Well

abandonment
and suspension
 Plug and Abandonment Procedures
• Every well will eventually have to be plugged and
abandoned. If the process is done correctly, it is a
permanent procedure, if not, the well fluids can leak to the
surface, allow surface fluids to leak into the reservoir or
allow cross flow of produced fluids from one zone to
another. Any of these problems can cause extensive
environmental and/or reservoir damage.
• Leaks of brine and hydrocarbons into domestic water
drinking supplies are an increasing problem from producing
or abandoned oil and gas wells.
• The information in the following paragraphs was generated
in the U.S. by the General Accounting Office and reflects a
problem which must be settled by effective completion,
monitoring, and abandonment of all types of wellbores.
• The most common sources of contaminant entry into
underground drinking water supplies were:
1. Cracks in injection wells. This may be splits in the casing
with subsequent matrix injection or uncontrolled fracturing
that has linked perforated productive zones to fresh water
aquifers or other zones that allow cross flow to the aquifers.
(It may also be channels in the cement).
2. Injection directly into drinking water zones. This is generally
the case where corrosion or accidental perforations have
opened up a channel into underground freshwater zones.
3. Injected brine cross flow through improperly plugged and
abandoned wells. This method of entry is most common in
old fields where channels exist along the outside of casing
through a poor primary cement job or where wells not in
use allow brine to flow from the injected zone up to
freshwater zones.
The EPA (Environmental Protection Agency) under the UIC
(Underground Injection Control) program has placed
the following designations on disposal wells.
Class 1 - hazardous waste, non-hazardous industrial
waste, and municipal waste. The disposal zones of
these wells must be located deeper than the deepest
source of drinking water.
Class 2 - oil and gas operations.
Class 3 - special processes such as mineral production
Class 4 - hazardous waste above underground zones of
drinking water. (These wells are now strictly iIlegal .)
Class 5 - All other injection wells which do not fit in one
of the above categories.
 Legal Requirements
• Abandonment of any type of well is covered by very specific
(and often inconsistent) regulations designed to prevent
pollution. Because this is a book on well completion, the
reader is left to dig out the local laws on plug and
abandonment. The true purpose of the abandonment job is
to stop all the individual permeable zones from flowing or
accepting fluid.
• There are a number of procedures and products that can be
used for P&A jobs.
• The correct type to use will depend upon local regulations
and how long the well is to be abandoned and the severity of
the well conditions.
 Setting Cement Plugs
• Downhole plugs of cement are usually set to seal off, either
temporarily or permanently, a zone of unwanted production
or a zone that will be held in reserve for an extended period
of time.
• Plugs are also useful in sealing off an entire well when the
well is to be plugged and abandoned.
• Cement plugs are only one of several methods of sealing
off a zone; however, if they are placed correctly, they
represent the most leak proof and mechanically trouble free
of the alternatives.
• Plugging a well with cement may seem to be an easy task, but
the correct use of cement to obtain a usable plug requires
skilled operators and good equipment. There are three basic
factors which influence the setting and permanence of a cement
plug:
(1) the condition of the mud or drilling fluid in the hole,
(2) the volume and type of cement used, and
(3) the placement technique used to set the plug.
• The first thing considered in a plugging operation is the
conditioning of the mud or wellbore fluid currently in the
hole. Conditioning the mud in this case means bringing it to
the correct density, viscosity and chemical content. If fluids
are not properly conditioned prior to contact of cement, the
cement plug may never be successfully set.
• The composition of the mud in the hole must be known along
with its general condition before a plug job can be successfully
designed. Most failures of plugging operations are due directly
to mud contamination or inability to “float” the cement on the
mud in the hole.
• The best fluid that can be used to set a cement plug is a
freshly prepared, gelled mud slurry which has sufficient
density or a high enough viscosity to keep the cement plug
from migrating up or down the hole due to the difference in
density between the cement and the mud.
• Cement has a density of approximately 16.4 Ib/gal for
regular Class G and H cement blends down to 11.5 Ib/gal
or less for light weight blends. Mud in most P&A jobs
range from 9 to 10 Ib/gal.
• If the density of an ungelled mud is more than the cement
used for the plug, the cement slurry will finger down the
wellbore and a plug will not be created. The fingering of the
cement through the mud also mixes the mud with the
cement (contaminating the cement) and may prevent the
plug from setting, even when very large volumes of cement
are used.
• The only way to float a heavy cement on top of a lighter
mud is to increase the viscosity of the mud high enough to
resist the intrusion of the cement. This “resistance to
intrusion” of the cement is similar to the yield point, or first
resistance to flow, the yield point or “initial stiffness” of the
Bingham plastic fluids will be an advantage in plug setting
without requiring unneeded and expensive viscosity.
