Flexibility in slurry design
Don Weisinger
BP America
B.R. Reddy
Richard Vargo
Bob Sepulvado
Halliburton
Some of the highest risks encountered in deepwater cementing operations are related to low temperatures and shallow water flows (SWFs). These conditions present special challenges in the Gulf of Mexico.
For example, with normal temperatures in GoM operations, Class A or Class H Portland cements are generally preferred, whereas cementing in low-temperature zones usually involves specialized high early strength blends with set accelerators. SWF zones, on the other hand, often require a relatively low density and short transition time to help maintain well control.
A disadvantage of using a variety of cement types and blends in offshore environments can be that unused bulk-blended materials designed for a special application must be discarded or offloaded and new cement loaded for subsequent operations. This practice can increase material costs and require additional boat trips. It can be both economical and environmentally beneficial, therefore, to use the same neat cement for cementing all casing strings.
In four cementing operations in the GoM, Halliburton used cementing systems that meet all these design criteria for delivery of deepwater well cementing without jeopardizing the environment while increasing cost effectiveness. A foamed cement system using a neat cement and liquid additives handled the SWF control, and the remaining portions of the wells were cemented with the same neat cement using different liquid additives.
Challenges
In deepwater (greater than 1,000 ft) wells, temperatures are often as low as 40° F at the mudline and in excess of 200° F at total depth. Cement slurries at these low-temperatures require set accelerators to gain compressive strength without excessive rig delays or waiting-on-cement (WOC) time. With slurries on the higher end of this temperature range, cements need to be retarded to enable safe placement time. Compressive strengths at the end of WOC time need to be adequate to support the casing, but historically, compressive-strength development is slow for these single-cement-based slurries at mudline temperatures. To meet these various needs, the slurry design has required special blends capable of developing early strengths.
In the GoM, zones where the conductor casing is cemented are often unconsolidated and are geologically young. Consequently, these shallow formations have the potential for abnormally pressured saltwater sands, or SWF zones. Weak formations and pressured sands present narrow margins between the pore pressure and fracture gradient, which often can equate to a loss of cement returns.
The presence of SWFs introduces special challenges to drilling operations. Staying within tight drilling margins (as low as 0.2 to 0.5 lbm/gal) has proven difficult because of problems such as managing weighted mud, controlling equivalent circulating densities (ECDs), and drilling shallow overpressured formations with a riser. Since 1985, when SWFs and their disastrous results were first encountered in the GoM, operators have learned to successfully drill riserless through these zones using a variety of cementing systems.
Water migration from geopressured saltwater sand into annulus between surface hole and 20-in. conductor pipe presents special challenges in the Gulf of Mexico.
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Lightweight blend
Low temperatures present further complications with SWFs. A special cement blend composed of Portland cement, ultra-fine cement, and microspheres was first used successfully in 1992. This lightweight blend exhibited short transition time and would rapidly develop compressive strengths at low bottomhole temperatures (BHTs). In 1994, a special foamed-cement blend still utilizing the ultra-fine cement, but without the microspheres, was implemented as a solution to SWFs in the GoM. In 1997, large-scale model studies were performed where water-extended slurries, special nonfoamed slurries, and special foamed slurries were compared.
The studies concluded that foamed slurries were superior to nonfoamed slurries for dealing with SWF problems. Operators also began using another special foamed blend that incorporated Portland cement and calcium sulfate. By simply changing the amount of nitrogen injected, foamed cement slurries offered flexibility in slurry design, including last-minute density changes and gradual variations in density during the job. To date, several hundred foamed-cement jobs have been performed in the GoM to successfully control SWFs.
Additive technology innovation
Although successful, special SWF blends tend to be expensive. They require dry blending of the components and additives onshore before being transported to the offshore rig. Offshore logistics often require this blending to take place at least one week before the job. This process does not provide any flexibility for redesigning or modifying the slurry during the final few days when well conditions may require such changes. When changes are required, the unused blend has to be discarded. Additionally, rig space is often not available for storing many different dry blends that could be used to cement casings at different depths.
In one of four cementing operations in the GoM, innovative cementing systems met all of the design criteria for delivery of deepwater well cementing without jeopardizing the environment while increasing cost effectiveness.
