Their are a variety of additives which may be added to cement.. These additives may be delivered to the rigsite as liquid or dry additives. The amount of additive is generally quoted as a percentage of the cement powder used. Since each sack of cement weighs 94 lbs, the amount of additive can be quoted in weight (lbs) rather than volume. This can then be related to the number of sacks of additive. The number of sacks of additive can be calculated from:

Number of sacks of additive = No. sxs Cement x % Additive
Weight of additive = No. sxs of Additive x 94(lb/sk)

The amount of additive is always based on the volume of cement to be used.

The mixwater required to hydrate the cement powder will be prepared and stored in specially cleaned mud tanks. The amount of mixwater required for the operation will depend on the type of cement powder used. The volume of mixwater required for the cement slurry can be calculated from:

Mixwater Vol. = Mixwater per sack x No. sxs

Although cement and other dry chemicals are delivered to the rig site in bulk tanks the amount of dry cement powder is generally quoted in terms of the number of sacks (sxs) of cement required. Each sack of cement is equivalent to 1 cu. ft of cement.

The number of sacks of cement required for the cement operation will depend on the amount of slurry required for the operation (calculated above) and the amount of cement slurry that can be produced from a sack of cement. The amount of cement slurry that can be produced from a sack of cement, known as the yield of the cement, will depend on the type of cement powder (API classification) and the amount of mix water mixed with the cement powder. The latter will also depend on the type of cement and will vary with pressure and temperature. The number of sacks of cement required for the operation can be calculated from the following.

No. of Sacks = Total Volume of Slurry/Yield of Cement

Sufficient cement slurry must be mixed and pumped to fill up the following (see Fig 29):
A – the annular space between the casing and the borehole wall,
B – the annular space between the casings (in the case of a two stage
cementation operation)
C – the openhole below the casing (rathole)
D – the shoetrack

The volume of slurry that is required will dictate the amount of dry cement, mix water and additives that will be required for the operation.

In addition to the calculated volumes an excess of slurry will generally be mixed and pumped to accommodate any errors in the calculated volumes. These errors may arise due to inaccuracies in the size of the borehole (due to washouts etc.). It is common to mix an extra 10-20% of the calculated open hole volumes to accommodate these inaccuracies.

The volumetric capacities (quoted in bbls/linear ft or cuft/limear ft or m3/m) of the annuli, casings, and open hole are available from service company cementing tables.. These volumetric capacities can be calculated directly but the cementing tables are simple to use and include a more accurate assessment of the displacement of the casing for instance and the capacities based on nominal diameters.

In the case of a two stage operation (Figure 30) the volume of slurry used in the first stage of the operation is the same as that for a single stage operation. The second stage slurry volume is the slurry required to fill the annulus between the casing and hole (or casing/casing if the multi-stage collar is inside the previous shoe) annular space.

(a) Cement bond logs (CBL)
The cement bond logging tools have become the standard method of evaluating cement jobs since they not only detect the top of cement, but also indicate how good the cement bond is. The CBL tool is basically a sonic tool which is run on wireline. The distance between transmitter and receiver is about 3 ft (Figure 26). The logging tool must be centralised in the hole to give accurate results. Both the time taken for the signal to reach the receiver, and the amplitude of the returning signal, give an indication of the cement bond. Since the speed of sound is greater in casing than in the formation or mud the first signals which are received at the receiver are those which travelled through the casing (Figure 27). If the amplitude (E1) is large (strong signal) this indicates that the pipe is free (poor bond). When cement is firmly bonded to the casing and the formation the signal is attenuated, and is characteristic of the formation behind the casing.

(b) the Variable Density Log (VDL)
The CBL log usually gives an amplitude curve and provides an indication of the
quality of the bond between the casing and cement. A VDL (variable density log),provides the wavetrain of the received signal (Figure 28), and can indicate the
quality of the cement bond between the casing and cement, and the cement and
the formation. The signals which pass directly through the casing show up as
parallel, straight lines to the left of the VDL plot. A good bond between the casing
and cement and cement and formation is shown by wavy lines to the right of the
VDL plot. The wavy lines correspond to those signals which have passed into and
through the formation before passing back through the cement sheath and casing
to the receiver. If the bonding is poor the signals will not reach the formation and
parallel lines will be recorded all across the VDL plot.

