MUD TO CEMENT TECHNOLOGY PROVEN IN OFFSHORE DRILLING PROJECT

Feb. 16, 1993
Kazem Javanmardi, Ken D. Flodberg Shell Offshore Inc. New Orleans James J. Nahm Shell Development Co. Houston A new mud-to-cement conversion technology has eliminated mud displacement problems, improved zonal isolation, and eliminated mud/cement incompatibility common with conventional cementing practices. Shell Offshore Inc. has successfully applied the technology to development wells at its Auger prospect in the Gulf of Mexico.
Kazem Javanmardi, Ken D. FlodbergShell Offshore Inc. New Orleans

James J. NahmShell Development Co. Houston

A new mud-to-cement conversion technology has eliminated mud displacement problems, improved zonal isolation, and eliminated mud/cement incompatibility common with conventional cementing practices.

Shell Offshore Inc. has successfully applied the technology to development wells at its Auger prospect in the Gulf of Mexico.

Ground, granulated blast furnace slag (hereafter referred to as slag) and common inexpensive alkaline activators are added to any water-based mud to form the cementitious slurry, designated "Slag-Mix" by Shell. Slag-Mix is simple to use, cost effective, and can significantly reduce mud-disposal costs in environmentally sensitive and zero-discharge areas because most of the mud is used as the base for the cement instead of being disposed or discarded.

Shell no longer uses Portland cement in wells at Auger; Slag-Mix is used for all cementing operations. To date, approximately 112,000 cu ft of Slag-Mix has been successfully used on several wells at Auger.

The Auger tension leg platform is scheduled for installation in 2,860 ft of water in Garden Banks Block 426 in 1993. The Auger field has approximately 220 million bbl of oil reserves.

The use of Slag-Mix technology at Auger has been proven cost effective and successful in improving zonal isolation between the casing and borehole and in providing an effective casing-to-casing annular seal. Shell has since used Slag-Mix successfully in several other fields, including Mississippi Canyon Block 705, South Timbalier Blocks 295 and 300, and Eugene Island Block 187.

Shell has licensed the Slag-Mix technology to several drilling mud and cementing service companies.

BACKGROUND

Conventional cementing practices cost the petroleum industry millions of dollars annually, and much of this money goes to ineffective operations. Conventional cements may not always achieve zonal isolation, provide an effective casing-to-casing annular seal, and support the casing adequately.

One problem with conventional cements is the incompatibility, of Portland cement and the drilling mud. Expensive preflushes and spacer fluids have been used, often with limited success, to attempt to separate mud and Portland cement effectively. Under downhole conditions, most spacers are ineffective in preventing high viscosities and cement contamination problems which lead to poor primary cement jobs.

One solution to this problem is to convert the drilling mud into a cementitious slurry, thereby eliminating the mud/Portland cement incompatibility. The existing mud solidification technologies have received limited acceptance because of high costs, complex design, and difficult field use.

Some mud solidification technologies use Portland cement as a cementitious agent along with polymers. Other applications are limited to specific mud systems (that is, magnesium-based mud).1-3

Shell Development Co.'s mud solidification technology, (Slag-Mix) uses finely ground, granulated blast furnace slag as the cementitious agent.4-5 The slurry is activated with predetermined amounts of common alkaline chemicals (caustic or soda ash) and a thinner/retarder, such as lignosulfonate.

Slag is only slightly reactive with water.6 Thus, the slag can be mixed in the mud through the mud hopper. At Auger, the slag was mixed and pumped with a conventional cementing unit. On two other operations (South Timbalier Blocks 295 and 300), the slurry was mixed in the mud pits and pumped down the well with the rig pump, thus eliminating the costs associated with conventional cementing units and services.

SLAG

Slag is a nonmetallic by-product of the iron-making process. When the slag is water quenched and quick cooled, it becomes a glassy, granulated material of sand-like consistency. The slag is composed primarily of calcium silicate, which is a chemical combination of lime, silica, and other oxides (such as CaO, SiO2, Al2O3, and MgO).4-5

Table 1 lists the chemical composition of slag.7

For granulated slag to be hydraulically active, it must be cooled very rapidly from its molten state in the blast furnace at a temperature of about 1,500 C. to a solid, glassy granule at less than 100 C. The glassy slag is then dewatered and finely ground to about 5,300 cu cm/gm (usually finer than Portland cement) to ensure complete hydration.

