15/01/2019
Chapter 2
Well Servicing
Khafiz Muradov
Institute of Petroleum Engineering, EGIS School, Heriot-Watt University, Edinburgh, U.K.
Learning Objectives
• Explain the need for well servicing and intervention, and their benefits.
• List and describe the equipment and operational concepts involved in wireline, coiled tubing and
hydraulic workover interventions.
• Describe the typical applications of wireline, coiled tubing, and snubbing interventions.
• Describe well pressure control, tool conveyance problems, and safety issues associated with the
different types of well interventions.
• Explain the reasons for and principles of the tubing retrieval.
• Analyse a given well problem and propose the optimal intervention program to rectify it.
• List, locate and describe the measurements routinely taken in wells. Explain their applications.
• Explain the objectives of production logging. Propose the logging tools suitable for these
objectives.
• Describe the production logging tools’ types and measurement principles. Explain how these
measurements are related to the fluid and formation properties as well as to flow phenomena.
1
15/01/2019
Production Well: A Complex System
Classes of Well Problems
3. Completion
string
2. Wellbore
casing
1. Reservoir &
bottomhole
completion
+ complex (interactive) problems! A->B->C
2
15/01/2019
Well completion (string) associated problems
Temporary or
permanent
Reasons for Completions Equipment Malfunctions or Failure:
(1) effects of pressure
(2) effects of thermal stress
(3) applied and induced mechanical loadings can cause the tubing to part or
unset packers. They can also be induced by temperature & pressure
changes.
(4) internal corrosion failure due to O2, CO2, H2S and acids. External casing
corrosion can result from corrosive formation waters.
(5) erosion due to high rate flow and/or sand production.
The General Well Completion
Scheme
3
15/01/2019
The nature and consequence of completion
failure
Example failure events
Well completion problems: vertical lift
performance problems
Depends on the
completion Mechanical or
complexity chemical plugging
Remedial measures:
• Wireline fishing
• Tubing retrieval
• Chemical treatment
• Scraping
vs • Hot wash
4
15/01/2019
Benefits of well servicing
Enhancement or restoration of safety
Reduction in operating costs
Acceleration of the production revenue
Increase in the ultimate recovery
Wireline Interventions
Wireline interventions: low cost, fast, operation on live wells possible,
lightweight surface equipment
Typical wireline applications are:
Well logging.
Installation of completion equipment prior to running the production
tubing e.g. a packer and a tailpipe assembly.
Installation or retrieval of equipment within the tubing string e.g. valves,
pressure gauges, etc.
Operation of downhole equipment to either divert or shut off fluid flow
e.g. open a sliding side door or install a bridge plug.
Removal of (rather soft) materials, which have built up in the tubing
string, such as wax, debris or sand.
Well kick-off e.g. by swabbing.
Adjustment of the completion interval e.g. perforating.
5
15/01/2019
Basic Wireline Toolstring
Tools and Functions:
Rope socket: Connection to the
wire (slickline, braided wire, e-line)
Stem(s) for weight (to lower in live
well and for better jarring effect)
Jars (mechanical, hydraulic): jolting
effect to shear a pin, latch onto a
device, clean debris, etc.
Optional knuckle joint for tool
positioning and orientation
Specialist Equipment Examples
Used for:
checking and cleaning (incl. ‘fishing’ = removing items
blocking the well).
taking measurements;
setting, pulling or operating tools
Examples:
a) Gauge Cutter; b) Swage; c) Wireline spear
6
15/01/2019
Surface Equipment
Key Components:
Winch
Hydraulic Power Unit
Lubricator
Wellhead Equipment
Key Components:
Lubricator
Stuffing box
Wireline BOP
Bleed-off Valve
Pulleys
Subsea wells:
Lubricator extends to surface or
ROVs / divers or
Through Flow Line (TFL)
technology
Optional class activity:
Propose the sequence or RIH or POOH
steps
7
15/01/2019
Coiled Tubing (CT) Operations
Objectives include:
Mechanical operations (similar to wireline)
Circulation operations such as
- well clean-out
- placement of fluids or cement
- kick-off (e.g. via gas lift) or kill the well;
- as a velocity string
Drilling or milling operations.
Perforating
Benefits compared to wireline operations:
Circulation is possible
Can be used in highly deviated wells (> 65 degrees or so from vertical)
Can put weight on the tool
Support heavier equipment
Surface Equipment
Surface equipment:
A reel of continuous tubing
A tubing straightener or
gooseneck
An injector head which will drive
the tubing into the well even if it is
under pressure, as well as
support it
A stripper (or pack-off) which is a
dynamic seal, and below it:
A BOP
Bottom hole assembly:
- Nozzles for spotting fluids or
- Drilling tools
- Overshot for latching onto fish
- Etc.
8
15/01/2019
Snubbing
Equipment:
A pipe handling system (travelling
and standing jacks with slips)
A wellhead safety system
consisting of a BOP and a
stripper.
A hydraulic power unit to drive the
jack(s) and auxiliary pumps.
Hydraulic Workover Mast
Reasons for Tubing Retrieval
Mechanical failure of the tubing string, for example a leak or collapse of
the tubing
Inability to conduct tubing wireline equipment replacement through the
internal bore of the string due to ID limitations - small tubing size.
The necessary replacement of the completion string by an alternative
design, for example to allow the installation of artificial lift.
The replacement of the tubing string to optimise reservoir performance,
for example to change the tubing string to a smaller size to better
accommodate declining reservoir productivity.
9
15/01/2019
Data Acquisition in Wells
At the wellhead:
- Pressure and Temperature (tubing, casing)
- Flowrate (test separator, MPFM)
- Other: erosion, artificial lift, etc.
