Cement plugs are used to separate the cement slurry from the spacer or mud and prevent contamination. On long casing strings additional plugs are pumped ahead of and between the spacer train to minimise contamination caused by different flow regimes within the different spacers and to maximise their effectiveness when they exit into the annulus.
Plugs are normally manufactured out of rubber. Various proprietary devices are used to “lock”
plugs together to make them easier to drill out (often termed PDC drillable).
The bottom plug has a thin diaphragm in it’s centre. After it lands out on the float collar, the diaphragm ruptures when a pre-determined differential pressure is reached. It is normally dropped ahead of the spacer or cement.
The bottom plug has a solid centre.
10.0 Cementing Practices 10.1 Primary Cementing
• Ensure that a computer simulation of the cement job has been performed to establish minimum and maximum flowrates and ECDs.
• Condition the mud to reduce rheology (YP, gels) prior to final trip out to run casing.
• Confirm that the plugs are correctly placed in cement head – bottom (diaphragm) plug below top (solid) plug
• Run the casing in until a few feet off bottom. Break circulation if required on the way in.
• Circulate at least one casing volume to ensure that there is nothing to plug the shoe and to remove any trip gas.
• Pump spacers, release bottom plug and pump cement slurry (lead and tail).
• Release top plug, clear cement line and begin to displace.
• Displacement rate should be altered depending upon what is in the annulus (mud, spacer or cement). Most spacers and cement require turbulent flow (if possible) to maximise mud removal and reduce mud contamination.
• When the bottom plug reaches the float collar, the diaphragm should rupture allowing continued pumping
• The displacement volume to land the top plug should have been calculated previously.
• Displacement rate should be reduced as plug bump is approached to prevent excessive pressures and any shock as plug lands.
• If no bump is seen, it is common practice to further displace up to half of the shoe track (note that some operators have adopted a pump till bump philosophy).
• All mud returns should be closely monitored throughout for losses, which could be evidence of fracturing formation.
• If losses are observed, the displacement rate can be adjusted to reduce the ECD i.e.
annular pressure losses.
• The plug should be bumped with about 1000psi differential having first confirmed that the casing burst pressure less safety margin would not be exceeded.
• If required the pressure can be increased at this point and a casing pressure test performed (note it is necessary to confirm the pressure rating of all components before performing this test)
• The pressure should then be bled off to confirm that the float valve is functioning and supporting the differential back pressure due to heavy cement in the annulus.
10.2 Stage Cementing
Used in applications where long sections of casing require cementing but concerns exist because of:
• Long pumping times
• High pump pressures
• Excessive hydrostatic pressure due to column of cement – exceed fracture gradient First stage
Repeat of primary cementing operation above.
DRILLING PRACTICES COURSE
Second stage
This necessitates the inclusion of a DV collar in the original casing string at a pre-determined depth. The first stage places cement in the annulus from the bottom up to the DV collar. The ports on the DV collar can then be opened by dropping a special dart (bomb) and shearing the retaining pins (1000-1500psi). Circulation is then established through the DV collar. The above primary cementing procedure can then be repeated but without any pipe reciprocation. Further stages could be included if necessary.
10.3 Inner String Cementing
Conventional cementing approach with large diameter casing will result in:
• Large displacement volumes
• Extended displacement duration
• Significant volume of cement remaining in shoe track
As an alternative, the casing can be cemented through tubing or drill pipe. A special float shoe is used which allows the drill pipe to stab in providing a hydraulic seal. The casing is run as normal and then the inner string is run and stabbed into the float shoe. The cement job proceeds as before using smaller drill pipe plugs. After displacement and confirmation that the float shoe is holding back the differential pressure, the pipe can be retrieved.
Care needs to be taken with this technique as the possibility of collapsing the casing is significantly increased.
10.4 Liner Cementing
A liner string usually comprises a shoe and float collar as with larger casings along with a liner hanger (hydraulic or mechanically set) to secure the upper end. The entire assembly is run on drill pipe and then the hanger set typically 300-500ft within the previous casing. Once set, mud is circulated to ensure an obstruction free cementing path around the liner. Prior to cementing, the running tool is backed off the liner hanger to guarantee removal of the drill pipe afterwards.
Liner cement recipes usually contain extra additives for fluid loss control, retarding, possible gas blocking etc. Since the mix proportions are critical and there is no lead slurry, it is usually batch mixed prior to the job. This guarantees the quality and density throughout the job.