The specific gravity of slag is about 2.92. Hydration of the slag is a function of its composition, its surface area, and the concentration of the activators.

A mixture of 10-90% slag with Portland cement has been used worldwide in the construction industry with great technical and economic success. These construction applications have overcome Portland cement problems related to thermal stability, saltwater contamination, and sulfur and CO2 chemical attacks.'

Slag-Mix is very resistant to chemical attacks because no calcium hydroxide is produced when the material hydrates.

Field evidence supports the material's corrosion resistance:

Two diatomite wells in the Belridge field in California were cemented with Slag-Mix, perforated, and fractured in 1990 and are still producing without problems.

Although slag has a high concentration of noncrystalline glassy silicate material, the slag is less abrasive than barite (Fig. 1).

The slag can be stored in bulk at the rig site in the same manner as barite or Portland cement.

SLAG-MIX FORMULATIONS

The Slag-Mix slurries are designed to provide the desired density, rheology, fluid loss, thickening time, and compressive strengths to suit the cementing application.

The drilling mud to be converted is first isolated. If necessary, the mud is then diluted with water to obtain a specific final slurry density or for rheological control. The mud is then treated with a lignosulfonate thinner/retarder and alkaline activators. This treated mud is pumped to the cementing unit where slag is added, and the mixed slurry is then pumped down the well. The order of addition of materials may be altered depending on whether Slag-Mix is mixed and pumped on the fly with a cementing unit or batch mixed and pumped with rig equipment.

The Slag-Mix formulations are pilot tested in the same manner as conventional Portland cement slurries. Thickening time and compressive strength can be adjusted by varying the concentration of retarder, activators, and slag.

CHALLENGES AT AUGER

The initial Auger cementing program was based on the use of available conventional cementing technology to achieve the following objectives (Fig. 2):

  • Provide effective zonal isolation across the productive intervals

  • Enhance well integrity and reduce production casing damage from subsidence and formation compaction problems while the wells are produced

  • Place adequate cement in the casing annuluses to minimize fluid (gas) migration which can cause pressure buildup below the subsea wellhead seal assemblies

  • Provide efficient mud displacement in the larger casings

  • Provide adequate cement throughout the length of the conductor casing to minimize flow (gas, saltwater, and silt) which could lead to sediment buildup around the guide base and jeopardize the structural foundation of the wells

  • Eliminate the need for subsurface release (SSR) plugs on the 16-in. casing to reduce drilling time and difficulties inherent in drilling out these large plugs.

Achieving most of these objectives was complicated by the use of a subsea wellhead system. Because the design required mandrel-style casing hangers, no casing movement was permitted once the casing was landed in the wellhead. Additionally, the small clearances prevented optimum centralization of the 20-in., 16-in., and 11 3/4-in. casing strings. Because of the lack of casing movement and good centralization, mud/cement contamination was a major concern.

An hydraulically actuated subsea stage collar was initially used in the 11 3/4-in. casing just above the 16-in. casing shoe. This stage collar provided adequate cement in the 16-in. x 11 3/4-in. casing annulus to enhance the prevention of potential gas migration and annular pressures. The stage tool was also necessary to reduce potential lost circulation because of the tall column of cement in the annulus.

Subsea cementing stage collars are by design more complex, more expensive, and less reliable than conventional stage collars. Shell encountered numerous difficulties opening and closing the stage tools. The tools would sometimes open at very low pressures (1,000 psi), and in other cases the tools would require excessive pressures (7,200 psi) to open.

On one well, the stage tool prematurely opened during the first stage cementing job. After significant operational difficulties, the associated trouble-time costs, and several design modifications on four subsequent wells, other cost-effective alternatives were considered to accomplish similar cementing objectives without the subsea stage tool.

Shell then decided to cement the wells with a lead slurry of Slag-Mix and a tail slurry of Class H cement for the following reasons:

  • Slag-Mix is simple, cost effective, and easy to use in the field.

  • The slurry has excellent compatibility with the drilling mud.

