Squeeze Cementing Design
Perforation Washing
For a cement squeeze to be effective, the perforations must be open and free of debris that could prevent the formation of a layer of dehydrated cement within the perforation tunnels. This can be accomplished by mechanical and/or chemical means prior to a squeeze.
Mechanical perforation washing involves running a tool on a work string (tubing or drill pipe) that includes an assemblage of rubber sealing elements that permit a wash fluid to be selectively pumped into one set of perforations while being produced back though another set. The tool is slowly moved upward in the casing until the entire squeeze zone has been cleaned.
Another option is to use a surge tool, essentially a chamber at atmospheric pressure to which the perforations are exposed, creating a high differential pressure that expels any debris in the perforation tunnels into the chamber for removal.
Chemical perforation cleaning involves pumping acids or solvents into the perforations ahead of the cement slurry.
Injection Tests
Before a squeeze cementing slurry is mixed and pumped, an injection test is performed. This procedure pumps a fluid, typically a completion fluid such as brine solution, into the well to ensure that the perforations are open and capable of accepting fluid, and to estimate the rate and pressure at which the job will be performed.
Clean completion fluid is injected for several minutes at a typical squeeze job rate taking care to avoid fracturing the formation. The bottom hole injection pressure is estimated based on pumping pressure, hydrostatic pressure, and estimated friction pressure loss. The injection proceeds until the pressure stabilizes, typically 10-15 minutes.
Slurry Design
Fluid loss control should be tailored to the formation permeability so that competent filter cakes form within a reasonable time period. For example, a high permeability formation would require a high fluid loss slurry (300 to 500 
under 1000 psi differential). If there are large voids behind pipe or fractures in the formation, a hesitation squeeze can be carried out with a high fluid loss lead slurry initially pumped to allow a fast filter-cake buildup, followed by a tail slurry with a lower fluid loss.
- In formations with unimpaired natural permeability, a slurry with a water-to-solids ratio of 0.4 (by weight) and a low fluid loss of 50 to 150
minutes under 1000 psi differential should provide satisfactory caking for most low-pressure squeeze jobs. - If the voids behind casing are narrow and deep, very tight fluid loss control (<50 ) is required to prevent premature slurry dehydration.
- When squeezing against shales, dense limestone, dolomites, or permeable formations where natural permeability is plugged with mud, a low fluid-loss cement may not be desirable. In these situations, a high-pressure squeeze job is usually performed, and high fluid-loss slurry is desirable because it promotes good filter-cake development.
A squeeze cementing slurry should generally be of lower viscosity than primary cementing slurry, to allow it to penetrate the smaller spaces being squeezed without an inordinate increase in pump pressure. Low viscosity slurries with dispersants are commonly used, as long as the viscosity does not get so low as to cause formation of a free water phase.
The slurry should be designed to have lower gel strength during placement so that increases in surface pressure related to gelling do not conflict with surface pressure indications of the squeeze job’s progress. Gelation can prevent the application of pressure through the filter cake and impair cake buildup.
Higher density slurries tend to result in high set strength but can be viscous. The best approach is to select particle sizes that optimize strength without increasing viscosity. Further, the slurry should have no free water.
Cement Volume
For filling unknown voids behind pipe, the injection test can be used to estimate the required volume of cement. Plugging perforations requires only enough cement slurry to build a cement filter cake in each perforation tunnel. In many cases, less than a barrel of cement is sufficient, but a 5 to 15 barrel batch is typically mixed. Another guideline is two sacks of cement per foot of perforated interval with a minimum of 50 sacks per job.
A high-pressure squeeze requires a higher volume of slurry, due to the higher volume created by the fractures. In extreme cases injected volumes may exceed 100 barrels of slurry.
The volume of cement slurry should not be so excessive so that it cannot be reversed out at the end of the job.
Spacers and Washes
Spacers can be used in squeeze cementing to prevent contamination of the cement slurry, which can be a particular problem in squeezes because the cement volumes are so small.
Washes can be employed to help clean perforations and to help remove rust, debris, scale or organic deposits from tubulars that have been in place for a long time.
Displacement Volume
The maximum displacement volume for a squeeze operation corresponds to the volume of the work string from the surface down to the perforations, plus a safety margin of a few barrels. Fluid compressibility may become a factor because the pressures attained are greater than during primary cementing operations.
Squeeze Pressure
For low-pressure squeezes, the dynamic fluid pressure at the formation face must be less than the formation fracturing pressure. It must be higher than the formation pore pressure to prevent fluid production. In a high pressure squeeze, the applied pressure exceeds the formation fracture pressure.
The differential pressure across various tubulars must be less than their burst or collapse pressures.
