SLIDE 6 Casing Introduction
Embed Size (px)
DESCRIPTION
Casing Introduction
Citation preview
Slide 1Introduction to Casing
© The Robert Gordon University 2006
After each section of hole has been drilled a string of casing is
run to line the inside of the wellbore. The main reasons for casing
off the open hole are:
to prevent caving of the wellbore, as if unsupported, the hole may
be liable to fall in upon itself;
to provide support for weak or fractured formations from mud
weights which may cause these zones to break down;
to isolate zones of abnormal pressure;
to seal off zones of lost circulation (i.e. thief zones where mud
or cement slurry etc enters the formation);
to provide a means of completing and producing the well
efficiently;
to provide structural support for wellheads and blow-out prevention
equipment
© The Robert Gordon University 2006
Different Casing Strings
Types Of Casing
Conductor Casing
Conductor casing is the first casing string to be run, and
consequently has the largest diameter. With continuous fluid
circulation seabed formations would be eroded away leaving large
washouts. The surface formations may also have low frictional
strength which could easily be exceeded by the hydrostatic
pressures exerted by the drilling fluid when drilling a deeper
section of the wellbore. Conductor pipe may not be necessary in
areas where the surface formations are stronger and less likely to
be eroded away. The conductor provides a conduit for mud returns.
Where conditions are suitable the conductor may be driven into the
formation and is then called ‘stove pipe’.
Surface Casing
The main functions of surface casing are to seal off any fresh
water sands and to provide structural support for the blow-out
preventer (BOP) and wellhead equipment. In an area where abnormal
pressures are expected the setting depth is important, as the
control equipment will need to be capable of withstanding that
pressure. If the casing is set too high there may not be sufficient
formation strength at the shoe to handle a kick when drilling the
next section.
Intermediate Casing
Intermediate, or ‘protection’ casing is used to isolate any
troublesome formations which would/could cause drilling problems in
the wellbore. Sloughing shale, lost circulation, and high pressure
zones are just a few examples. Several strings of intermediate
casing may be required depending upon the number of problems
encountered, if rock formations are stable, drilling can be
undertaken for relatively long periods of time prior to the setting
of casing. The setting depth depends on a knowledge of pore
pressures and fracture gradients. During drilling operations the
mud weight controls pore pressures, but must not exceed the
fracture strength of shallower zones not cased (and protected) from
the drilling fluid.
Production Casing
This is usually the last string of casing to be run in the wellbore
and is either run through the pay zone (reservoir) or set just
above the pay zone for an ‘open hole’ completion. The main purpose
of production casing is to isolate the production interval from
other formations such as water bearing sands, and to protect the
completion tubing (through which the reservoir fluids will be
transported to the surface). Production casing should be thoroughly
pressure tested. Usually the casing will be pressure tested to
around 60% of its specified burst pressure, if the casing
withstands that pressure it is expected to be capable of
withstanding higher pressures.
Liner
A liner is a short string of casing which does not extend back to
the surface. It is run back inside the previous casing string to
provide some overlap. Liners may be used either as an intermediate
or production string. The liner has the advantage of being much
cheaper than a full length casing string. If required, a tie back
string can be run to extend the liner back to the wellhead. A liner
is usually less than 5,000 feet long. Overlap is somewhere in
region of 200–500 feet and it is essential that the liner overlap
is gas tight.
© The Robert Gordon University 2006
Casing Programmes
Casing makes up a considerable part of the overall well cost
(<20%), so great care must be taken to design a casing string
which will satisfy the requirements of each well. eg:
Must withstand expected loadings
Operational problems in running the casing
Be able to handle expected formation pressures
Be functional for the anticipated lifespan of the well
Be flexible enough to allow additional string to be run if
operational difficulties are unexpectedly encountered
With a view to ultimate abandonment
© The Robert Gordon University 2006
Casings have to be cemented in place to ensure a competent pressure
control system
More about cementing later...
