Choosing the best cementing solution for a crude oil or natural gas well is an essential part of achieving the optimal combination of well-construction efficiency, rate of production, and ultimate recovery.

But because the old petroleum industry axiom about no two oil and gas wells being alike is literal fact, no single base cement slurry can be modified to provide a competent cement sheath in all downhole environments. In fact, both the chances of choosing the wrong cementing system and the potential consequences of a poor choice have escalated because of the deeper, evermore complex, more challenging geological horizons being targeted for drilling and development, including some in deep water.

Oilfield cementing technology has kept pace with the escalating complexity of downhole environments by developing three basic categories of lightweight cementing systems to deal with the more variable temperatures and more dynamic pressures in deep wells and deepwater and ultra-deepwater wells: water-extended, microsphere, and foam cements.

Collectively, these three primary lightweight cements can be modified with performance enhancing additives to achieve varying levels of effectiveness and reliability in wells with severely depleted zones or zones with low fracture gradients, lost circulation problems, or shallow water flow.

Water-extended lightweight slurries typically are recommended whenever cost of cementing is the primary concern and downhole conditions not too demanding.

If creating a lightweight cement sheath with high strength or low permeability are the main drivers, microsphere slurries usually are the best choice. Foam slurries are recommended when the potential is high for gas migration or shallow water flow, or when longterm stress resistance is desired.

Determining the best cementing solution
Designing the optimal lightweight cement for a given application is not easy even in the best circumstances, both because of the variability of conditions encountered downhole in more challenging wells as well as the many additives and methods that can be used to manipulate a cement's properties to enhance its performance characteristics.

Though surprising to consider, the best cementing solution might not even be considered in some situations. Oilfield service companies that can't provide all three basic lightweight cementing solutions typically recommend the best option they can provide; it just might not be the optimal cement for the application at hand. Some companies base their cementing decisions upon subjective factors, such as personal preferences or local practices.

Halliburton is the only oilfield service company capable of providing water-extended, microsphere, and foam base cements in all of its locations around the world. Moreover, it can tailor these primary lightweight cements with specially developed additives and highly engineered methods of application to offer the petroleum industry's most complete portfolio of lightweight cementing solutions.

Within the past few years, the company has brought together an array of lightweight cementing assets—including its complete line of primary lightweight cements and additives, the latest in slurry design software, and an extremely accurate and efficient cement-delivery system-to create an entirely new oilfield cementing concept: Tuned Cementing Solutions™ process.

Tuned Cementing Solutions process recognizes the inherent complexity of every well and bases slurry designs on a combination of well conditions and job objectives, including potential hazards, cost, production objectives, and longterm zonal isolation goals.

Tuned Cementing Solutions process selection tool
The heart of the Tuned Cementing Solutions process is the new selection tool, an easy-to-use questionnaire that tunes lightweight slurry designs based upon a user's responses to a series of multiple choice or yes-no questions about well conditions, logistics, and job objectives. The application is web-based and can be run on any computer with access to the internet and the proper security clearance.

A user simply clicks on the answers to the questions and submits a completed questionnaire to get an answer in a matter of seconds. That recommendation may be analyzed and individual selection criteria changed based upon expectations of contingencies that could come up during the job, to get a second recommendation.

The selection tool software can enable a user to create and analyze several scenarios, taking into account all the possible contingencies, in a few minutes and come up with the best lightweight cementing solution based upon specific well conditions and producer objectives. Since Halliburton can provide a water-extended, microsphere, or foam base slurry, it can design a lightweight cementing solution with the optimal characteristics for providing a competent cement sheath in essentially any well.

The selection tool methodology takes into account and models the entire sequence of events at the well site, including drilling, cement hydration, well completion and production, to design a cement sheath optimized for the expected stresses. Conventional slurry design practices do not take into account such factors as the hydrostatic changes that occur when drilling mud is exchanged for completion fluid or during pressure testing, stimulation or production cycles.

