Wells – Processes – Assembling well part
Reexamination Certificate
2000-03-29
2003-09-30
Bagnell, David (Department: 3672)
Wells
Processes
Assembling well part
C166S072000, C166S085400, C166S090100
Reexamination Certificate
active
06626245
ABSTRACT:
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 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'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.
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'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'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'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.
Applicant's co-pending United States Patent application entitled BLOWOUT PREVENTER PROTECTOR AND METHOD OF USING SAME which was filed on Jan. 28, 2000 and has been assigned Ser. No. 09/493,802, describes 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. The blowout preventer protector seals against a bit guide of the well and provides full access to the casing of the well to permit downhole tools to be run in or out of the casing. However, there are certain types of wellheads which do not include a bit guide. Such wellheads are generally referred to as “Larkin-type” wellheads. In Larkin-type wellheads, the top of the casing is threaded and the wellhead is screwed to the top of the wellhead using a high-pressure sealing compound, or the like. Consequently, the blowout preventer protector described in Applicant's co-pending patent application filed Jan. 28, 2000 cannot be used to service such wells. In addition, as wells age and are stressed by extended use, the seal between the bit guide and the casing cannot always be relied on to withstand elevated fluid pressures.
There therefore exists a need for a blowout preventer protector that seals off in the casing of the well while providing access to tubing in the well or permitting tubing to be run into or out of the well.
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 we
Dougherty Jennifer R
Nelson Mullins Riley & Scarborough LLP
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