Premium lightweight cement systems are routinely used in a variety of specialized applications in low fracture gradient environments where specific design performance parameters must be met. These applications include shallow water flow
mitigation, annular gas control and high induced-stress states.

Two types of lightweight cement designs — foamed cement and high-performance

Figure 1. An automated foam cementing system assured precise control of nitrogen and liquid additive injection rates during a foamed cementing operation in the Caspian Sea. Here, the automated nitrogen unit is rigged up and ready for action. (Photo courtesy of BJ Services)
sphere- containing cement — have proven successful in controlling fluid influx in applications with low, medium and high flow potential. Both foamed cement and high- performance sphere- containing cement have demonstrated success in maintaining zonal isolation under high induced wellbore stress conditions.

A third technology, based on a modified extended lightweight design, has been successfully deployed in environments with low to medium flow potential.

Foamed cement
The process of foaming well cement entails injecting a known quantity of nitrogen gas under pressure into a given volume of base cement containing a foaming agent and stabilizer. Foamed cements are normally designed with a gas fraction (foam quality) of 15% to 30%. This allows the density of foamed cement to be up to 4.0 lbm/gal less than the base cement being foamed.

Foamed cement, although used in onshore operations for many decades, came into prominence for use in mitigating shallowwater flow in the early 1990s. Specialized process control and metering (largely borrowed from foam hydraulic fracturing operations) are used to ensure the design criteria are met during mixing and placing the slurry.

Foam cements maintain an internal pressure, which counteracts the loss of volume as the slurry undergoes the transition between a liquid and set state. The compressible nature of foamed cement allows it to offset the hydrostatic pressure loss that initiates water or gas flow. Foamed cement is also mechanically more resilient than the Portland cement from which it is generated.

Foamed cement was recently employed to curtail a severe shallow water flow in a high-value project in the Caspian Sea. In this particular case, the operation was conducted on a production platform situated in shallow water, less than 500-ft (152.5 m) water depth, with a production riser in place.

Normally, foamed cement could not be employed in this shallow marine environment because without the benefit of a large water column (and its associated pressure contribution), small changes in wellbore pressure (at low pressure) produce large variations in foam density. Also, due to the riser being in place, contingency planning required the possibility of energized fluids being returned to surface be addressed.

An automated process foam cementing system (AFCS) was used to ensure precise control of the nitrogen and liquid additive injection rates. An 11.9-lbm/gal foamed lead cement (DeepSet cement) with specific performance characteristics was placed across a shallow water flow interval whose pore pressure was measured at 10.9 lbm/gal. The flow was successfully mitigated and the well drilled without incident.

In another application, foamed cement was used to minimize the occurrence of sustained casing pressure in a high-deliverability gas field offshore Middle East. Foamed cement was chosen due to the high occurrence of sustained casing pressure in previously cemented wells. The mechanical resistance to sustained casing pressure provided by the foamed cement has been demonstrated in that none of these high-pressure gas wells have exhibited sustained casing pressure after 10 years of production.

Sphere-containing cement
High-performance, sphere-containing lightweight systems have been used in well-cementing applications since the mid-1980s. These systems are based on low specific gravity hollow pozzolan spheres (0.75 sg) or borosilicate glass spheres (0.35-0.45 sg). Incorporating these spheres into the slurry allows density reduction without substantially increasing the water/solids ratio of the design.

In the late 1990s, designs incorporating lightweight spheres for density reduction and meeting the same flow-mitigating performance characteristics of foamed systems were introduced. The hollow pozzolan spheres can be used to produce high-performance lightweight designs at a practical minimum density of ±9 lbm/gal. The borosilicate glass spheres are used at a density as low as ±7.0 lbm/gal.

A typical example of the use of high-performance sphere-containing lightweight cement can be found offshore West Africa. In this deepwater environment, the operator faces several challenges, including a very low fracture gradient and a low wellbore temperature. For this application, the gas tight slurries must achieve sufficient compressive strength to allow drilling operations to commence within 24 hours.

Furthermore, because the wellbore’s hydrostatic pressure is higher than the collapse rating of hollow pozzolan spheres, the high-performance borosilicate sphere–based lightweight LiteSet 7 cement was employed. The specific composition of the borosilicate sphere-based system was determined using the newly developed LiteView software program that optimizes the liquid/solids ratio of the design.

Low fracture gradients were found in both the top-hole section and deeper in the well bore. At the productive intervals, the cement sheath was expected not only to support the casing and provide zonal isolation, but also to withstand the stresses encountered during the completion process, including frac-packing.

The induced pressure expected during the frac packs and other wellbore information were used as inputs into a proprietary stress model to predict the radial and tangential stress imparted on the cement sheath. The simulator predicted a compressive strength requirement on the order of 2,200 to 2,400 psi and a tensile strength of approximately 200 psi. Wellbore hydraulic simulation predicted that the highest cement density that can be pumped while maintaining the equivalent circulating density below the formation fracture pressure was 10.5 lbm/gal.

In this application, the high-performance borosilicate sphere-based lightweight cement provided the operator flexibility in establishing and maintaining isolation in a low-pressure environment, which allowed the well to be completed as intended.

Modified lightweight cement
A third lightweight system, recently introduced in areas with low to moderate shallowwater flow potential, is a modified conventionally extended system that meets the same flow-mitigating criteria (limited critical static gel strength period, low fluid loss, and the ability to set at low curing temperatures) as foamed cement and high-performance sphere-containing lightweight systems. The newer system can be applied to non-critical areas of the wellbore where zonal isolation requirements are not stringent and in intervals with critical flow potential. The density range of this system is 11.5 lbm/gal to 13.5 lbm/gal.

This system has been used successfully in deepwater as a lead slurry for surface and conductor strings in the Gulf of Mexico. Nearly a dozen deepwater operators have had successful results since the introduction of this technology in mid 2006. The system has proven to be a reliable and economic substitute for foamed and high-performance sphere designs in areas with low to moderate flow potential.

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