Advances of lightweight cementing technology
Halliburton has been creating lightweight cements for many decades, beginning with water-extended slurries for relatively low-pressure, straightforward applications.

The company has been designing slurries containing lightweight solids since the late 1970s, and in the early 1990s introduced a beaded lightweight cement with a short transition time and good compressive strength known as Deepwater Flow Stop® 1 cement, specifically to deal with shallow water flow in wells in the Gulf of Mexico.

The technology underlying Deepwater Flow Stop 1 cement diverged along two evolutionary paths during the 1990s, one involving advances in microsphere slurries and the other advances in foamed cement formulations.

Advances in microsphere technology culminated about the turn of the century in so-called Tuned Light® cement, which can provide better compressive strength development more quickly than any other lightweight cementing system. Available in premium cementing formulations as light as 6.7 ppg, Tuned Light cement is recommended in situations where a high-performance lightweight cement is required and key objectives include minimizing the amount of equipment needed on location, maximizing the strength-to-weight ratio of the cement sheath, or minimizing the permeability of the sheath.

High strength, low density Tuned Light cement can reduce lost returns effectively in downhole pressures as great as 25,000 psi, and can be tuned to provide designs for high mechanical strength to prevent gas migration and to handle shallow water flow or sustained casing pressure.

Lightweight foamed cement advances
Halliburton began marketing a new foamed lightweight cement with the introduction of Deepwater Flow Stop® 2 cement, in which foam was substituted for beaded lightening agents. The evolution of Deepwater Flow Stop lightweight cements continued throughout the 1990s and into the New Millennium, culminating about two years ago with Deepwater Flow Stop® 5000 cement.

In Deepwater Flow Stop 5000 cement, Halliburton has replaced the complicated cement blend with a liquid additive. This strategy enables Deepwater Flow Stop 5000 cement to be delivered to the well site in pure form. If excessive problems are not encountered, neat cement can be foamed on location to achieve the appropriate density, to create a relatively inexpensive cementing solution. If shallow water flow zones are encountered, the required slurry can be created using the more costly liquid additive to achieve the desired cement properties only when needed.

About six years ago, Halliburton introduced the ZoneSeal® isolation process, which is the premier process for delivery of foamed cement. A highly elastic cementing solution, ZoneSeal process helps improve mud displacement while helping prevent gas migration and shallow water flow. ZoneSeal® cement slurry also can be modified on-the-fly to alter density, if required by evolving job conditions.

For a foam cement job to be considered delivered using the ZoneSeal isolation process, Halliburton requires the job to be designed using OptiCem™ software, to be sure nitrogen is being added at the right concentrations and the right cement volumes. In addition, automated nitrogen and surfactant injection systems must be used on the job, under direction of a certified specialist for the ZoneSeal process, supported by radio-equipped technicians manning the various pieces of equipment required at the well site.

Tuning cement for the best results
Halliburton has conducted more than 900 successful tuned cementing jobs around the world, targeting wells on every continent, both onshore and off. Job sites have been located in Australia, Brazil, Japan, Kazakhstan, the Mediterranean, Mexico, Middle East, Norway, North Africa, Scotland, the U.S. Gulf of Mexico, Trinidad, and West Africa.

Because the essence of the Tuned Cementing Solutions process is to determine the best lightweight cementing solution for any given well, a properly tuned cement can be based upon a water-extended, microsphere or foam base slurry. The objectivity of the tuned cementing selection tool prevents subjective preference of any base slurry, which means the costs of the effective options can drive the ultimate recommendation of a lightweight cementing system for a given job.

Cost comparisons show water-extended slurries are the lowest cost lightweight cementing option, regardless of job size. Yet water-extended slurries lack performance characteristics for all but the most basic applications.

Among premium, high-performance lightweight cements, microsphere slurries normally are less costly than foamed cements for small cementing jobs. Yet the cost of microsphere cementing solutions can escalate faster than foamed counterparts, because of the relatively high cost of beads. So on bigger jobs with high-performance cement, a foamed solution in most cases will be the more cost efficient choice.