Micro-annuli - Causes, prevention and cure

Micro-annuli is a very small gap between casing or liner and cement sheath that can be created in primary cementing operations. Micro-annuli could run along the pipe or formation and connect the reservoir all the way to the top of cement and compromise well integrity. Primary cementing operations include circulating to condition the well and mud prior to displacing with cement slurry, bumping the plug, and leaving cement in the well for certain hours to set. Once the cement is set, the casing is cleaned out and pressure tested to ensure casing integrity. During the whole process and the operations after cementing, the well goes through several temperatures and pressure variations, which can cause develop micro-annuli.

It's always recommended to circulate and condition very well before displacing cement. It helps in the removal of cuttings from the annulus and stabilizes downhole temperature in the well. Rheological simulation carried out in the lab considers wellbore temperature to design the slurry with an appropriate concentration of additive chemicals. During the cement job and during the setting process, the slurry goes through several thermal and pressure variations which could cause cement shrinkage or cracking, which could result in the creation of microcannulas. After ‘Wait On Cement’ (WOC) time, a clean-out assembly is run to clean out the cement inside the shoe track. The casing is also pressure tested to ensure that there is no leak in the casing string that could jeopardize the integrity and future operations in the well. However, pressurizing casing after the cement is set, could cause ballooning of the pipe, exert additional stress on the cement behind the pipe and cause minute cracks. Similarly, if the well is displaced to lighter fluid for the next phase of operations in well, it causes reverse ballooning of pipe causing contraction, but since the cement is already set, this could result in minute gaps between pipe and cement sheath.

Some of the steps that can be taken to avoid Micro-annuli are part of the cementing best practices:

·        Proper centralization of the casing string to achieve the desired stand-off. Proper centralization helps in placing the cement slurry all around the pipe rather than leaving the mud on the low side.

·        Properly designed spacer volume helps in creating good separation between mud and cement slurry, avoiding cement contamination. Appropriate selection and composition of spacer ensure removing any thin mud film that might be sticking to pipes, especially in oil-base mud. If a film is left on the pipe surface, it could result in poor cement bond and develop Micro-annuli.

·        Good hole cleaning and conditioning of well prior to displacing with cement ensure that pressure regime remains within predictable range and all additives work appropriately to achieve desired results.

·        Designing future operations in the well to minimize pressure shocks created by ballooning and contraction of casing pipes. Evaluate the possibility of a green cement pressure test rather than testing the casing after WOC.

·        Using anti-gas leak additives like latex could help minimize or prevent gas communication through the channel.

·        The new technology of self-sealing cement could help in ensuring well integrity in the short as well as long term. The cement additives of self-sealing cement react to hydrocarbons and repair cement ensuring hydraulic sealing.

·        Use External Casing Packer (ECP) to create an annular barrier to any gas migration.

After completing the WOC period, the casing is cleaned out and pressure tested as required by the plan, and evaluation of the cement bond is carried out through sonic and ultra-sonic logging tools. Micro-annuli can be detected by cement evaluation logging tools. However, it may be very hard to repair it with secondary cementing. Once the micro-annuli segment of the well is identified, there are a few remedies that can be tried that must be extensively planned and risk assessed with all stakeholders.

·        Perforate the casing to create pathways. Set a cement retainer above perforations.

·        Try to squeeze cement slurry with ultra-fine cement particles. In most cases, it might be hard to get even this slurry into the Micro-annuli. Some gaps might be filled but many areas might be left.

·        Alternatively, squeeze low-viscosity resin material through the perforation. This particle fee resin is a more viable choice for sealing off micro-annuli as it flows like water and sets after left in place.