Guidelines for Design of Cement Plugs

General Guidelines for Setting Cement Plugs (job design, volume and excess Requirements, slurry requirements)

Job Design

In deviated wells (deviation > 30 °, a solid base should be provided for the cement plug (bridge plug or a highly viscous and pumpable pill).

In potential loss zones, it is better to set abandonment cement plugs in two stages. The bottom cement plug should be have a relatively short thickening time to support the other abandonment plugs.

If the hole is washed out, it is recommended to set two short plugs over the washed out section, rather than a single large plug.

When setting cement plugs across a casing window (eg., for sidetracking) a technique has been successfully applied using a ca. 20 ft blanked off slotted stinger which is reciprocated across the window while setting the plug to create a jetting action at the window.

A kick‑off cement plug should have approximately 25 ft of cement on top of the window, however, enough excess has to be pumped and circulated out immediately after placing the plug.

Volume and Excess Requirements

The Drilling Supervisor and the Cementer shall calculate the relevant volumes (slurry volume, displacement volume, etc.) independently.

As a minimum, plug size must meet or exceed the following requirements:

For open hole plug backs, calliper information available should be used to calculate the slurry volume and consider addinganother 10% exccess if long circulation times have elapsed since the calliper has been run.

If no calliper is available, make a best guess of hole volume (suggest 2in - 3in enlargement), depending on local experience.

If a cement plug has to be set over perforations, 15 % extra excess should be used.

To ensure that uncontaminated cement is placed across the zone of interest, 50 ft excess, inside the open hole or casing, of cement slurry shall be pumped.

Plug lengths should be limited to 500 ft, although longer plugs can be accepted in special cases (e.g. the last plug of a series).

Slurry Requirements for Cements Plugs

To ensure cement hardening within reasonable time after being placed, the cementing recipes shall be formulated such that:

pumpable time = mixing time + displacing time + 60 to 120 minutes (depending on job total duration and slurry thickening time repeatability)

A fluid loss control additive shall be used to prevent dehydration of the cement slurry, and subsequent flash setting, when plugging back over very permeable formation, previous completion intervals, etc. Losses must be cured before plugging back.

Slurry samples to be put in hot water bath.

The use of thixotropic cement may be considered in loss zones.

Inside casing or across massive shale sections the cement slurry shall contain no other additives other than retarders and thinners, unless excess cement is to be circulated out across depleted formations, in which case, fluid loss control additives shall also be added