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

single stage cementing

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.

two stage cementing

 

(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.

CBL tool

receiver signal

(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.

CBL VDL
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.

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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.

balanced plug

(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.

dumb bailer plug

 

The disadvantages are:
• 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.

(a) Bradenhead squeeze
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.

bradenhead

(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)

squeeze cementing packer

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

In a high pressure squeeze the formation is initially fractured (broken down) by a solids free breakdown fluid. A solids free fluid is used because a solids laiden fluid such as drilling mud will build up a filter cake and prevent injection into the formation. Solids free fluids such as water or brine are recommended. The direction of the fracture depends on the rock stresses present in the formation. The fracture will occur along a plane perpendicular to the direction of the least compressive stress (Figure 19). In general, the vertical stress, due to the overburden, will be greater than the horizontal stresses. A vertical fracture is therefore more likely. In practice the fracture direction is difficult to predict since it may follow natural fractures in the formation. Since squeeze cementing is often used to isolate various horizontal zones a vertical fracture is of little use (vertical fluid movement is not prevented).

fracturing

After the formation is broken down a slurry of cement is spotted adjacent to the formation, and then pumped into the zone at a slow rate. The injection pressure should gradually build up as the cement fills up the fractured zone. After the cement has been squeezed the pressure is released to check for back flow. The disadvantages of this technique are:
1. No control over the orientation of the fracture
2. Large volumes of cement may be necessary to seal off the fracture
3. Mud filled perforations may not be opened up by fracturing, so the cement may not seal them off effectively.

Squeeze cementing is the process by which hydraulic pressure is used to force cement slurry through holes in the casing and into the annulus and/or the formation. Squeeze cement jobs are often used to carry out remedial operations during a workover on the well. The main applications of squeeze cementing are:
• To seal off gas or water producing zones, and thus maximise oil production
from the completion interval
• To repair casing failures by squeezing cement through leaking joints or corrosion hole
• To seal off lost circulation zones
• To carry out remedial work on a poor primary cement job (e.g. to fill up the annulus)
• To prevent vertical reservoir fluid migration into producing zones (block squeeze)
• To prevent fluids escaping from abandoned zones.

During squeeze cementing the pores in the rock rarely allow whole cement to enter the formation since a permeability of about 500 darcies would be required for this to happen.

There are two processes by which cement can be squeezed:
• High pressure squeeze – This technique requires that the formation be fractured.
which then allows the cement slurry to be pumped into the fractured zone.
• Low pressure squeeze – During this technique the fracture gradient of the formation is not exceeded. Cement slurry is placed against the formation, and when pressure is applied the fluid content (filtrate) of the cement is squeezed into the rock, while the solid cement material (filter cake) builds up on the face of the formation.

Institute of Petroleum Engineering, Heriot-Watt University

The main cause for poor isolation after a cement job is the presence of mud channels in the cement sheath in the annulus. These channels of gelled mud exist because the mud in the annulus has not been displaced by the cement slurry. This can occur for many reasons. The main reason for this is poor centralisation of the casing in the borehole, during the cementing operation. When mud is being displaced from the annulus the cement will follow the least path of resistance. If the pipe is not properly centralised the highest resistance to flow occurs where the clearance is least. This is where mud channels are most likely to occur (Figure 18).

In addition, field tests have shown that for a good cement bond to develop the formation should be in contact with the cement slurry for a certain time period while the cement is being displaced. The recommended contact time (pump past time) is about 10 minutes for most cement jobs. To improve mud displacement and obtain a good cement bond the following practices are recommended:

• Use centralisers, especially at critical points in the casing string
• Move the casing during the cement job. In general, rotation is preferred to
reciprocation, since the latter may cause surging against the formation. A specially
designed swivel may be installed between the cementing head and the casing to
allow rotation. (Centralisers remain static and allow the casing to rotate within
them.)
• Before doing the cement job, condition the mud (low PV, low YP) to ensure
good flow properties, so that it can be easily displaced.
• Displace the spacer is in turbulent flow. This may not be practicable in large
diameter casing where the high pump rates and pressures may cause erosion or
formation breakdown.
• Use spacers to prevent mud contamination in the annulus.

chanelling

Institute of Petroleum Engineering, Heriot-Watt University