Types of Casing
Conductor Casing
Surface Casing
Intermediate Casing
Production Casing
Conductor Casing
Conductor casing (sometimes called “pipe”) is the first casing
string to be run – often driven into the formation using large
hydraulic jack-hammers. It ...
consequently has the largest diameter
protects seabed formations - continuous fluid circulation will
likely create large “washouts” through erosion
also protects surface formations which may have low frictional
strength which could easily be exceeded by the hydrostatic
pressures exerted by the drilling fluid when drilling a deeper
section of the wellbore
provides a conduit for mud returns
© The Robert Gordon University 2006
Surface Casing
The main functions of surface casing are to seal off any fresh
water sands, and ...
to provide structural support for the blow-out preventer (BOP) and
wellhead equipment
In an area where abnormal pressures are expected the setting depth
is important, as the control equipment will need to be capable of
withstanding that pressure. If the casing is set too high there may
not be sufficient formation strength at the shoe to handle a kick
when drilling the next section
© The Robert Gordon University 2006
Intermediate Casing
Intermediate, or ‘protection’ casing is used to isolate any
troublesome formations which would/could cause drilling problems in
the wellbore - a few examples:
sloughing shale
lost circulation
high pressure zones
Several strings of intermediate casing may be required depending
upon the number of problems encountered, if rock formations are
stable, drilling can be undertaken for relatively long periods of
time prior to the setting of casing
The setting depth depends on a knowledge of pore pressures and
fracture gradients
During drilling operations the mud weight controls pore pressures,
but must not exceed the fracture strength of shallower zones not
cased (i.e. not protected) from the drilling fluid
© The Robert Gordon University 2006
You’ll hear a lot about .. “Pore Pressure”
It is .... The pressure of fluids within the pores of a reservoir
(usually hydrostatic pressure) or the pressure exerted by a column
of water from the formation's depth to sea level
When impermeable rocks (such as shales) form as sediments are
compacted, their pore fluids cannot always escape and must then
support the total overlying rock column, leading to anomalously
high formation pressures
© The Robert Gordon University 2006
Production Casing
This is usually the last string of casing to be run in the wellbore
and is either run through the pay zone (reservoir) or set just
above the pay zone (for an ‘open hole’ completion). Its main
purpose is to:
isolate the production interval from other formations such as water
bearing sands, and
to protect the completion tubing (through which the reservoir
fluids will be transported to the surface)
Production casing should be thoroughly pressure tested. Usually the
casing will be pressure tested to around 60% of its specified burst
pressure, if the casing withstands that pressure it is expected to
be capable of withstanding higher pressures
© The Robert Gordon University 2006
Liner
A liner is a short string of casing which does not extend back to
the surface. It is run back inside the previous casing string to
provide some overlap
Liners may be used either as an intermediate or production
string
The liner has the advantage of being much cheaper than a full
length casing string
If required, a tie back string can be run to extend the liner back
to the wellhead
A liner is usually less than 5,000 feet long. Overlap is somewhere
in region of 200–500 feet and it is essential that the liner
overlap is gas tight
© The Robert Gordon University 2006
Casing vs. Bit Size (illustrative)
Casing Size
Tugwell (AG)
Selection of bit and casing sizes can mean the difference between a
well that must be abandoned before completion and a well that is an
economic and engineering success. Recently there has been pressure
on designers to develop slim hole designs based upon a conductor
with (for example) a fourteen inch diameter. This means
operationally there is no ability to run an intermediate or backup
casing string if any hole problems are encountered. Small holes may
therefore have to be abandoned due to drilling or completion
problems. A well drilled in an area of high pressure will usually
require additional casing strings or liners
© The Robert Gordon University 2006
Casing setting Depth
The initial selection is based on the pore pressures and fracture
gradients anticipated in the wellbore – pressures at which fluid
will come out of the formation and the pressures at which pumped
fluid will fracture the formation. The drilling engineers should
ensure, as far as possible, that relevant offset data has been
considered in the estimation of pore pressures and fracture
gradients, and the effect of hole angle on offset fracture gradient
data has also been considered
The total depth of the well, and hence the setting depth of the
production casing or liner, is driven by logging, testing, and
completion requirements. The shoe must be set deep enough to give
an adequate sump for logging, perforating, and to enable testing of
the reservoir fluids
© The Robert Gordon University 2006
4.3) Move across to Point C which identifies the mud weight
requirement for that depth
4.6) Point E is the normal pressure range and no further casing is
required to withstand the associated mud weight. However, a
structural and conductor casing are required, and the setting depth
criteria for those strings are discussed later
1) Draw the mean pore pressure gradient curve along with the
lithology, if available
2) Draw the mud weight curve. The mud weight curve should include a
200 to 400 psi trip margin
4.2) Move up to Point B which determines the initial estimated
setting depth for the intermediate casing (actually run it 300–400
feet deeper)
4) To determine initial estimates of casing setting depths
–First:
Enter the mud weight curve at Point A (Total Depth (TD))
4.4) Move up to Point D which determines the preferred setting
depth for the surface casing/ intermediate string
3) Draw the predicted fracture gradient curve
4.5) Move across to Point E to identify the mud weight required at
that depth
© The Robert Gordon University 2006
Other factors may affect the casing design programme...
