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

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