production control chokes
DESCRIPTION
N/ATRANSCRIPT
Production Control Chokes
• Types
• Reasons
• Basics of Operations
• Application
8/25/2015 1 George E. King Engineering
GEKEngineering.com
Adjustable Restriction
“Needle and Seat” for this type of choke.
Rough schematic of an adjustable choke
A choke is a restriction in a flow line that causes a pressure drop or reduces the rate of flow. It commonly uses a partially blocked orifice.
8/25/2015 2 George E. King Engineering
GEKEngineering.com
Most Common Chokes
• Positive:
– Fixed orifice
– Shut in well (or divert flow) and disassemble choke housing to change the restriction or “flow bean”
• Adjustable
– Provides variable orifice size through external adjustment without choke disassembly.
8/25/2015 3 George E. King Engineering
GEKEngineering.com
Variable Chokes - good for bringing wells on gradually.
Prone to washouts from high velocity, particles, and even droplets or bubbles in severe cases.
Solutions - hardened chokes (diamond and carbide), chokes in series, dual chokes.
8/25/2015 4 George E. King Engineering
GEKEngineering.com
Beans are fixed (non adjustable) orifices – ID size is in 64ths of an inch.
This type of choke is used on wells that require almost no adjustments to flow.
ID
8/25/2015 5 George E. King Engineering
GEKEngineering.com
Choke Uses
• Control Flow – achieve liquid lift
• Maximize use – best use of gas (lift?)
• Protect equipment – abrasion and erosion
• Cleanup – best use of backflow energy
• Control circulation – holds a back pressure
• Control pressures at surface (during flow)
• Control injection – on injection line
8/25/2015 6 George E. King Engineering
GEKEngineering.com
What Happens as Choke Provides a Pressure Drop and What Happens to the pressure?
Energy from pressure drop is lost in:
• Increased velocity (from gas expansion)
• Vaporization (flashing) of light (short carbon chain) hydrocarbon liquids to gas
• Vaporization of water
• Cavitation
• Heat production (usually liquid friction)
8/25/2015 7 George E. King Engineering
GEKEngineering.com
Detriments
• Flashing – hydrocarbon light ends lost (value lost)
• Cavitation – erosion of surfaces in and around choke
• Erosion– solids, droplets and bubbles in high velocity flow
• Freezing – expansion of gasses cools the area – refrigeration principle
8/25/2015 8 George E. King Engineering
GEKEngineering.com
Pressure around the choke
Inlet or well pressure, P1
Pressure drop through the orifice
Pressure “recovery” , P2 8/25/2015 9 George E. King Engineering
GEKEngineering.com
Distance Flow Traveled
Delta P
Recovery
P1
P2
Pressure
VENA Contracta Phenomenon
The consequences of the low pressure region in the choke can lead to severe problems with cavitation and related flashing (vaporization).
8/25/2015 10 George E. King Engineering
GEKEngineering.com
Problems
• The larger the difference between the inlet and outlet pressures, the higher the potential for damage to the internals of the choke.
• When delta P ratio (i.e., (P1-P2)/P1) rises above 0.6, damage is likely. Changes in choke type, materials of construction, or choke arrangement may be needed (multiple chokes in series for high pressure drops?)
8/25/2015 11 George E. King Engineering
GEKEngineering.com
Cavitation During Liquid Flow
Ultra low pressure region in and immediately below choke causes bubble to form from vaporizing liquid, Recovery of pressure causes bubble to collapse; i.e., cavitation
The rapid collapse of the bubbles causes high velocity movement of liquid and damage around the site.
Pressure recovery line – limit of damage
Imploding bubbles and shock waves
8/25/2015 12 George E. King Engineering
GEKEngineering.com
Flashing During Liquid Flow
Vaporization of light ends, but no significant damage in this region since pressure recovery not above vapor pressure, hence bubbles don’t collapse.
Pressure recovery occurs downstream, damage location?
8/25/2015 13 George E. King Engineering
GEKEngineering.com
Freezing
• Expansion of gas (and solutions containing gas) cools the surroundings. Can form an ice plug and block flow.
