Design for operational events throughout well life maintains zonal isolation

Preliminary results for varied temperature and pressure-loading conditions suggest that operators should analyze wells on an individual basis and apply forward-looking design techniques when selecting an annular sealant to meet the operational requirements.

Laboratory analysis and limited field results indicate that finite-element analysis can improve sealant design. Initial studies indicate a strong interaction between the long-term effectiveness of the sealant and its sheath properties, the formation properties, the sealant in situ stresses, completion and stimulation operations, and other events.

Particularly for a high-risk well, sealant design and placement may now include a study of operational events throughout the expected productive life of the well that may subject the sealant to significant changes in temperature or pressure.

Design limitations

Current drilling environments present many unique challenges such as deepwater, high pressure, high temperature, and completions in unconsolidated formations. These challenges require engineers to employ new practices to achieve their drilling goals.

Operators should design annular sealants for use in these wells to accommodate loads such as changes in pressure and temperature occurring during operational events.

Naturally developing stresses (subsidence and depletion), operational stresses (production and gas storage), and human interventions (injection, testing, perforating, sidetracking, and changing fluids) are events that exert these loads.

Zonal isolation is essential to the long-term success of a well. The US Department of the Interior's Materials Management Service (MMS) reports that operators observe sustained casing pressure (SCP) in more than 11,000 casing strings in more than 8,000 wells on the Gulf of Mexico's Outer Continental Shelf.¹

Operators may attribute these problems to incomplete annular sealant placement, sealant cracking, or debonding.

Casing collapse, sustained casing pressure, early-water production, interzonal communication, or annular-fluid flow after displacing the wellbore to a lighter fluid following sealant placement may indicate sealant failure.

These problems can cause safety or environmental concerns and can result in reserves loss. Such problems can be costly and difficult or impossible to repair.

As drilling objectives become more challenging and require new drilling practices, traditional cementing practices may no longer prove sufficient.

Casing design practices often consider pressure or temperature changes that are possible during well operations. Operators should apply a similar design approach to annular sealant selection and design.

Mechanical properties

Focused on the slurry, traditional annular sealant design procedures consider thickening time, rheology, density, and gas migration phenomena that occur soon after crews have placed the sealant in the well.

Frequently, engineers consider a sealant with high compressive strength an appropriate choice for zonal isolation.

The compressive strength of a sealant, however, is not a sufficient criterion to determine zonal-isolation success. Recent field experience indicates that other mechanical properties of the annular sealant can be critical to a well's success.

Several documented industry studies indicate cement performance is a function of Young's modulus and other mechanical properties.^2-4

A preliminary study indicates that a rigorous technique based on finite-element analysis, combined with methods to improve sealant placement, can significantly increase the likelihood that the annular sealant will withstand the rigors throughout the life of the well.

Engineers can improve annular sealant designs by analyzing anticipated events and quantifying how these events will impact the cement sheath.

Long-term zonal isolation

To help ensure zonal isolation, operators should consider the following factors:

Cementing operations should make every effort to achieve 360° cement sheath coverage in the annulus. Leaving portions of the pipe exposed may make the sealant's mechanical properties a moot point.

Engineers should optimize the properties and formulation of the set sealant, to withstand the various operations occurring during the well's life.

Cement placement

Many factors contribute to the success or failure of cement placement. The wellbore stability, extent of hole washouts, hole cleaning, drill-cuttings removal, equivalent-circulating density (ECD), centralizer placement, and treatment with flushes or spacers can all impact the cement placement.

A software-based mathematical model, known as Erodibility, has proven to be a valuable tool for achieving effective cement placement.⁵

A wellbore cementing analysis software that includes the model, which Houston-based Halliburton Co. calls OptiCem, simulates mud removal during the cementing process.

Optimize sealant properties

Goodwin, et al. discussed experimental studies to investigate the effect of operational events on the wellbore annular sealant and Bosma, et al. developed a mathematical model to describe the problem.^{2 4}

The model takes into account the operational events from the time the cement is pumped into the annulus.

Using the finite-element analysis technique to find the solution to the mathematical problem, Bosma, et al. employed a method of solving complex engineering issues that adapted readily to annular-sealant design.

To use this model to design an annular sealant, engineers must have the following information:

• Rock properties.
• Cement slurry and sheath properties.
• Casing properties.
• Operational details for completion, stimulation, production, and injection.

When using finite-element analysis to simulate the loading of the well, it is important to determine the borehole condition, sealant-hydration characteristics, and the resultant in situ stresses in the sealant.

Preliminary modeling indicates the in situ stress condition in the sealant affects the ability of sealant to provide zonal isolation during the life of the well.

In situ stresses in the sealant depend on the condition after cement curing and on borehole stability. The other important factors for estimating sealant integrity are the casing-cement and cement-rock interfaces.⁶

Sealant failure can occur three ways:

1. A microannulus forms at the casing-cement interface.
2. A microannulus forms at the cement-rock interface.
3. The sealant sheath cracks due to changes in temperature or pressure.

Sealant failure by microannuli forming at the casing-cement or cement-rock interfaces might occur when the drilling operations displace the wellbore to a lighter density fluid inside the casing, resulting in a negative pressure change within the casing. The pressure decrease inside the casing may debond the sealant from the casing or formation, leaving a flow channel.