Achieving lasting cement jobs is demanding.

In order to handle tough downhole conditions, it's a given that the cement job has to be durable. It has to bond to metal and formation. It needs the appropriate dynamic slurry and zonal isolation properties as well as long-term compressive strengths and sheath integrity. Sometimes, however, cement sheath durability is a less obvious requirement. Nevertheless, the sheath must be capable of withstanding all manner of stresses and preventing its failure is the key to maintaining cement and production longevity.
As operators push for production in increasingly harsh environments, demands on zonal isolation operations are increased. Unconventional challenges are encountered when deepwater, high pressure/high temperature (HP/HT), deep gas, thermal or injection wells are planned. These tougher conditions involve higher temperatures and pressures that can potentially increase rates of cement isolation failure. More conventional stresses are generated through different stages of the well; impacts caused by perforation; variations in downhole pore pressures, future stimulation jobs (i.e.frac jobs), change of fluids. The cement job has to handle this.

Effectively isolating zones is a two step process, which involves cement placement and stress analysis. Firstly, the cement must be placed correctly. This means proper mud conditioning, correct spacer selection, proper centralization, pipe rotation and effective displacement. Secondly, consideration must be given to differing scenarios. This means changing temperature and pressure profiles, pressure testing, completions, stimulating and changes in wellbore conditions. For example, when mud is changed to completion fluid or injection water, fluid pressure differentials in the pipe can cause pipe contraction and result in an annulus between the cement and the pipe.

• It is well known that worldwide many wells lose annular integrity during their life cycle. This usually manifests itself in sustained annular casing pressure which is caused by a failure in zonal isolation. When the cement sheath in the annulus fails this provides a flow-path for gas or water to enter the annulus. Depending on the volume and chemical nature of the flow the well can be rendered unsafe or uneconomical through loss of zonal isolation or structural wellbore failure.

• In the case of smaller-hole geometries - between pipe and hole or pipe and pipe - thinner cement sheaths are more prone to failure when compared to thicker sheaths of cement. Consequently, it is important to understand completion/temperature cycling scenarios to avoid cement sheath failure and causing annular cracking. Where the cement sheath is damaged this can cause the well to become uneconomical as sheath damage results in non-productive costs. These are usually linked to different issues such as safety and environmental occurrences, remedial jobs or well loss, or early water production among others.

Halliburton's cementing specialist Alberto Valencia said, "a good cementing job will focus not only on dynamic slurry properties, compressive strengths, mud displacement but also on the cement sheath isolation capacity to ensure it does not fail in the later stages of the well lifecycle. We have developed technology that can determine whether a cement sheath has the necessary dimensions and physical properties to cope with the demands made of it over the life of the well. The 'Well-life' process uses advanced finite element analysis to characterize mechanical and other stresses that can affect the integrity of the cement sheath and consequently the cement job. We can model stresses that will be seen according to drilling, completion or intervention operations. We do this by breaking down the formation, casing and cement matrix into individual elements and characterizing their interaction over a range of scenarios."

Candidates for this type of analysis are wells that are subject to large changes in mud weight, pore pressure, temperature or formations. These types of wells are inherently riskier as the likelihood of cement sheath damage is high. Therefore, applications to improve well safety and economics exist in well such as HP/HT Deepwater Steam injection.

Valencia said, "the analysis we conduct considers well parameters, casing, cement sheath and formation properties. It evaluates the risk of damage to cement sheath from different scenarios and operations. This analysis in combination with operator's risk tolerance can be used to select the cement system that best provides for safe and economic production. Based on this approach we can project a well's economic value or even the additional benefit that the customer can realize using a fit for purpose cement system. The economic analysis takes into account the cost associated with remedial operations, loss of production due to inter-zonal communication, non-productive time and early water production. That means the customer can compare different solutions and judge their cost-effectiveness over the lifespan of a given well or field development."