The interpretation of CBL logs is still controversial. There is no standard API scale
to measure the effectiveness of the cement bond. There are many factors which can
lead to false interpretation:

1. During the setting process the velocity and amplitude of the signals varies significantly. It is recommended that the CBL log is not run until 24 – 36 hours after the cement job to give realistic results.
2. Cement composition affects signal transmission.
3. The thickness of the cement sheath will cause changes in the attenuation of the signal.
4. The CBL will react to the presence of a microannulus (a small gap between casing and cement). The microannulus usually heals with time and is not a critical factor. Some operators recommend running the CBL under pressure to eliminate the microannulus effect

(a) Temperature surveys (Figure 24)
This involves running a thermometer inside the casing just after the cement job. The thermometer responds to the heat generated by the cement hydration, and so can be used to detect the top of the cement column in the annulus.

(b) Radioactive surveys (Figure 25)
Radioactive tracers can be added to the cement slurry before it is pumped (Carnolite is commonly used). A logging tool is then run when the cement job is complete. This tool detects the top of the cement in the annulus, by identifying where the radioactivity decreases to the background natural radioactivity of the formation.

At some stage during the life of a well a cement plug may have to be placed in the wellbore. A cement plug is designed to fill a length of casing or open hole to prevent vertical fluid movement. Cement plugs may be used for:

1. Abandoning depleted zones
2. Seal off lost circulation zones
3. Providing a kick off point for directional drilling (eg side- tracking around fish)
3. Isolating a zone for formation testing
4. Abandoning an entire well (government regulations usually insist on leaving a series of cement plugs in the well prior to moving off location).

The major problem when setting cement plugs is avoiding mud contamination during placement of the cement. Certain precautions should be taken to reduce contamination.

1.  Select a section of clean hole which is in gauge, and calculate the volume required (add on a certain amount of excess). The plug should be long enough to allow for some contamination (500′ plugs are common). The top of the plug should be 250′ above the productive zone
2. Condition the mud prior to placing the plug
3. Use a preflush fluid ahead of the cement
4. Use densified cement slurry (ie less mixwater than normal)

After the cement has hardened the final position of the plug should be checked by running in and tagging the cement. There are three commonly used techniques for placing a cement plug:

(a) Balanced plug (Figure 22)
This method is aimed at achieving an equal level of cement in the drillpipe and annulus. Preflush, cement slurry and spacer fluid are pumped down the drillpipe and displaced with mud. The displacement continues until the level of cement inside and outside the drillpipe is the same (hence balanced). If the levels are not the same then a U-tube effect will take pace. The drillpipe can then be retrieved leaving the plug in place.

(b) Dump bailer (Figure 23)
A dump bailer is an electrically operated device which is run on wireline. A permanent bridge plug is set below the required plug back depth. A cement bailer containing the slurry is then lowered down the well on wireline. When the bailer reaches the bridge plug the slurry is released and sits on top of the bridge plug. The advantages of this method are:
• High accuracy of depth control
• Reduced risk of contamination of the cement.

• Only a small volume of cement can be dumped at a time – several runs may be necessary
• It is not suitable for deep wells, unless retarders used.

After the cement has hardened it must be pressure tested. The tests should include both positive and negative differential pressure. The following should be considered when making a test:
• A positive pressure test can be performed by closing the BOPs and pressuring up on the casing. (Do not exceed formation fracture gradient.)
• A negative pressure test (or inflow test) can be performed by reducing the hydrostatic pressure inside the casing. This can be done using a DST tool or displacing with the well to diesel. This test is more meaningful since mud filled perforations may hold pressure from the casing, but may become unblocked when pressure from the formation is applied.

The high pressure and low pressure squeeze operations can be conducted with or without packers.

This technique involves pumping cement through drill pipe without the use of a packer (Figure 20). The cement is spotted at the required depth. The BOPs and the annulus are closed in and displacing fluid is pumped down, forcing the cement into the perforations, since it cannot move up the annulus. This is the simplest method of placing and squeezing cement, but has certain disadvantages:

•      It is difficult to place the cement accurately against the target zone
•      It cannot be used for squeezing off one set of perforations if other
perforations are to remain open
•      Casing is pressured up, and so squeeze pressure is limited by burst resistance
A Bradenhead squeeze is only generally used for a low-pressure squeeze job.