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A recent innovation in additive technology can allow the simple base cement normally on the rig to be used in combination with only liquid additives and surfactant solutions to produce lightweight foamed cement (LFC). This system can provide a low-density slurry with short transition times to help prevent SWFs while maintaining zonal isolation, adequate placement time, and short WOC times. The system can simplify the cementing process by enabling operators to limit bulk-cement inventory to a single dry cement and improves the success rate by allowing modification of the cement design based on the latest information for each specific job by adjusting the slurry formulation onsite.
Laboratory results
Generally acknowledged requirements for SWF-preventative cement compositions include thickening times of 3-5 hours at 65° F (18° C), 24-hour compressive strengths of 400–500 psi at 45-55° F, and thixotropic characteristics. The LFC system, when formulated with any Class A, H, or G Portland cement, accomplishes these objectives. In regard to rheological character, Class H cement slurries with densities in the 16-16.6 lbm/gal range are shear-thinning at high shear rates. However, at low shear rates, they show thixotropic properties. The beneficial effects of the slurry's thixotropic nature were revealed by its uniform density distribution.
Transition time is the amount of time a cement slurry takes to thicken from liquid state, where it has the ability to transmit full theoretical hydrostatic pressure, to a highly gelled state, where it is now so thick that the gel strength alone will prevent the migration of fluid through the column. Transition times were determined by measuring the static gel-strength development times after simulating the slurry's placement time and placement conditions.
The slurry's gel-strength development was measured at downhole pressure and temperature under static conditions to simulate post-placement changes to slurry gel strength. With this test, the transition time (gel-strength increase from 100-500 lb-f/100 sq ft) is measured. Studies have shown that the migration of gas and water through a cement column will not occur at gel strengths above 500 l-bf/100 sq ft.
Another measure of transition time, measured under dynamic conditions, is based on the interpretation of a thickening-time test run on a high-temperature/high-pressure (HTHP) cement consistometer (often called the time to right-angle set). Values under 15-20 minutes for slurry consistency to increase from 70-100 Bc (Bearden units) are preferred to help prevent migration of fluids through the cement column. The interpretation of transition times measured under dynamic conditions is not recommended; nevertheless, results for the current slurry system show short transition times under static and dynamic conditions.
Field evaluations
Four cases from the Mississippi Canyon area of the GoM have demonstrated how unblended API Class H cement can be used as a base slurry to foam and protect against SWF. SWF zones were identified in this area, did flow while drilling, and were subsequently killed with mud. Casing was run and cemented, and no flow was encountered after the jobs. Risk assessment of the problem was high enough to justify running an SWF prevention cement system. By using unblended API Class H cement and liquid additives, a specially blended cement did not have to be run when an SWF was encountered. After the casings were cemented, the remaining casing strings were cemented with the same bulk cement and suitable liquid additives.
Cost savings
Use of a single-source cement on these deepwater wells in the GoM provided savings that may exceed $150,000 per well. Approximately $140,000 in cost savings can be realized with regard to materials when flow-control cements are required. The additional versatility of the liquid-additive system can prevent operators from making early decisions that could drastically impact the cost of the cement work for the SWF. The new technology used can eliminate the need for highly reactive API Class A and microfine cements and the problems associated with them.
Additional savings can be realized from the reduction in health, safety, and environment (HSE) events resulting from reduced bulk losses that could have exceeded 35%, but were reduced to less than 10% with the use of neat Portland cements. Savings also were seen in a reduction in rig and transportation vessel tank cleaning from using a single blend of cement, reduction in time needed to remove the cement from the tanks, reduction of the environmental impact caused by cement disposal, and reduction of human exposure to harmful dust particulates from tank cleaning. Although not calculated here, these HSE concerns can significantly reduce costs. As the government tightens restrictions on overboard disposal, these types of innovation that save time, money, and exposure to HSE incidents can become more important.
Use of this additive system can provide:
- A significant cost reduction in the development cycle of a well
- Elimination of multiple cements and less impact on the environment
- Flexibility in slurry design that can allow last-minute density changes and gradual variation in density during the job
- Elimination of the need for dry blending of materials onshore before transport to the offshore rig, reducing the need for additional rig space and extra boat runs
- A viable low-density slurry with short transition times that helps prevent SWF while maintaining zonal isolation, adequate placement time, and a shorter WOC time
- At low shear rates, the thixotropic properties are beneficial for the prevention of SWF
- Cost savings to the operator and improved environmental performance by reducing cement waste, reduced trips to offshore rigs, decreased need for multiple vessel cleaning, and exposure to cement dust. ;
Acknowledgements
The authors would like to thank David Kulakofsky of Halliburton for his comments.
