A novel concept utilizes field case histories to match solutions to well conditions.

The new lightweight cementing solutions (LCS) concept takes advantage of lessons learned from case histories. Rather than being a one-size-fits-all approach, the system includes a broad range of solutions capable of fitting the specific requirements of any well. LCS can be broken down into all three primary groups (or categories) of lightweight cement slurries:

Group I: Conventional water extended systems

• Gel
• Sodium silicate
• Fumed silica
Group II: Hollow microsphere systems
• Pozzolan
• Borosilicate
Group III: Foamed cement systems
• Nitrogen
• In-situ gas generators
• Compressed air

Using the LCS system, the solution is tailored to meet the needs of specific well performance requirements and expected life cycle requirements. Because no two oil or gas wells are alike, there is a wide variety of specific well requirements, including cost.

When cost or low ultimate production is the main driver, solutions from Group I provide economical results. The density of these slurries is reduced with the addition of extra water. Water is normally free or obtainable at a relatively low price, making this class of slurries the most economical, at least in the short run. Since compressive strength and permeability are directly related to the water ratio, these slurries traditionally have low strength and high permeabilities. The lightweight additives used in this category do not reduce the density themselves, but rather tie up the extra water that was added to reduce the density, providing slurry stability. These slurries make good filler cements, but perform poorly when confronted with gas migration, shallow water flows, or when used for the prevention of sustained casing pressure.
Group II additives can have densities as low as 3 lb/gal (0.32 sg). With the addition of hollow microspheres, lightweight slurries from this category can be designed with a minimum amount of water. By minimizing the water content, these slurries can be designed with higher compressive strengths and lower permeabilities than other lightweight cements. When well conditions are such that superior compressive strengths or minimal permeabilities are required, high solids, hollow microsphere slurries can provide the optimal solution.

Traditional foamed cement or cement made compressible with any of the Group III technologies can have several desirable characteristics, including:

• Compressibility, that minimizes the effect of hydration and fluid loss volume reduction thus helping to maintain full hydrostatic pressure during the transition time.
• Lower Young's Modulus than conventional cements. A low Young's Modulus allows foam cement to absorb more stresses without cracking.

If avoiding gas migration, shallow water flow, sustained casing pressure, or stress cracks in the cement sheath are primary drivers in slurry selection for a given string of pipe, optimal tuning of the cement slurry design can yield foamed cement for that purpose.

For slurries to be properly tuned, well life considerations should play a role. In the first development phase of Shell's Shearwater field, all but one well exhibited annular pressure. As a result, rigorous design methods were developed to evaluate the integrity of the cement sheath using finite element analysis. With this methodology, the entire sequence of events was modeled, including the drilling phase, cement hydration, well completion and production. The result of this analysis was a cement sheath optimized for the expected stresses. Traditional slurry design does not take into account factors such as the hydrostatic change when the mud is swapped for the completion fluid, pressure testing, or stimulating and production cycles. Thus, an initially successful cement job may not actually provide long-term zonal isolation.

A recent study showed that of the 15,773 active wells in the Gulf of Mexico, 5,620 currently have casing pressure in at least one annulus. Another study showed that in excess of US $150 million was spent in both 1998 and 1999 remediating shallow water flow problems. The study also showed that 60% of Gulf of Mexico wells older than 30 years suffered from this annular pressure problem. In addition, each of the past 5 years has shown a reduction in casing pressure problems. When tuning in the correct solution for your well, ignoring the long-term aspects of zonal isolation can be a costly mistake.

New Design Tool

As mentioned, different well conditions require different solutions. To reduce the learning curve and help provide the capability to properly tune slurry recommendations from the appropriate LCS category, a simple software package was developed consisting of a series of simple questions that drive the new design tool for any given set of conditions. The answers to the questions are compared to the historical database in decision-tree logic to arrive at the best recommendation for the well under consideration. The application is Web-based and can be run on any computer with an Internet connection and proper security clearance. Based on well conditions, logistics and key concerns during the cementing job, all possible solutions pass through a tuning process and are presented in an easy-to-read format.

Any solution choice that makes it through the tuning process is prioritized into one of three levels of recommendation:

• Recommended Design
• Primary Alternative(s)
• Other Choices

When an unrealistic request is made, no choice is displayed. When no choice is presented, at least one of the constraints requires modification to receive a recommendation. In all other cases, there will be at least one suggestion for the recommended design. Slurries designed from this category are intended to provide the best solution for the set of well conditions entered. Sometimes one or more primary alternatives will be offered. If offered, solutions from this category are good choices for the long-term zonal isolation of the well. When other choices are offered, they may be confidently recommended, but will not be the best solution under the given constraints.