A typical liner cementation operation would proceed as below:
• Position liner at required depth
• Circulate bottoms up – ensure low rheology (minimal YP and gels); rotate liner
• Set liner hanger
• Release setting tool and slack off weight (10-20Klbs)
• Pump spacer
• Pressure test surface lines
• Pump pre-mixed slurry
• Release plug
• Pump spacer
• Displace cement out of liner into annulus – rotate liner if possible
• Pump down plug releases liner wiper plug
• Both plugs pumped down liner until they latch into landing collar
• Bump plugs with 1000psi
• Bleed off and check for back-flow
• Pick up, position tail pipe at top of liner and circulate excess cement out from above liner
10.5 Squeeze Cementing
Use of hydraulic pressure to force cement into an annulus or formation. Usual applications:
DRILLING PRACTICES COURSE
• Seal off gas or water producing zones to improve production
• Repair casing failures
• Seal off lost zones
• Remedial work on primary cement jobs e.g. top up jobs
• Prevent vertical reservoir fluid migration into production zone
• Prevent fluids escaping from abandoned zones
To pump cement into a formation, a permeability of 500darcies would be required. Since this does not occur normally, use several techniques to compensate.
10.5.1 High Pressure Squeeze.
• Formation is broken down first and then cement is squeezed (dense, impermeable formations preferred.
• Use solids-free fraccing fluid. Mud filter cake build up would prevent fraccing.
• Since the overburden generally provides the maximum principal stress (acting vertically), fractures initiated would be vertically oriented i.e. pushing the rock apart horizontally against the direction of the minimum principal stress
• Once fracturing had occurred, cement would be spotted against the fracture zone and then pumped away into the formation after closing in the well.
• The injection pressure should gradually rise as the cement fills up the fractures.
10.5.2 Low Pressure Squeeze
• Here the pressure always remains below fracture pressure.
• Perforations should be flushed clean – free from mud and other plugging materials.
• An injectivity test using water should be conducted first to confirm feasibility of a squeeze.
• A build up of pressure would force fluid from the cement into the pores leaving a filter cake to form on the surface gradually inhibiting the process.
• As the injection process ceases in one location, it can commence at a different site and will continue until an impenetrable seal has blocked off the loss zone.
• Fluid loss additives are important. The use of neat cement alone would result in dehydration of the slurry due to the high fluid loss of neat cement. This in turn would create bridging before all the permeable zone could be sealed.
• Preferred slurry properties: fluid loss 5—200mls; water: solids ratio of 0.4 by weight
10.5.3 Running Squeeze
• Cement is pumped slowly and continuously until final pressure obtained. Used for repairing damaged casing.
10.5.4 Hesitation Squeeze
• Pumping is stopped periodically to allow the slurry to dehydrate and create a filter cake.
Usually pumping in increments of 0.25-0.5bbls every 10-15 minutes.
10.5.5 Bradenhead Squeeze
• Cement is pumped through drillpipe/ stinger (no packer), spotted and squeezed after closing the BOP’s.
• Since the cement can not move up the annulus, it is forced into any loss zones.
• Low pressure squeeze option
• Difficult to place cement accurately
• Cannot be used for selectively squeezing perforations
• As the casing is pressured up, restricted by burst specification.
10.5.6 Packer Squeeze
• Packer enables cement squeeze to be more accurately targeted
• Since the annulus is sealed off, can use higher pressures (not limited by casing burst).
DRILLING PRACTICES COURSE
• Setting depth important – too high and cement contaminated with mud and excess fluid pumped into formation before cement. Too low – risk of cementing packer in.
• Packer normally set 30-50ft above zone of interest with or without a tailpipe.
• Drillable packer (e.g. Halliburtons EZSV or Fasdril)
• Single use only
• Back pressure valve prevents back-flow after squeezing
• Retrievable packer
• Multiple use
• If back-flow occurs after releasing packer, re-set and apply squeeze again.
10.6 Cement Plugs
These are used to fill a section of hole and prevent movement of fluids within. Typical applications are:
• Abandoning depleted zones
• Seal off lost circulation zones
• Provide a kick-off platform for side-tracks
• Isolating a zone for formation testing
• Abandoning an entire well – provision of barriers (Government regulations specify that plugs are required to seal off production zones, aquifers etc)
The biggest problem setting plugs is mud contamination which can be minimised by:
• Use a gauge section of hole
• Use a plug volume sufficient to allow for some contamination - typically 500ft height
Surface hole Surface plug
timed to DRIFT into Loss Zone as slurry thickens Bit is walked off cement plug into softer formation to sidetrack well bore
Sidetracking
DRILLING PRACTICES COURSE
• Condition the mud beforehand
• Use a pre-flush ahead of the cement
• Use a weighted slurry which contains less water
10.6.1 Plug Placement
Balanced plug – attempt to displace sufficient cement out of the drillpipe such that the column of cement in both pipe and annulus are of equal height. The drill pipe or stinger can then be withdrawn leaving the plug in place.
Bridge plug – this can be set on depth and a 500ft cement plug spotted above it. This method gives better depth control and reduced risk of contamination.
Dual plug – setting an initial balanced plug which can then be tagged to give a reference base upon which a second plug can be spotted (height of plug dependent on position of initial plug.
Note: When setting a series of cement plugs, it is advisable to pump a wiper dart or ball after each plug to ensure that the pipe/ stinger does not become plugged itself with cement.