if shallow gas zones are encountered whilst drilling, operations
must stop and casing must be set;
if lost circulation zones are encountered, drilling should stop,
mud returns should be checked and Lost Circulation Material (LCM)
pumped downhole before the mud is conditioned to a more suitable
rheology. The zone will then need to be drilled before casing can
be set;
formation stability: if the formation is sensitive to the mud
weight over time, casing may have to be set
© The Robert Gordon University 2006
directional well profile: it is important to line out the well
trajectory before setting casing and attempt to achieve a
consistent survey ahead of a tangent. Long open hole sections may
require casing to reduce the occurrence of stuck pipe, and the
level of torque;
side tracking requirements as specified in the drilling programme.
For example, the 13-3/8” casing may be set high enough to allow
9-5/8” casing to be cut and pulled, enabling a side-track in
12-1/4” hole;
fresh water sands: i.e. have to protect drinking water, and not
contaminate it with drilling fluid;
hole cleaning, particularly if a long section of 17-1/2” hole is
required
© The Robert Gordon University 2006
salt sections, invariably associated with a good reservoir, should
be drilled round rather than through;
high pressure zones;
casing shoe shall, where practicable, be set in competent
formations;
uncertainty in depth estimating e.g., require a margin related to
confidence limit when setting close to a permeable formation. Best
source for this information is offset well data or area experience,
but this should be documented
© The Robert Gordon University 2006
Kick tolerance
Once the initial casing setting depths are selected, the kick
tolerance associated with those depths should be determined. Start
from TD up to the surface to determine the kick tolerance and
preferred setting depth for each casing string. The acceptability
of kick tolerance values of less than 100 bbls should always be
justified – by a review of the type of well, capacity of rig
equipment for kick detection and operator/driller’s experience,
area experience and geology
© The Robert Gordon University 2006
Design “Bottom-to-Top”
A design should be developed by well planning that provides for
economic production from the pay zone consistent with safety
requirements. The pay zone should be analysed for its flow
potential and the drilling problems that will be encountered upon
reaching it. The well should be designed from bottom-to-top. The
opposite approach can result in a well that limits the production
capacity of the pay zone
© The Robert Gordon University 2006
Design of the Completion Tubing
The design of completion tubing strings must be given consideration
relative to its ability to transport oil or gas to the surface at
economical rates. Small diameter tubing chokes the flow rate due to
high frictional pressures, therefore small diameter tubing is to be
avoided. Large diameter strings are preferable for good flow rate.
Small completion tubing can also lead to operational problems due
to reduced radial clearances making tool placement operations more
difficult, and workover activities more complicated. The required
completion characteristics must therefore be taken onto account
when designing the casing string
© The Robert Gordon University 2006
Further Reading
SPE’s Applied Drilling Engineering (Red Book) :–
Chapter 7 “Casing Design”