Case Histories

In the Ship Shoal field (OCS-G-1028), Dominion Oil Co. cut a window in its F-18 well at 2,840 ft (866 m) measured depth (MD) in its 75/8-in. casing to drill a 71/2-in. sidetrack hole section from ENSCO 87. Plans for this hole section called for a 51/2-in. production liner to be set 8,096 ft (2,469 m) MD (5,264 ft or 1,605 m total vertical depth). Because this was to be the production string, top quality cement was required. The formation integrity test (FIT) was 11.6 lb/gal. With a mud weight of 10.2 lb/gal, the maximum cement density was set at 12 lb/gal.

To tune in the correct lightweight cementing solution, in addition to the 12-lb/gal density limitation, the solution also required production-quality cement able to achieve rapid compressive strength development with a bottomhole static temperature (BHST) of only 143°F (61°C). Tuning the slurry for this well's conditions and requirements yields a Group II recommendation. By building the 12-lb/gal slurry with hollow microspheres, a slurry was created that achieved a compressive strength of 1,540 psi in just 12 hours at the specified BHST and was capable of providing the required feet of fill as well as yielding a good bond log. During the job, equivalent circulating density (ECD) was continuously calculated and compared to design data to help ensure that at all times pumping was conducted at a safe circulating pressure.

The job data for this operation can be viewed in Figure 1. The completion engineer remarked that for the Ship Shoal 247, F-18 ST, he was very pleased with the results of the primary cement job. He believed that very good zonal isolation was established across all of the objectives and that the bond log resembled a "textbook example" of a very good cement bond, i.e., no channeling, good formation arrivals, etc. "But the real proof is in the production," he said. "The F-18 ST has been producing the 4,700-ft (1,433-m) sand at 2,000 MMcfg/d with zero water production since March 2003. The 4,700-ft (1,433-m) sand consists of 10 ft (3 m) of gas on top of 50 ft (15 m) of water," he explained. This production was achieved with a 12-lb/gal cement. When high strength and light weight are both needed, hollow microspheres are recommended.
In a different section of the Gulf of Mexico (OCS-G-19996), Dominion was drilling its Devils Tower project. Again, the company faced low fracture gradients while drilling 30-in. and 24-in. hole sections. To further complicate the low fracture gradient situation, these wells were being drilled in a known shallow water flow (SWF) area. Dominion's own subsurface surveys confirmed that the potential for SWF was present at this site. For the upper strings in the Devils Tower prospect, the tuning process from our lightweight cementing solutions indicated that the compressible attributes of foamed cement would be the ideal solution for this lightweight situation. All six 26-in. and all six 20-in. casings (12 total) were successfully cemented through these low-fracture gradient, SWF zones with no evidence of flow to the sea floor.
The Devils Tower 103/4-in. x 97/8-in. production casing required a low-density cement flexible enough to handle life-of-the-well stresses. Tuning the design for the #9 well resulted in the well being cemented with a foamed-cement slurry. On well #9, the surface density was 17.7 ppg when nitrogen was injected into the slurry. Nitrogen was injected at rates to yield a downhole density of 14 ppg. For foam to be used successfully in an oil or gas well, the bubbles should be discrete and well dispersed. When foamed correctly, the result is a stable cement with a lower Young's Modulus that can provide an increased level of stress protection. A stress-protected cement can provide the well with long-term zonal isolation. The Young's Modulus of conventional Class H cement is in the range of 1,200,000 psi. The foam from this job was around 650,000 psi.

The rest of the production casings in these Devils Tower wells were foamed using an in-situ foam generator. The creation of gas in-situ can provide slurry expansion in the plastic state. This process counteracts hydration volume reduction and fluid loss volume reduction that contribute to the loss of hydrostatic pressure during the transition time. If pressure can be maintained during the transition time, at or near the theoretical hydrostatic pressure, gas migration problems can be avoided. This plastic state expansion can also provide outward forces, resulting in improved slurry-to-casing and slurry-to-formation bonding.

Summary

The following recommendations are provided to tune lightweight cement slurries.
• Consider choices from all three categories.
• Determine the ultimate properties actually required from the slurry.
• Determine the obstacles that must be overcome to successfully place the slurry.
• Determine the stress that will be placed on the cement sheath during the life of the well.
• Take into account relevant logistical issues.
• Consider whether the low price of filler slurry or more expensive premium slurry will add more value to the ultimate production of the reservoir.

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