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Remedial cementing placement techniques
Remedial cementing is performed to correct problems associated with the primary cementing job. This article describes some common techniques for placing the cement in the correct location to correct the problem. These techniques are described in basic terms; they can be custom-designed for specific situations, and depend upon the type of remedial cementing operation being undertaken.
Dump bailers are used for placing very small volumes of plugging material precisely and economically. Different types of dump bailers, including gravity and positive-displacement bailers, are shown in Fig. 1. These are generic dump bailers and are indications of various types. Generally, any company in the business or setting wireline plugs (both slickline and e-line) will have some type of dump-bailer service.
These tools can be run on wireline, slickline, or sandline, depending on the tool. Both through-tubing and through-casing sizes are also available. Placed with this technique are:
- Thermal-set resins
- Cement slurries
The use of dump bailers for spotting materials that thermally depend on set times (such as polymers, resins, and cement slurries) has historically been limited to shallow depths because of temperature concerns, but modern polymer and retarder technology allows for broader use.
One of the following is often placed below the desired plugging location to provide a solid bottom in the wellbore:
- Limit plug
- Cement basket
- Permanent bridge plug
- Sand pill
The dump bailer, containing a measured quantity of plugging material, is lowered to the desired depth. The bailer is opened either electronically by the wireline operator or mechanically by tagging the bridge plug and then raised to release the plugging material at this location. At times, the job is performed either with a lubricator on the wellhead or under overbalanced conditions so that the plugging fluid may achieve limited forced entry through gravel packs, perforations, and other passages into formation matrix.
Coiled tubing (CT)
Probably the most technically efficient way to spot fluid in a wellbore is to lay it in with CT, but it is not always the most economical or logistically efficient way. The process consists of placing the end of the CT at the bottom of the planned plug depth, and while the cement or other plugging slurry exits, the nozzle at the end of the coil slowly extracts the coil so that the pull-out rate matches the fluid-pump rate and keeps the end of the coil just below the top of the slurry. This placement method results in a volume of plugging fluid with little or no contamination in the wellbore. After placement, the operator may wash out the wellbore above the plug to establish a very accurate top of cement or apply squeeze pressure in some prescribed manner.
Bullheading or bradenhead placement
The bullheading or Bradenhead placement method consists of injecting a plugging slurry into a formation with the intent to leave some portion of the plugging material in the wellbore. Typically applied in cases of well control, lost circulation, or abandonment, this method is probably one of the less accurate placement methods because of the uncertainty of the fluid path.
The general assumption is that the fluids will follow the path of least resistance, but that is not always reliable. Consequently, when a slurry goes into the annulus rather than down into a lower portion of the wellbore, work strings are sometimes unintentionally cemented in the wellbore. Despite the uncertainty involved, this plugging method has been used successfully when it is executed with caution.
Probably the most common technique in both drilling and abandonment operations, the balanced-plug method involves pumping the slurry through drillpipe or tubing until the level outside is equal to that inside the string. The volume and hydrostatics of wellbore fluids, preflushes, spacers, and plugging fluids must be carefully calculated to ensure that the system is being correctly balanced in the hole. The pipe or tubing is then pulled slowly from the plugging material before it sets, leaving the plug in place. The method is simple in theory, but depending on wellbore conditions, the fluid mechanics can be extremely complex. Wellbore, fluid, and hardware constraints must all be considered during the design and execution of the job.
Preparing a drilling fluid for cementing can be difficult before a primary cementing operation; it can be next to impossible when preparing to spot a balanced cement plug because of time, economic, and technical constraints.
When these conditions exist, a simple way to ensure maximum mud removal is to wash across the interval where the plug will be placed, typically with a diverter tool (such as a drillpipe, CT, or a specialized tailpipe assembly) on the end of the work string. This approach ensures that the wellbore fluids are as close as possible to being 100% mobilized across this critical interval.
The operator should consider running a sacrificial tailpipe in cases where well control is a concern, such as:
- Placement of thixotropic slurries
- Short slurry-thickening time
- Other instances with a high risk of compromising plug stability/integrity when the work string is removed
This tailpipe can be released by either shearing it off at the end of the job or leaving the work string in place until the plugging material has set and cutting or backing off the pipe at the first free connection. In extreme situations, a sacrificial string may include the bottomhole drilling assembly, but if the pipe can be tripped, releasable tailpipe assemblies can be quickly fabricated out of locally available tubing and hardware. If necessary, such assemblies can be constructed out of any drillable material, such as aluminum.