Annular Pressure Overview
Annular Pressure Overview
Annular Pressure Overview
Where did it come from? How can you control it? Annular pressure is not always associated with leaks, malfunctions or problems The highest pressures are often generated with little more than heat transfer.
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North Slope wellhead with surface safety valve. The annular access ports are below the master valve. Each spool usually indicates access to a different annulus. These annular areas are most often monitored by gauges.
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How do well fluids leak into the outer annulus on some wells?
No, its not supposed to happen. However:
Thread leaks in inside tubulars Wellhead seal leaks Corrosion holes in inside tubulars Packer seal leaks Mechanical damage Shallow zone charging of annulus (rare!)
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Thread Leaks
Many different types of threads can rely on mechanical seal(s) or thread dope, depending on the pipe thread type.
Many older connections, such as API and buttress threads were selected for availability or strength, but are difficult to seal gas tight. These threads can be subject to very small leaks if the pipe dope seal deteriorates. Patented threads, with much better elastomer and metalto-metal seals, are available for situations requiring seals.
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Thread Dope
If gaps between threads are larger than particles in the dope, a long lasting seal may not be created. Dope must maintain elasticity
Patented Thread - A flush connection (not upset) with two-step thread, metal-to-metal seal and a torque shoulder at the base of the thread. This thread uses dope or make up grease to prevent galling
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One-Way Leaks?
Yes one-way leaks are very common. The liquid level height is one of the keys
Some leaks will allow gas to pass, but are too small for liquids. If the leak is below liquid level, the gas comes through and is trapped in the annulus because the liquid cant be forced back through the leak.
Also, annular debris can act as a check valve for some leaks.
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One-Way Leaks?
Liquid level as a one-way leak path:
Gas permeates the leak, rising through the liquid to gas cap Liquid is much slower to travel back to inner tubular when the tubing pressure is Accumulated Gas lowered. Gas is trapped.
in Annulus
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One-Way Leaks?
Annular particle-containing fluid as a check valve:
Any particle containing fluid may seal the leaks NOTE this is often an intermittent problem.
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One-Way Leaks?
If well head seal leak is created when the pipe is heated (and expanded length increases), gas may leak into the annulus. When the pipe cools, the seal may re-engage, trapping the gas.
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Tubing hanger. Note the lockdown screw and small seal isolating the tubing from the annulus. Other seals are above the hanger. This is one of many types of wellheads.
Note that the top part of the hanger is threaded to allow pickup of the tubing string.
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Seal assembly on an outer casing string in the wellhead. The seal stacks in most wellheads depend on a smooth pipe contact with the seal body. A small amount of upward pipe movement does not generally cause a problem.
Seals
Ring Gasket
Slips
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Lockdown screw
Annular Valve
Casing Tubing
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Parts of a Well
Cellar
Conductor pipe not cemented, keeps the loose soil and rock out of the cellar area.
Surface casing Casing shoe (shows a seal either packer or cement fill) Production Casing
A well is a pressure vessel with 200+ connections. How much risk is there that a leak will form?
Inside Annulus (also called A annulus) Gas Lift Valves Production Packer
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One type of annular valve. The valves come in many pressure ratings 2,000 psi, 3,000 psi and 5,000 psi are the most typical rating. Sizes range from 2-1/16 to over 7-1/8.
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Never throttle with a gate valve! - washouts will ruin seal ability. Valves in series give repair opportunities.
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Wellhead Construction
Most wellheads are made up of flanged spools and valves. The flange contains a metal-to-metal seal that is very reliable if properly made-up and routinely inspected.
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A gasket sealing area in a flange, prior to cleaning, lubrication and assembly. The seal depends on a metal-to-metal seal that is accomplished by deforming a new seal ring during flange makeup.
George E. the King Engineering The type of flange and gasket determine how seal is made. 3/14/2009 GEKEngineering.com
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Flanges with Ring Grooves, API Type 6B, for 5000 psi working pressure
Nominal Size of Flange 2-1/16 2-9/16 3-1/8 4-1/16 5-1/8 7-1/16 11 3/14/2009 13-5/8 Diameter Diameter of Bolt Number of Ring Type of Flange Bolts Circle 8-1/2 5-1/2 8 R-24 9-5/8 7-1/2 8 R-27 11 8-1/2 8 R-37 12-1/4 9-1/2 8 R-38 14-3/4 11-1/2 8 R-44 15-1/2 12-1/2 12 R-46 23 19 12 R-54 George E. King Engineering 23 GEKEngineering.com 30-3/8 26-5/8 16 BX-160
6 BX Flanges Those flanges for which API Spec. 6A specifies BX Ring Gaskets.
Made-up 6 BX Flanges 6 BX flange raised faces shown in contact after assembly. In actual field situations any small gap present after achieving specified torque should appear uniform all around.
These flanges have raised faces that the design permits to meet or touch when the connecting bolts have reached the required torque.
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6 B Flanges
Those flanges for which API Spec. 6A specifies R or RX Ring Gaskets These flanges (usually without raised faces) have designs that leave a stand-off (gap) between the flanges after bolts have reached the required torque. See illustration. Select flange size to display stand-off between flanges using R and RX Gaskets in standard Ring Grooves.
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6 B flanges must always stand apart after assembly. Raised faces on 6 B flanges make the stand-off (gap) space difficult to measure accurately but field construction of a simple feeler gage will usually give a satisfactory approximation of the measurement. This stand-off should appear uniform all around
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Well Construction
Each part of a well is built to establish pressure isolation adequate to control any pressure that the well will see during further drilling, completion, production, intervention, stimulation, or abandonment. The pressure containment depends on long-term pipe, cement and seal integrity. Every well segment where trapped fluids have accumulated can build pressure as well conditions change.
