Squeeze Cementing Definitions and Tools
There are two traditional squeeze cementing classifications, the low pressure squeeze and the high pressure squeeze, depending on whether the bottomhole squeeze pressure is less than or more than the formation fracturing pressure.
Low Pressure Squeeze
The objective of a low pressure squeeze is to fill the perforation tunnels and void behind the casing without fracturing the formation. To accomplish this, it is essential that the tunnels be clear of mud or debris before the cement is pumped. In general, a low pressure squeeze is the preferred approach and is the most likely to succeed.
There are two low-pressure pumping methods, running squeeze and hesitation squeeze, and two basic cement placement techniques, Bradenhead and squeeze-tool technique.
Running Squeeze
With a running squeeze, the cement slurry is pumped continuously until the desired final squeeze pressure is obtained and then pumping is stopped. Pressure is monitored, and if it begins to fall more slurry is pumped to maintain the final squeeze pressure. This continues until the pressure is maintained for several minutes without additional pumping. It is common for slurry volumes of 10 to 100 barrels to be pumped in a running squeeze. In some cases where very small micro-annular gaps must be filled, modified pumps that can maintain very low pump rates (0.06 
) have been used.
Hesitation Squeeze
Generally speaking, the minimum slurry pump rate for most oilfield pumps will be higher than the rate of slurry filtrate leak-off (fluid loss) into the formation, so maintaining a constant differential pressure without fracturing the formation can be a challenge.
The hesitation squeeze applies intermittent pump pressure: pumping at a rate of 1/4 to 1/2 barrel per minute for a period of time followed by 10 to 20 minute pump shut down intervals for pressure falloff as filtrate is lost to the formation. Generally, the leak-off time period increases with successive cycles as the filter cake builds up (Figure 1).
Bradenhead Squeeze Placement
A Bradenhead squeeze is a means of placing the cement slurry without using a packer in the tubing work string. In this approach, open-end tubing is used to spot the cement in the casing while taking returns on the annulus (Animation 1). The tubing is pulled up above the top of the cement, the annulus is sealed off, and pressure is applied by the fluid in the tubing. This approach is simple and requires no special equipment, but because there is no packer sealing off the annulus above the cement, the operator must be certain that the entire casing and casing head is capable of withstanding the pressure being applied. (Note: Braden was an early casing head manufacturer, and thus Bradenhead became another name for casing head.)
High Pressure Squeeze
A low pressure squeeze may not be possible if the passages to be filled (perforation tunnels or cracks in primary cement) are plugged or too small. A larger passage must be created by actually fracturing the formation. The fracture will be oriented perpendicular to the least principal stress and fractures induced in formations of any significant depth are nearly always vertical. The risk of high pressure squeezes is that vertical communication between zones may be established through the fracture created.
Squeeze Tools
Retrievable Squeeze Packers and Bridge Plugs
A number of retrievable squeeze packers and bridge plugs are available for running on tubing or drill pipe (Figure 2). These tools can be combined to isolate casing perforations or other openings and permit a relatively small volume of cement to be squeezed into the openings under elevated pressure without exposing either uphole or downhole intervals to the cement or pressure.
Retrievable packers can be set, released, and reset multiple times, providing a greater degree of flexibility. Bridge plugs are normally set to isolate the casing below the zone being squeezed by preventing flow downward. They are available in both retrievable and drillable options. In a situation with multiple zones that must be squeezed sequentially, a retrievable bridge plug and packer may be run in tandem.
Drillable Cement Retainers
Cast iron drillable cement retainers can be placed in the casing permanently or for drilling out post-squeeze. Cast iron retainers are often used instead of retrievable packers because they prevent flow in either direction.
Coiled tubing through-tubing squeeze
Since the 1980s, coiled tubing that can be run through production tubing has been used to place relatively small volumes of cement downhole without the need for a workover rig. This approach can be used in special situations such as when it is cost-prohibitive to set up a rig.