Press
Distance Traveled
Recovery Recovery
Freezing Pt
Temperature
dP
P1 T1
T2 P2
8/25/2015 14 George E. King Engineering
GEKEngineering.com
P2 is outlet pressure
P1 is inlet pressure
Flow rate through the choke
dP is press drop thru the choke
Measurements used in Choke Calculations
8/25/2015 15 George E. King Engineering
GEKEngineering.com
Calculations
• delta P = P1 – P2
• delta P ratio = delta P/P1
• These values are use to measure the capacity and recovery of the choke
8/25/2015 16 George E. King Engineering
GEKEngineering.com
Throttling Methods
• Needle and seat
• Multiple orifice
• Fixed Bean
• Plug and Cage
• External Sleeve
8/25/2015 17 George E. King Engineering
GEKEngineering.com
Needle and Seat
• Simplest and least expensive adjustable
• Best for pressure control
• High Capacity
8/25/2015 18 George E. King Engineering
GEKEngineering.com
Multiple Orifice
• Quick open and close
• Good rate and pressure control
• An in-line instrument – not usually used on the wellhead
8/25/2015 19 George E. King Engineering
GEKEngineering.com
Fixed Bean
• Best when infrequent change needed
• Used mostly on trees
8/25/2015 20 George E. King Engineering
GEKEngineering.com
Plug and Cage
• High capacity
• Good control
8/25/2015 21 George E. King Engineering
GEKEngineering.com
External Sleeve
• Superior Erosion Resistance
• Minimizes Body Erosion
8/25/2015 22 George E. King Engineering
GEKEngineering.com
Choke Sizing
• Control the flow – maximize production
• Minimized vibration damage
• Minimize erosion damage
8/25/2015 23 George E. King Engineering
GEKEngineering.com
Choke Selection
• Based On:
– Application (lift, deliquifying the well, erosion control, solids production prevention, etc.)
– Rate or flow and range of flow rate
– Presence of solids
– Maximum velocity
– Total pressure drop
8/25/2015 24 George E. King Engineering
GEKEngineering.com
Choke Selection (continued)
• Fluid – liquid, gas, or GOR of mix.
• Pressure – both pressure drop and total pressure
• Temperature – range of acceptable temperatures during service
• Occurance and timing of solids in flow
• Droplets, bubbles
• Scale and organic deposit potential
8/25/2015 25 George E. King Engineering
GEKEngineering.com
How would you set a choke with minimum monitoring equipment?
• One way is by measuring temperature at the surface……
– Producing a well at maximum rates means lifting more liquids. Using the high heat capacity of liquids (3 to >10x most gas heat capacities), the max lift in a well would be achieved very near the maximum wellhead temperature.
8/25/2015 26 George E. King Engineering
GEKEngineering.com
Choke Sizing
• Cv = coefficient value
– Number of gallons of water per minute that will pass through a restriction with a pressure drop of 1 psi at 60oF.
– Used as the “flow capacity index”
– Does not correspond to a specific throttling method.
8/25/2015 27 George E. King Engineering
GEKEngineering.com
8/25/2015 28 George E. King Engineering
GEKEngineering.com
8/25/2015 29 George E. King Engineering
GEKEngineering.com
Choke Size
(inches)
Bore Diam
(inches)
Choke Coefficient
MCF/D/PSIA
4/64 0.0625 0.08
6/64 0.0938 0.188
7/64 0.1094 0.261
8/64 0.1250 0.347
9/64 0.1406 0.444
10/64 0.1563 0.553
12/64 0.1865 0.802
16/64 0.2500 1.470
24/64 0.3750 3.400
32/64 0.5000 6.260
Example: a well is flowing through a 10/64 choke at 2175 psig WHP. What is the dry gas flow rate? (This is a very rough estimate!) 2175 psig = 2190 psia. Choke coeff. for 10/64 = 0.553 Gas rate = 2190 x 0.553 = ~1200 mcf/d
Choke Calculation Example
Note: for accuracy – the upstream press must be twice downstream press.
8/25/2015 30 George E. King Engineering
GEKEngineering.com
Choke Operations
• Problems with Erosion
• Solutions
8/25/2015 31 George E. King Engineering
GEKEngineering.com
Erosion is damage caused by impingement of particles, droplets, bubbles and even liquid on any solid surface at high velocity.
To reduce erosion, slow down the velocity.
8/25/2015 32 George E. King Engineering
GEKEngineering.com
Erosion in a positive of bean choke from micron sized fines and high velocity gas flow.
8/25/2015 33 George E. King Engineering
GEKEngineering.com
Typical flow patterns (and erosion) in a bean choke.
8/25/2015 34 George E. King Engineering
GEKEngineering.com
Advanced corrosion is often in the exit end of the choke from higher gas velocities after gas has expanded.
8/25/2015 35 George E. King Engineering
GEKEngineering.com
Erosion at the exit flange
JPT, March 1998
8/25/2015 36 George E. King Engineering
GEKEngineering.com
The velocity profile and pressure drop across a choke with a large pressure drop – opportunity for erosion is very high.
JPT, March 1998
8/25/2015 37
George E. King Engineering GEKEngineering.com
One solution to the problem is to take the pressure drop in series and hold a slight backpressure. For example, a 1000 to 0 psi pressure drop produces a 68 fold expansion in gas volume, while a 1500 to 500 psi pressure drop produces a 3 fold gas volume expansion.
JPT, March 1998 8/25/2015 38
George E. King Engineering GEKEngineering.com
Choke Conclusions
• Production chokes help unload and produce the well through pressure management.
• Choke setting requirements change as pressure drops, rate changes and fluid composition varies.
• Good production engineering requires regular design and setting checks for production chokes.
8/25/2015 39 George E. King Engineering
GEKEngineering.com