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Wells are drilled in stages with casing strings run and cemented to control formation pressures, to seal off unwanted fluids and to isolate sections of the formation.
16 to 17 Drilled Hole
13-3/8 Casing
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While the steel casing provides the initial strength, the cement provides the seal between zones. It also supports and protects the casing.
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The deeper parts of the well are drilled and the deeper casing string (the production casing in this example) is run through the upper strings (the surface casing) after the upper casing is cemented and tested to the maximum pressures expected in drilling the lower sections.
9-5/8 casing
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At the surface the small amount of cement fallback due to gravity, leakoff and other factors may leave a few feet with poor cement coverage.
Regardless of intent cement is never perfect. And it only extends to ground level.
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If the uncemented section is in a permeable formation, any annular pressure in the outside annulus can bleed off to the formation. However, unless the active annulus is monitored for fluid height, a leak in the tubing will go undetected and the vent zone can be pressured up.
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If the open lap is cemented sealed with cement or covered with mud or other fluid loss control material, the ability is lost to bleed off the pressure and the outside annulus becomes a sealed pressure chamber.
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Example 1 increasing pressure slightly by A sealed fluid filled heating. chamber builds up pressure as it is heated by the heat from the produced fluids.
There is a very big difference in the rate of pressure build up when the chamber is all liquid filled compared to a gas filled chamber.
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Example 2 heating an all liquid closed chamber system increases pressure rapidly. Pressure buildup as a liquid filled or nearly liquid filled system is heated will be very rapid. Diesel, for example, increases volume at 0.0004 vol % increase per 1oF temperature rise (0.00072 vol% increase per 1oC). Pressure rise to very high levels is almost immediate when heated even a small amount.
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Example 1a Effect of Temperature Large Gas Volume 10 bbl chamber, 5 bbl gas and 5 bbl diesel, initial pressure 0 psig (1 bar) and initial temperature is 32oF (0oC). What is the pressure when the fluids are heated to 212oF (100oC)?
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Approximate Solution
Initial Temperature = 460 + 32 = 492oR Final Temperature = 460 +212 = 672oR Diesel volume increase = (212-32)*(0.0004)*(5) = 0.36 bbl., Total final volume of diesel is 5.36 bbl Ideal gas law pressure rise: P1V1/T1 = P2V2/T2 P2= [(1)*(5)*(672)] / [(5-0.36)*492] = 1.47 bar or 22 psia or 8 psig. Assumption: gas compression is a minor influence in opposing diesel expansion.
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Example 1b The effect of decreasing gas volume Consider the same problem where the starting condition is 9.3 bbl diesel and 0.7 bbl gas. Same starting pressure (0 psig). Same temperature rise from 32oF (0oC) to 212oF
Consequence? The less initial gas volume you have in the system, the higher the pressure goes and the faster it increases.
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Approximate Solution
Initial Temperature = 460 + 32 = 492oR Final Temperature = 460 +212 = 672oR Diesel volume increase = (212-32)*(0.0004)*(9.3) = 0.67 bbl., Total final volume of diesel is 9.97 bbl Ideal gas law pressure rise: P1V1/T1 = P2V2/T2 P2= [(1)*(0.7)*(672)] / [(0.03)*492] = 31.9 bar or 469 psia or 454 psig. Assumption: gas compression is a minor influence in opposing diesel expansion.
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Example 3 - importance of the starting pressure! Consider the same problem where the starting condition is 9.3 bbl diesel and 0.7 bbl gas. But, the starting pressure is 100 psig. Same temperature rise from 32oF (0oC) to 212oF
Consequence? The higher the initial pressure you have in the system, the higher the pressure goes and the faster it increases.
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Approximate Solution
Initial Temperature = 460 + 32 = 492oR Final Temperature = 460 +212 = 672oR Starting Pressure = 100 psig = 114.7 psia or 7.8 bar Diesel volume increase = (212-32)*(0.0004)*(9.3) = 0.67 bbl., Total final volume of diesel is 9.97 bbl Ideal gas law pressure rise: P1V1/T1 = P2V2/T2 P2= [(7.8)*(0.7)*(672)] / [(0.03)*492] = 248.6 bar or 3654 psia or 3640 psig. Assumption: gas compression is a minor influence in opposing diesel expansion.
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2. 3. 4.
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Compressible Materials
One of the approaches taken for subsea wells where venting is difficult. The problem is maintaining the gas cap from being displaced (preventing filling the annulus with liquid). Compressible, closed cell foams are good but are usually an option only for new wells. Annular pressure testing is very difficult when any part of the annulus is filled with gas.
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The pipe values also heavily depend on the quality of cement filling the annulus.
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Consider Two Pipes 5-1/2 and 9-5/8 Strength from pipe tables
Yield or Pipe Collapse Diameter Weight Resistance Burst psi in. psi Grade lb per ft 5-1/2" 5-1/2" 5-1/2" 9-5/8" 9-5/8" 9-5/8"
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Note that both increases in grade (alloy) and weight per foot (wall thickness) increase collapse and burst, but in different proportion. Also note that a larger pipe diameter, although with a thicker wall, has less collapse and burst strength.
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2500 psi???? Or has the gauge wrapped around? Pressure, PSI Remote sensing measurements with a wide measurement range is a safer way.
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Without a stop pin at maximum gauge reading, the indicator hand can sweep around, showing one dial reading and really measuring twice (or more) the indicated pressure.
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