26  is preferably glycol, although it will be understood that other types of inert liquid may be used, for example, water. Connected to the snubbing stack  18 , there may be a bleed off valve  28  between the upper sealing element  20  and the lower sealing element  22 . As shown, bleed off valve  28  is located at the bottom of snubbing stack  18 , and is intended to be used to drain any fluid in snubbing stack  18  as the inert liquid is pumped in. There may be a second bleed off valve  31  located at the top of snubbing stack  18 . Either or both of these valves  28  and  31  may be used to relieve pressure in snubbing stack  18  between sealing elements  20  and  22 . 
     Pressure sensor  30  is connected below the lower sealing element  22  and provides a pressure signal to the source of inert liquid  26 . The source of inert liquid  26  pressurizes the snubbing stack  18  between the upper sealing element  20  and the lower sealing element  22  to the desired pressure relative to the pressure within casing  14 . This is preferably done in response to the pressure signal from the pressure sensor  30 . Source  26  may be regulated in various ways. For example, there may be a microcontroller that regulates the pressure. There may also be other mechanical or electrical sensors or switches that control source  26  in response to the pressure within casing string  14 . Pressure sensor  30  allows for the inert fluid to be added into the cavity  44  between the upper sealing element  20  and the lower sealing element  22 , pressurizing the inert fluid within the cavity  44  until the pressure of the inert fluid is equal to the pressure of the wellhead  12 . Pressure sensor  30  may attach to snubbing stack  18  below lower sealing element  22 . In the depicted example, snubbing stack  18  has a tubing spool  32  below lower sealing element  22  that houses lower sealing element  22  and to which pressure sensor  30  is attached. As can be seen, there is a valve, such as a casing valve  33 , between pressure sensor  30  and tubing spool  32 . This is optional, but may be useful, for example to isolate pressure sensor  30  if a problem arises. There may also be a casing bleed off valve  42  connected to tubing spool  32  by another casing valve  33 . This bleed off valve  42  allows downhole fluid pressure to be relieved below lower sealing element  22 . 
     Snubbing stack  18  may also include other features, such as a blind ram blowout preventer (BOP)  46  and a pipe ram BOP  48  as well as a stripping ram BOP  50 . These BOPs are include for safety purposes to ensure safe operation and to allow the operator to control the well in the event of a failure. Stripping ram BOP  50 , port  29  and bleed off valve  31  may all be carried by an equalizing spool  52 , depending on the preferences of the user. 
     Operation: 
     The construction of a well is well known in the art and the downhole assembly will not be discussed in detail. When it is desired to snub a tubing string into or out of a well, snubbing stack  18  is attached to wellhead  12 . Snubbing stack  18  may be as shown in  FIG. 1 , or may be any reasonable variation based on the principles described above. During the process of assembling snubbing stack  18 , a rubber insert  22  or other similar sealing element is place in tubing spool  32  in order to form lower sealing element  22 . The pressure sensor  30  installed on tubing spool  32  in the drawings connects to a device that can pump the inert liquid  26  inside the snubbing stack  18 , such as a skid mounted triplex pump, and this device is attached to the pressure sensor  30 . 
     When placing the snubbing stack  18  with an upper sealing element  20  and a lower sealing element  22 , the tubing string  16  will be inserted such that the upper sealing element  20  and the lower sealing element  22  form a seal about tubing string  16  within snubbing stack  18 . Once properly installed, downhole pressure will be contained below lower sealing element  22 , although snubbing stack  18  may be filled with downhole fluids. Source of inert liquid  26 , such as a pump, is then be turned on and glycol or another inert liquid is added to the cavity  44  formed between upper sealing element  20  and lower sealing element  22 . Casing bleed off valve  42  may be used to allow any well fluids to be drained out of cavity  44 , allowing it to be filled with inert liquid. Source of inert liquid  26  continues to pump liquid until such point as the pressure in this cavity  44  is equal to the pressure in the wellhead  12  below lower sealing element  22  as detected by pressure sensor  30 , or matches some other predetermined pressure relationship as described above. The use of glycol or other inert liquid ensures that, in the event of an equipment or seal failure, the inert liquid will be released prior to the wellbore fluids, allowing operators the opportunity to shut in the well or close a BOP. Once properly installed and arranged, tubing string  14  may be snubbed in or out using string driver  24 . 
     In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. 
     The scope of the following claims should not be limited by the preferred embodiments set forth in the examples above and in the drawings, but should be given the broadest interpretation consistent with the description as a whole.