In the tubing:
- Tubing wall state (caliper, magnetic field, etc.) – Production Logging
- MPFM
- Other: pump sensor package, fibre-optic, etc.
At the bottomhole (wellbore):
- Wireline tools, including Production Logging Tools (PLT)
- Gauge cutter
- P and T during testing (flowing, shut-in – well test, multi-rate test, …)
- DST (Drill stem test)
- Other (e.g. geophones or EM surveys)
Behind tubing or casing:
- Fibre-optic sensors: T, stress, acoustic
Openhole Logging
Many logs describe formation, fluid, and the wellbore state, including
Sonic
Spontaneous Potential (SP)
Gamma Ray (GR)
Neutron logs
(Electric) resistivity
(Magnetic) Induction
Nuclear Magnetic Resonance
Caliper
Formation Imaging
…
Other measurements:
Formation Testers
Cased holes: Cement Bond Logging (or similar e.g. Cement Evaluation Tools),
Casing Collar Locator CCL
In cased holes, as part of PL: GR and Neutron can ‘see through’ casing
10
15/01/2019
Production Logging Objectives
Commonly, PL investigates:
Status of well completion.
Anomalous fluid flow between zones.
Performance of the production or injection interval.
Success of stimulation treatment.
Reservoir performance.
Optional class activity:
Discuss examples of the above
Production Logging Measurements
Fluid velocity measurements
Routine
Informative
Needs calibration
Sensing envelope
Optional class activity:
Tutorial 3
11
15/01/2019
Production Logging Tools
Flowmeters
Spinner
– Poor performance in multi-phase flow
– Continuous type vs Diverter type
– Multi-pass calibration
– Flow regime - affected
Downhole MPFMs
– Venturi
– Non-intrusive: reverse, fibre-optic
Virtual flow metering as an option
Production Logging Measurements
Wellbore pressure
Absolute pressure
– Manometer
– Used for wellbore flow model match, well
testing, PVT corrections
Pressure gradient
– Gradiometer
– Recognises changes in fluid density –
relates to flowing phases (how?)
VG
VL
Slippage and liquid holdup
12
15/01/2019
Production Logging Measurements:
Electromagnetic
Fluid capacitance logging
– Electrodes under oscillating potential (e.g.
radio frequency). Measures fluid
capacitance. Differentiates HC from water
CCL
– Measures change in magnetic field against
collars. (Alternatively – caliper-type)
Wall thickness
– Change in electromagnetic field against
thinner tubing (corrosion / erosion?)
Production Logging Measurements: T
Wellbore temperature
Wellbore thermal state depends on
– Heat convention (examples?)
– Heat conduction (examples?)
– Thermodynamic effects (examples?)
Situations
– Flowing and shut-in gradients
– Thermal slugs
– Cement hardening
Can detect
– TOC
– Cross-flow
– Flow behind casing
– Inflow/outflow intervals
– Stimulation assessment
Radial Temperature – flow behind casing
13
15/01/2019
Production Logging Measurements: Gamma Rays
GR can be either natural or induced by e.g.
(pulsed) neutron bombardment and
‘activation’
Types: Example: gun positioning
Conventional GR
– Formation props
– Tracers (ejected for V; added for placement allocation)
– Tool positioning
Spectral GR
– Better differentiation or composition
Focused GR
– Wellbore fluid density
Thermal neutron decay
– Formation water saturation
Pulsed Neutron: activation of elements
– Oxygen for flow behind casing
– Silicon for GP assessment, …
Production Logging Measurements: Acoustic
Noise
– What makes noise in the well? Does it sound
different for various events?
Ultrasonic logs
– Small leaks
From ““Production Operations”, Volume 1, by Allen, T., and Roberts, 1997
Optional class activity:
Propose an example, well performance problem and list the PLTs suitable to detect or even quantify it.
14
15/01/2019
Fibre-Optic Sensors
Principle: light back-scattering in an optic fibre. The back scattered light ‘remembers’
the fibre state in the point of scattering
Interferometer at surface to compare the source, laser pulse spectrum and the back-
scattered one
Sensing either distributed or discrete (Bragg gratings)
Can measure continuously: temperature, strain, acoustic signal. MPFM
Non-intrusive, no electronics downhole
Problems: ageing, splicing, dry and wet connections
Well Abandonment
Required by law to ensure there is no fluid spill to surface nor contamination of fresh
water reservoirs by saltwater
All production/injection and liner hanger (if any) intervals are then cemented. Cement
plugs at least 500’ long with the top of cement at least 100’ above each interval are
placed and tested. Bridge plugs (i.e. mechanical plugs installed permanently e.g. on
a wireline) are installed at the bottom of every next cement plug.
The near-surface freshwater reservoirs are also cemented.
Finally, all the surface equipment is removed and the casings are cut a few meters
below surface (or seabed) and retrieved. (Optionally, no casing retrieval from offshore
wells may be required, as long as nothing is protruding above seabed).
15
15/01/2019
Learning Objectives
• Explain the need for well servicing and intervention, and their benefits.
• List and describe the equipment and operational concepts involved in wireline, coiled tubing and
hydraulic workover interventions.
• Describe the typical applications of wireline, coiled tubing, and snubbing interventions.
• Describe well pressure control, tool conveyance problems, and safety issues associated with the
different types of well interventions.
• Explain the reasons for and principles of the tubing retrieval.
• Analyse a given well problem and propose the optimal intervention program to rectify it.
• List, locate and describe the measurements routinely taken in wells. Explain their applications.
• Explain the objectives of production logging. Propose the logging tools suitable for these
objectives.
• Describe the production logging tools’ types and measurement principles. Explain how these
measurements are related to the fluid and formation properties as well as to flow phenomena.
16