(b) Squeeze using a packer
The use of a packer makes it possible to place the cement more accurately, and apply higher squeeze pressures. The packer seals off the annulus, but allows communication between drill pipe and the wellbore beneath the packer. (Figure 21)

Two types of packer may be used in this type of operation:

(i)    Drillable packer (cement retainer)
This type of packer contains a back pressure valve which will prevent the cement flowing back after the squeeze. These are mainly used for remedial work on primary cement jobs, or to close off water producing zones. The packer is run on drill pipe or wireline and set just above or between sets of perforations. When the cement has been squeezed successfully the drill pipe can be removed, closing the back pressure valve. The advantages of these packers are:
• Good depth control
• Back pressure valve prevents cement back flowing
• Drill pipe recovered without disturbing cement
The major disadvantage is that they can only be used once then drilled out.

(ii)    Retrievable packer (cement retainer)
These can be set and released many times on one trip. This makes them suitable for repairing a series of casing leaks or selectively squeezing off sets of perforations. By-pass ports in the packer allow annular communication, but these ports are closed during the squeeze job. When the packer is released there may be some backflow, and the cement filter cake may be disturbed. If this happens the packer should be re-set and the squeeze pressure applied until the cement sets.

The basic procedure for squeezing with a retrieveable packer is:
1. run the packer on drillpipe and set it at required depth with by-pass open
2. pump the cement slurry (keep back pressure on annulus to prevent cement falling.

The packer setting depth should be considered carefully. If positioned too high above the perforations the slurry will be contaminated by the wellbore fluids and large volumes of fluid from below the packer will be pumped into the formation ahead of the cement. If the packer is set too low it may become stuck in the cement. Generally the packer is set 30 – 50 ft above the perforations.

Sometimes a tail pipe is used below the packer to ensure that only cement is squeezed into the perforations, and there is less chance of getting stuck (Figure 21). Bridge plugs are often set in the wellbore, to isolate zones which are not to be treated. They seal off the entire wellbore, and hold pressure from above and below. Bridge plugs can either be drillable or retrievable.

Institute of Petroleum Engineering, Heriot-Watt University

It is generally accepted that a low pressure squeeze is a more efficient method of sealing off unwanted perforated zones. In a low pressure squeeze the formation is not fractured. Instead a cement slurry is gently squeezed against the formation. A cement slurry consists of finely divided solids dispersed in a liquid. The solids are too large to be displaced into the formation. As pressure is applied, the liquid phase is forced into the pores, leaving a deposit of solid material or filter cake behind. As the filter cake of dehydrated cement begins to build up, the impermeable barrier prevents further filtrate invasion. The filtrate must then be diverted to other parts of the perforated interval. This technique therefore creates an impermeable seal across the perforated zone. Fluid loss additives are important to perform this technique successfully. Neat cement has a high fluid loss, resulting in rapid dehydration which causes bridging before the other perforations are sealed off. Conversely a very low fluid loss means a slow filter cake build up and long cement placement time. Key factors which affect the build up of cement filter cake are:
• Fluid loss (generally 50 – 200 cc)
• Water to solids ratio (0.4 by weight)
• Formation characteristics (permeability, pore pressure)
• Squeeze pressure

Only a small volume of cement is required for a low pressure squeeze. Perforations must be free from mud or other plugging material. If the well has been producing for some time these perforations have to be washed out, sometimes with an acid solution. The general procedure for a low pressure squeeze job is:
1      Water is pumped into the zone to establish whether the formation can be squeezed (injectivity test). If water cannot be injected the squeeze job cannot be done without fracturing the formation
2      Spot the cement slurry at the required depth
3      Apply moderate squeeze pressure
4      Stop pumping and check for bleed off
5      Continue pumping until bleed off ceases for about 30 mins
6      Stop displacement of cement and hold pressure
7      Reverse circulate out excess cement from casing

A properly designed slurry will leave only a small cement node inside the casing after removing the excess cement. Throughout the procedure squeeze pressure is kept below the fracture gradient. A running squeeze is where the cement is pumped slowly and continuously until the final squeeze pressure is obtained. This is often used for repairing a primary cement job. A hesitation squeeze is where pumping is stopped at regular intervals to allow time for the slurry to dehydrate and form a filter cake. Small volumes of cement (1/4 – 1/2 bbl) are pumped each time separated by a delay of 10 – 15 mins. This technique is dangerous if the cement is still in contact with the drillpipe or packer.

Institute of Petroleum Engineering, Heriot-Watt University