• The second factor of plug setting is the selection of the plug
material. A cement slurry for a plug must have durability in
contact with the drilling fluids in the hole as well as good
bonding characteristics to the pipe or formation. A long
enough column of cement should be used so that it is
capable of with standing the weight of drill pipe without
being pushed out of position. The bond with the formation
is critical to the success of a plug both from a adhesion
standpoint and from the elimination of any leakage into or
out of the permeable zones. Increasing the bonding to the
formation is accomplished by cleaning the mud cake from
the wall and selection of the proper place for setting the
plug.
• The primary objectives in plug and abandonment
procedures are: (1) to prevent the contamination or
depletion of any formation based resource, (2) to prevent
communication of subterranean fluids and surface fluids,
and (3) to safely secure the surface of the well so that
accidental entry is impossible.
• Many of the cave-ins around old wellheads result from the
setting of poor quality or insufficient length plugs. When the
steel casing corrodes sufficiently, surface water leakage is
possible and the flow of water may carry very large
volumes of soil into the well, creating surface and/or
underground washouts.
• Never expect the steel casing to be a long term, integral
part of the plugging system in the well unless it is both
surrounded by and filled with cement.
• Placement of the plug is the third critical factor of plug
procedures. The physical location of a plug may differ with
its intended location. The problem is usually caused by the
higher density cement fingering down through the mud. The
plug and abandonment laws and designs usually specify a
solid cement plug extending from 50 ft above to 50 ft
below any fresh water zone or other productive zone.
• In cases where there is no cement behind the pipe or
when the cement quality is unknown or even suspect, the
• pipe must be perforated 50 ft below the deepest usable
water zone and cement circulated through the perfs and up
the annulus to surface.
• Other considerations are setting plugs at liner tops and a
thick plug near the surface to prevent accidental entry or
flow when the wellhead is removed. Placement of a plug is
generally done by one of three methods.
• The balance method involves pumping the cement down
the drill pipe or tubing and up to a calculated height which
would balance the pressure exerted from the cement in and
outside the pipe. This pressure balanced tubing effect can
be used to create a stable plug; provided that the mud that
is currently in the hole will support the cement without
allowing the cement to move through the mud by density
• difference. When a light weight mud is to be used to spot a
16.4 Ib/gallon cement slurry, then the mud must be
gelled to prevent the cement from moving through the mud.
• An alternative to the open end tubing is the use of a cement
retainer. The cement is squeezed under pressure into the
retainer, and the retainer itself helps prevent density
segregation. Bridge plugs or packers may also be set prior
to the cementing operation to keep the cement from moving
through the mud. They are not commonly used due to their
expense but they are very effective.
• A second method of cement plug placement is with a dump
bailer. The bailer, which is run on wireline, is run to the
necessary depth, and cement is flowed out of the bailer into
the area used for the plug.
• Normally, several bailer runs are employed to get the
required volume of cement over the zone. Accurate depth
control is a necessity. Placement of cement with a bailer
often leads to severe mud contamination of the cement
since the operation of the bailer continuously stirs up the
mud and cement. The technique should only be used
where the mud in the hole will not affect the cement setting
characteristics.
• A thicker plug is also recommended. Time must be allowed
between the bailer runs for the cement placed by each run
to take an initial set or to build some gel strength. The
bailer method is commonly used in shallower zones or for
plugs other than those used for permanent abandonment of
the well.
• The two plug method is the third positioning method for
cement. This method involves a similar plug design to that
used in primary cementing.
• The bottom plug is used to isolate the cement and the mud
in the string as the cement is pumped down the pipe. At
some point where the plug is to be set, a plug catcher is
placed in the tubulars to catch the plug as it is pumped out
of the string.
• The cement following then fills the annulus around the pipe.
• The top plug which follows the cement causes a pressure
rise when it hits the bottom of the pipe ensuring that a
signal of cement positioning has been passed.
• This string may then be pulled up out of the cement plug
and the plug left to harden. Mud conditioning is still
required.
• Additional equipment is available to help in setting the plug.
This equipment usually involves the running of scratchers
through the plug setting interval to help remove mud cake.
Regardless of the plug setting methods used, two
operations should follow the plugging operation: (1) tagging
the plug to make sure it is in the right place and
• (2) pressure testing the plug to make sure that there are no
leaks.
An idealized case of plugs set during a plug and abandonment
operation. in real cases, plug thicknesses may be much in excess of
what is required by law. Cement tops are always tagged to make
sure of the location.