  • The slurry's rheology is reasonably close to that of the drilling mud, thus improving mud displacement and reducing the cement channeling problems.

  • Slag-Mix has low permeability and porosity. The slurry has no known chemical shrinkage, enhancing gas migration controls.4-6

  • Slag-Mix has low fluid loss and zero free water-these properties are mainly governed by mud properties.

  • The set slurry has higher compressive strengths than Portland cement at a low slurry density (2.92 sp gr compared to 3.15 for Portland cement). The low-density mixtures reduce or eliminate lost circulation problems common to long cement columns, especially in deepwater environments with low formation fracture gradients.

11 3/4-IN. CEMENT JOB

For the 11 3/4-in. cement job, 500 bbl of 11.5-ppg, partially hydrolyzed polyacrylamide (PHPA) mud reserved from a previous well were diluted to 11.2 ppg with seawater and treated with 4 lb/bbl caustic soda, 14 lb/bbl soda ash, and 11 lb/bbl lignosulfonate.

The treated mud was then pumped to the cementing unit where 140 lb/bbl slag (from bulk storage) was added for a final slurry weight of 13.4 ppg.

This material was pumped as the lead slurry for the 11 3/4-in. casing.

The Slag-Mix slurry was then followed by a tail of 146 bbl of Class H cement. The ultrasonic cement analyzer (UCA) compressive strength plot indicates good compressive strength results (2,470 psi) on a sample of the Slag-Mix retrieved during the job from the cementing unit mixing tub (Fig. 3).

The cement bond log evaluation indicates competent cement, especially in the 16-in. x 11 3/4-in. casing annulus at a depth of about 7,300-7,600 ft (Fig. 4). The formation weakness in this interval was a main reason for using the stage tools.

The elimination of the stage tool saved 2 days of rig time and the cost of the tool, which runs about $60,000.

After further lab testing and field evaluations, the use of the Slag-Mix was cautiously and gradually increased to the other Auger wells (Fig. 5). The Auger drilling team was then convinced that Slag-Mix could be used on all casing jobs (26 in., 20 in., 16 in., 11 3/4 in., and 7 in.) both as lead and tail slurries.

The team believed the slag cementing mixture could potentially eliminate other conventional cementing-related problems:

  • Portland cement contamination problems on the 16-in. casing

  • Expensive and undesirable planned tack-and-squeeze cement jobs on the 7-in. production liner

  • Expensive cementing spacers and mud treatment costs following the drilling out of cement in the casing strings.

16-IN. CASING

The Auger team wanted to eliminate the subsurface release (SSR) plugs on the 16-in. casing. Drilling out these large plugs was difficult and time consuming. In addition, polycrystalline diamond compact bits could not be used effectively because of their incompatibility with SSR plugs for large casing. Thus, eliminating the 16-in. SSR plugs would have direct economic benefits.

Without the SSR plugs, however, after the cement set for 15 hr the casing would have to be tested against the cement in the open (no plugs to bump) float equipment. Thus, the test would rely heavily on a good, uncontaminated Portland cement job. Because of significant cement contamination problems on several previous wells, costly cement plugs had to be balanced on top of the float equipment. These cement plugs allowed pressure testing of the 16-in. casing prior to the formation test.

The obvious solution was to eliminate the compatibility problem between the mud and the Portland cement. The Auger team therefore evaluated the Slag-Mix compatibility with the drilling mud on two consecutive 16-in. casing jobs. The Slag-Mix performed successfully and saved one-half day of rig time per well.

7-IN. PRODUCTION LINER

The main objectives for the setting of the 7-in. production liner were to achieve effective zonal isolation, enhance well integrity, and reduce potential casing damage from formation compaction problems. To achieve these objectives, the liner should be fully supported with an effective one-stage cement job.

Unfortunately, conventional cement had several potential problems: channeling, contamination, lack of a proper top-of-liner seal, and potential lost circulation in the lower pressure formations.

Thus, a tack-and-squeeze technique had to be used on the first eight wells.

The 7-in. production liners were typically run from 13,000 ft to 20,000 ft and landed in the 11 3/4-in. casing. Because the entire liner could not be cemented in one stage, the bottom of the liners had to be tacked with cement pumped around the shoe and across the pay zone, leaving the upper portion of the liner unsupported. The liner lap was then squeeze cemented in a subsequent operation.

This tack-and-squeeze operation was necessary because of the channeling problems and because of the weak formations which could not support an entire column of Portland cement. However, the results were often undesirable: cement may not cover the pay zones, most of the liner is left unsupported, washouts or formation subsidence could collapse the liner, and the top squeeze may not necessarily have a good seal over time.

The tack-and-squeeze jobs typically added one additional day of rig time, with a total cost of about $100,000.

The Auger team switched to Slag-Mix for the 7-in. production string because of the materials previous successes, its compatibility with the mud, and its effective gas migration control characteristics.

Slag-Mix was used to support the 7-in. production liner in a one-stage operation. To date, three 7-in. production liners have been successfully cemented (top of liner tested to 4,000 psi) with Slag-Mix. The planned tack-and-squeeze jobs were eliminated, saving about 1 day of rig time per liner job. Because the 7-in. liner has to be perforated in the future completion operation, the quality of the set cement behind the casing is critical. A 14.5-ppg Slag-Mix cured in Halliburton Services' full-scale model had excellent bonding properties: 30-135 psi shear bond and 400-1,800 psi hydraulic bond (Table 2).

In contrast, a compatible 14.5-ppg Class C cement has only 20-24 psi shear bond.

PLUGS

When Hurricane Andrew passed through the Gulf of Mexico in late August 1992, Well A-19 had to be temporarily abandoned and the rig evacuated. A Slag-Mix plug was set at 14,000-13,700 ft below a bridge plug inside the 11 3/4-in. casing. The slurry design for the plug was similar to the Slag-Mix formulation for the 7-in. production liner (Fig. 6).

When operations resumed 14 days later, the plug had excellent hardness. The plug was drilled out with two 10 5/8-in. bits and one cement mill with an average drilling rate of 8.8 ft/hr, 10,000-45,000 lb weight-on-bit, and rotational speed of 80-100 rpm. Because of the plug's low permeability, porosity, and shrinkage, gas migration was stopped. After the plug was drilled out, 2,000 units of gas were encountered below the plug.

Because of the long-term exposure of the open hole to the water-based PHPA mud, the hole could not be reclaimed (10 5/8 in. hole from 14,196 ft to 18,390 ft measured depth). A 15.7-ppg Slag-Mix sidetrack plug (with 3-ppg thinner/retarder, 8-lb/bbl caustic, 10-lb/bbl soda ash, and 320-lb/bbl slag mixed in 51 bbl of PHPA drilling mud) was set in the open hole from 14,605 ft to 15,130 ft. After the plug set for 26 hr, it was tagged at 14,605 ft.

The plug was very hard and allowed a successful kickoff. The new hole did not encounter any gas from the original hole, indicating the plug effectively sealed and isolated the original hole. The main reason for the success of these Slag-Mix plugs is the compatibility with the mud. Conventional open hole cement plugs are plagued with costly contamination problems, even with expensive spacers before and after the cement.

ENVIRONMENTAL BENEFITS

Slag-Mix technology can reduce mud discharges to the environment and reduce mud disposal costs. A reduction in the volume of mud for disposal is especially critical in environmentally sensitive areas and in zero-discharge areas, such as California and Mobile Bay, Ala.

Table 3 lists the costs to comply with Alabama Oil & Gas Board zero-discharge rules for the last three wells Shell drilled in the Fairway field in Mobile Bay. The cost to dispose of drilling mud is high, more than $34/bbl for the Mobile Bay wells.

The spent drilling mud constitutes about 75% of the final Slag-Mix slurry volumes used to cement all casing strings at Auger. Approximately 15,000 bbl of spent drilling mud has been used to make the Slag-Mix at Auger.

Instead of discarding this mud (PHPA mud with excellent fluid loss and Theological properties costs about $50/bbl), it is being recycled into a useful cementitious material. Fig. 7 shows the estimated whole mud saved since the application of Slag-Mix. Since the introduction of Slag-Mix, no mud has been discharged after a well has been drilled. Additionally, the average amount of mud saved at the end of each well is about 2,790 bbl.

Table 4 shows the amount of whole mud discarded (or run over a shaker because of excessive viscosity) because of contamination with Portland cement after the cement was drilled out of the casing strings. The contamination problems resulted in an average of about 1,730 bbl of mud discarded per well. Since the application of Slag-Mix as lead and tail slurries, no mud has been dumped because of contamination.

The calcium hydroxide generated during Portland cement hydration is the main cause of mud contamination. Slag-Mix does not generate calcium hydroxide during hydration and thus has eliminated this type of mud contamination.

The use of Slag-Mix at Auger has saved a total of about 4,500 bbl of mud per well.

The significant impact of the mud savings resulting from Slag-Mix use can be fully realized in zero-discharge environments. In addition, because slag is itself a by-product, its use is encouraged by Environmental Protection Agency (EPA) federal guidelines.8

FUTURE WORK

The Auger wells present a unique opportunity for further comparison of Slag-Mix and Portland cement because seven production liners were cemented with Portland cement and three liners were cemented with Slag-Mix. After the Auger wells are completed and put on production, the performance of Slag-Mix will be evaluated in the following areas:

  • Perforations and production characteristics

  • Bonding characteristics (bond logs will be run before the wells are completed)

  • Compressive strength relationship with time

  • Zonal isolation

  • Formation subsidence behavior and production liner integrity

  • Drillability of temporary abandonment plugs.

RESULTS

The Slag-Mix mud solidification technology has replaced conventional cements for all cementing requirements (26 in., 20 in., 16 in., 11 3/4 in., and 7 in. casing strings) including sidetracks and temporary abandonment plugs at Auger. The slag can be used to solidify virtually any water-based mud.

The Slag-Mix technology has proven simple and cost effective to use in the field. The savings on cement materials alone was about $111,000 per well.

The use of Slag-Mix has helped solve expensive conventional cement contamination problems and eliminated the use of expensive spacers at Auger. The slurry designs were a factor in eliminating the troublesome subsea cementing stage tool on the 11 3/4 in. casing strings.

Because of its good gas migration control characteristics and good compressive strength development at low densities, one-stage Slag-Mix slurries replaced the undesirable tack-and-squeeze cementing technique on the 7-in. production liners. In contrast to Portland cement, Slag-Mix does not generate calcium hydroxide during hydration. The use of Slag-Mix eliminated the mud contamination problems from drilling cement out of the casing.

ACKNOWLEDGMENT

The authors thank Shell Offshore Inc. (SOI) for permission to publish the article. The authors also thank Bob Romero with Shell Development Co., the SOI deepwater drilling operations personnel, and the drilling crews on Sonat Offshore Drilling Inc.'s semi-submersible George Richardson. The authors also thank Dowell Schlumberger and Michael Pendley and David Moon with MilPark Drilling Fluids for their participation in the use of this technology at Auger.

REFERENCES

1. Jones, F.T., and Oliver, D.C., "A new material to cement well casing," OGJ, Oct. 13, 1969, pp. 95-96

2. Wyant, R.E., and Van Dyke, "Method and Composition for Cementing oil Well Casing," U.S. patent No. 3,499,491, Mar. 10, 1970.

3. Wilson, W.N., Carpenter, R.B., and Bradshaw, R.D., "Conversion of Mud to Cement," SPE paper 20452, New Orleans, Sept. 23-26, 1990.

4. ASTM C-989, "Standard Specification for Ground Granulated Blast-Furnace Slag for use in Concrete and Mortars," American Society for Testing and Materials, Philadelphia, 1989.

5. American Cement Institute 226.1R-87, "Ground Granulated Blast-Furnace Slag as a Cementitious Constituent in Concrete," reported by ACI Committee 226.

6. Talling, B., and Brandstetr, J., "Present State and Future of Alkali-Activated Slag Concretes," paper No. 1525, Trondheim Conference, 1989, pp. 1519-1545.

7. Hogan, F.J., and Meusel, J.W., "Evaluation for Durability and Strength Development of a Ground Granulated Blast Furnace Slag," 1981.

8. Atlantic NewCem, "Good News for NewCem Users," profile No. 8, March 1983.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.