In a multilateral intelligent well completion from the Middle East, wellable CATs were cemented in place for additional assurance of lateral isolation in the heel section while the toe section of each well was produced first. (Figure courtesy of Halliburton)

It has been estimated that 25% to 30% of all wells may be affected by some type of annular pressure problem indicating lack of zonal isolation. Even in wells that do not exhibit sustained casing pressure (SCP), cross flow from higher-pressure zones to lower-pressure zones can create localized loss of zonal isolation that may not be evident as annular pressure at surface.

WellLife service

There are field-proven mechanisms that operators can use in combination to effectively minimize inter-zonal communication throughout the life of the well. Comprising the WellLife III service, these mechanisms include modeling and analysis for an engineered cement system design. Tuned cement systems for a cement sheath with an auto-seal feature and cement assurance tool (CAT) with a swelling packer element in the casing/liner string.

Modeling and analysis

Modeling and analysis account for the impact of initial geophysics and drilling operations on the formation as well as modeling the planned completion and the resulting impact of any subsequent stimulation, production and injection operations on cement sheath integrity.

Properly analyzing the effects of well operations on the durability of the cement sheath requires many input parameters, including such data as the physical properties and thermal properties of casing, cement and formation, the well design, and formation in-situ stresses. Specific well events like pressure testing, fracturing operations and temperature and pressure cycles are then modeled to determine their effect on cement sheath integrity.

Results include both the condition of the cement sheath after it is subjected to the various well operations and its remaining “useful capacity,” an estimate of how close the cement sheath is to its elastic limit (the point of failure). Useful capacity also can be considered a measure of the safety factor. If the value is larger, the probability is greater that the cement sheath will withstand predicted cyclic well operations. Based on these results, the cement sheath is then engineered to possess the necessary mechanical properties required to withstand the events without failure.

Design and test

The cement slurry is designed and tested to meet the conventional properties such as rheology, stability, pumping time and fluid loss. In addition, the cement slurry formulation is optimized so that the cement sheath mechanical properties meet or exceed the requirements as per the analysis. The cement sheath is designed and tested to withstand cyclic loads.

Tuned cement system

Even when a carefully engineered cement sheath has been designed and deployed for a specific well to withstand predicted events, there remains the possibility that unpredicted events may exceed the design parameters and cause damage to the cement sheath. Therefore the second mechanism, developed to help provide an extra level of zonal isolation assurance, is an “intelligent” solution in the form of cement sheath with the capability to react and respond in the event of damage to the cement sheath.

Consider a deepwater well in about 4,845 ft (1,477.7 m) of water, with a reservoir temperature exceeding 300°F (138°C). The temperature of the hydrocarbons flowing through tubulars is therefore about 300°F, while the seabed temperature is 45°F (13*C), so the cement sheath will be subject to a temperature increase of 255°F (123 m) when the well is put on production.

The challenge in this case is to design a cement system that will resist both an influx of any shallow water/gas flow as soon as the slurry is pumped, and will withstand the increase in temperature when the well is put on production.

The intelligent solution is in the form of slurry design that incorporates proprietary cement additives that remain chemically inert under normal circumstances, but react to an influx of formation fluid by swelling, automatically sealing the annulus without application of external intervention.

The cement sheaths are engineered with react and respond (RAR) capabilities to close micro-annuli or sheath cracks on the order of 100 to 250 microns, effectively eliminating flow pathways caused by cracking, debonding, or shear as a result of unplanned events like over-pressuring, unanticipated formation subsidence, and tectonic activity.

Since 2005, auto-sealing cements have been successfully deployed in more than 70 primary cementations onshore and offshore using conventional cementing equipment, with flowing temperatures ranging from 130°F (39°C) up to 500°F (152.5*C) . No inter-zonal communication has reported to date.

CAT

The third key mechanism is swelling-element tools run as integral parts of the casing string. No special handling, running precautions or actuating procedures are required, nor is a specialist required on location, so standard casing-running procedures can be used. The swellable CATs are simply placed in the casing at key intervals to separate formations of different pressures, and in locations where poor casing standoff is likely to occur.

Attached to the outside of the casing before it is run in the well and activated by in-situ resources like downhole fluids, the CAT swells to as much as twice its original size, filling in uncemented areas, incongruities, channels or micro-annuli to help re-establish the hydraulic seal without any manipulation or intervention from the surface.

Deploy, measure, optimize

With a system designed based on modeled analysis of the completion application, the WellLife III service work flow helps ensure cement slurry and CAT deployment is part of an ongoing continuous improvement cycle. Real-time measurements allow optimization of parameters during operations, and provide data for “lessons-learned” that contribute to ongoing improvement in solution design.

In a multilateral application in Asia Pacific, the horizontal wells were constructed such that the toe and lower portion of each well could be isolated in the event the gas cap should cone into the well and water out the zone. The completion design included production packers run on tubing to compartmentalize the reservoir, and a sliding sleeve between the packers for production control and reservoir management.

Due to concerns about potential channeling as a result of the tendency of casing to lie on the low side of the hole in a horizontal configuration, the service design incorporated use of two swellable CATs run as part of each liner string and cemented in the wells. Pre-positioning the CATs in combination with the primary cementing job would provide the operator the best opportunity to maximize reservoir performance.

During deployment, typical running speed was 100 ft/min (30.5 m/hr), and the CAT did not significantly contribute to pick-up or slack-off values. Once on bottom, with 50 rpm rotation and full circulation, the liner hanger was set and the primary cement job performed as usual, with no reported losses. The plug was bumped, and excess cement was circulated out of the well.

After perforation, the two production packers were landed in the liner at approximately the same depth as the swellable CAT, with the sliding sleeve assembly between packers providing control of the heel segment of the well as the toe portion of the well was produced first. The well was completed and brought on line as designed, with initial production of 100% oil.

By pre-positioning the CAT during the primary cementing job, the service approach helps ensure that reservoir performance will be maximized, even though the gas cap may invade the well bore as the well continues to produce.

Unlike costly workovers and squeeze cementing, or novel, unproven remediation techniques, the service addresses cement integrity before problems develop. Deployed individually and in combination, these technologies are proven to provide an effective means of preserving cement sheath integrity, even in wells prone to cement-sheath failure.

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