11.0 Evaluation of Cement Job
A cement job has failed and will require remedial work if any of the following situations exist:
• The cement does not fill the annulus to the required height
• The cement does not provide a seal at the shoe
• The cement does not isolate undesirable formations
The effectiveness of the job (and hence the need for additional work) can be measured by various means:
Temperature survey – running a thermometer inside the casing to detect the top of cement. The hydration process of setting cement is exothermic (gives out heat) and is detectable from within the casing.
Radiation log – radioactive tracers can be added to the cement before it is pumped (Carnolite an example).
Cement bond log (CBL) – this is a sonic log capable of both detecting the top of cement and determining the quality of the cement sheath. It is run on wire-line, emits sonic signals and must be centralised to yield credible results. These pass out through the casing and are picked up by a receiver some 3 ft away. Both the transit time and amplitude of the signal are used to indicate the cement bond quality. Since the speed of sound is greater in casing than in the formation or mud, the first signals to return are those of the casing. If the amplitude of this signal (E1) is large, this indicates that the pipe is free (poor bond). When cement is firmly bonded to the casing and formation, the signal is attenuated (weakened) and is characteristic of the formation behind the casing. The signal can also indicate where the cement is bonded to the casing but not the formation. The effect of channelling can also be detected.
DRILLING PRACTICES COURSE
The CBL usually gives an amplitude curve and a Variable Density Log (VDL) which indicates the strength of the signals by the intensity of dark and light streaks. The casing signals show up as parallel lines. A good bond is demonstrated by wavy lines. There is no standard API scale to measure the effectiveness of the CBL and many factors can result in false interpretations:
• During the setting process, the velocity and amplitude of the signals varies significantly. It is recommended that the CBL is not run until 24-36 hours after the cement job to give realistic results.
• Cement composition affects signal transmission.
• The thickness of the cement sheath will cause changes in the attenuation of the signal.
T
R
3ft
Schematic of CBL Tool
FORMATION
SHORTEST PATH LONGEST PATH
DRILLING PRACTICES COURSE
The CBL will react to the presence of a micro-annulus (a small gap between the casing and cement). This usually heals with time and is not a critical factor. Some Operators recommend running a CBL under pressure to eliminate this effect (the casing will balloon out under pressure and occupy any micro-annulus).
One of the limitations of the CBL is that it only gives a one dimensional view if the cement bond at a given depth. An alternative tool that can be run is the Cement Evaluation Tool (CET) which uses ultrasonic transducers and the principles of casing thickness resonance to give a full radial picture of the cement bond around the full circumference of the casing. This is extremely useful in identifying if a channel is present and, on directional wells, the exact orientation of this channel.
12.0 Cementing Calculations
The principal calculations required for a cement job are:
• The amount of slurry required to fill the annulus outside the casing to the programmed height.
• The amount of mud needed to pump to displace the cement i.e. bump the top plug.
In all cement calculations, it is necessary to know the yield per sack of the cement being used to be able to confirm that sufficient material is on site for the job (including contingency). The yield/sack depends on the amount of additives in the cement and the required final slurry density.
Schematics are invaluable in clarifying the volumes required including details regarding annular capacities (open hole and cased hole), different grade casings, section lengths etc.
12.1 Example
A 7” liner is to be set as per the attached schematic.
Calculate the following:
9.5/8" 47 lb ft Casing
Top of liner @ 10,555ft Wiper Plug @ 10,579ft
9.5/8 shoe @ 11,050ft 12¼ open hole @ 11,070ft
Float shoe @ 13,125ft Float collar @ 13,040ft 8½" open hole 7" 29 lb ft liner 5" 19.5 lb ft DP
RTE
DRILLING PRACTICES COURSE
• The amount of water per sack required to give a 16 ppg slurry
• The yield in cuft / sack
• The slurry volume required
• The tonnage of cement blend required
• The mud displacement to latch the wiper plug
• The mud displacement to bump the plug
• Required thickening time Assume the following:
− 30% open hole excess volume
− Static bottom hole temperature 270ºF
− Slurry formulation D603 is a liquid fluid loss additive
D109 is a high temperature liquid retarder
Freshwater is used as the mix water as seawater would accelerate the thickening time Calculations
The amount of water per sack required to give a 16 ppg slurry
Using a variation of the equation density = mass / volume it is possible to calculate the amount of water required.
First it is necessary to calculate the combined weight and volume of the slurry’s components per sack of dry cement.
This is best done in tabular form as shown below
Material Weight (lbs) Absolute Volume (gal /
From cementing tables (example Halliburton Red Book – Technical Data, Physical Properties of Cementing Materials and Admixtures) read off the absolute volume for all the slurry’s components.
One sack of cement weight 94 lbs
35% BWOC silica flour weights 35% x 94 lbs = 32.9 lbs All of the figures in black are taken from the slurry formulation
All of the figures in blue are calculated by dividing the volume by the absolute volume to give weight
All of the figures in red are calculated by multiplying the weight by the absolute volume to give the volume.
Y is the amount of water required.
So for a 16 ppg slurry the totals can be represented as: