BLAST FURNACE SLAG SLURRIES MAY HAVE LIMITS FOR OIL FIELD USE

O.G. Benge, W.W. Webster
Mobil Exploration & Producing Technical Center
Dallas

Thorough testing, economic evaluations, and environmental evaluations of blast furnace slag slurries revealed that replacing Portland cement with slag slurries may compromise essential properties in a cementing operation.

The use of blast furnace slag (BFS) slurries should be analyzed on a per case basis for oil well cementing operations. BFS slum, technology may be a viable mud solidification process, but the slurries are not cement and should not be considered as such.

Several slurries using field and laboratory prepared drilling fluids solidified with blast furnace slag were investigated to determine thickening time, compressive strength, free water, and other pertinent properties. The tests included an evaluation of the expansion of the set material and shear bond, as well as Theological compatibility studies of the finished slurries with the base muds. These additional tests are critical in the potential application of this process under field conditions.

One of the main environmental benefits from any mud solidification process is the reduction in the volume of mud discharged. Because of the mud's dilution requirements to add the BFS, the actual mud volume (undiluted) that would have been discharged or disposed of is considerably less than the volume (diluted) reported in the drilling literature.

This study evaluated the actual reduction in disposal volume while accounting for the dilution volume. The economic comparison included field operations and a theoretical comparison for zero-discharge operations in Mobile Bay.

The following points briefly summarize the analysis of the test results:

• BFS slurries can be designed to give good placement times and show compressive strength development over a wide temperature range.

• Stress cracking of BFS slurries is severe and occurs in atmospheric, pressurized, and constant temperature testing. The cracking indicates the potential for severe well problems.

• A BFS slurry is a complex slurry and not easily designed. Laboratory slurry designs must include all of the materials (samples of the BFS, mud, and all additives) planned for use in the field.

• The economical application of this technology, depends heavily on mud handling, storage, and treatment costs. The field use of BFS slurries can be more expensive than the use of conventional cements.

• The environmental impact of using BFS is not significant compared to operations with sound cement and mud programs. The BFS system requires mud dilution, added storage, and replacement of a portion of the active mud system while tests are conducted.

• Field personnel must be alerted to the safety hazards of mixing and doing BFS slurries.

• More information is needed on the effects of BFS on downhole corrosion rates and the effects of perforating on the annular seating ability of BFS cements.

• Mud solidification is a viable process and may offer some limited benefits; however, many of the noted properties of BFS slurries make them unacceptable as alternatives to cement slurries.

• BFS slurries are solidified mud and cannot be treated as cement.

SLURRIES

Three typical field muds were taken from wells to represent various parts of the drilling process: an 8.8-ppg, light-weight spud mud typically used for surface holes, a 12.6-ppg mud often used at intermediate casing points, and a 17.6-ppg mud representative of the final stages of a well. (The base properties of the field mud are found in Reference 1.)1 Five service companies were contacted to prepare blast furnace slag (BFS) slurries for each mud. The test used a bottom hole static temperature of 185 F., which is typical for a 10,000 ft well (with a 1.1/100 ft temperature gradient) in the Gulf of Mexico. The aim was to prepare a BFS slurry that would give 3-5 hr of thickening time at 150 F. bottom hole circulating temperature.

Table 1 lists the formulations used for preparing the BFS slurries. The BFS concentration is expressed in pounds of BFS per finished barrel of slurry. In earlier investigations, it is unclear whether the BFS was added to a barrel of diluted mud or expressed as pounds per finished barrel. 2-4 If the concentration of BFS is expressed in pounds of BFS added to a barrel of mud, the apparent concentration is higher, simply because of the additional volume of the BFS.

For example, in this study Mud 5 had 300 lb BFS per finished barrel, or 428 lb added to a barrel of diluted mud.

TESTS

Table 2 lists the results of several basic cement tests performed with standard American Petroleum Institute (API) procedures and recommended practices where possible. A BFS slurry that has an acceptable thickening time and compressive strength development could be produced.

Strength development was measured with nondestructive methods using an ultrasonic cement analyzer (UCA) instead of actual crush tests. Standard crush strengths are very difficult to perform and reproduce because of excessive cracking of the samples. Evidence of stress cracking appeared throughout the investigation.

It has been proposed that the reason for the cracking during the tests is the rapid release of the confining pressure, which may, not allow the pore pressure to decrease as the pressure is released. Although this claim may appear valid, this study used samples cured in 3,000 psi confining pressure and in atmospheric pressure.

No difference in the amount of cracking was noted in any of the tests. Samples cured in atmospheric conditions showed the same cracking and deterioration as those cured with a confining pressure (Fig. 1).

UCA strength tests confirmed that a BFS slurry will set over a wide temperature range. Rapid strength development over a wide temperature range is an advantage of these systems compared to conventional cements. Most conventional cement systems do not allow for the high-strength development and extended working' times found in the BFS slurries.

Fluid loss results indicate some fluid loss control, but the fluid loss is considerably higher than that in the base muds. These values are comparable to those reported in other investigations. The higher fluid loss values resulted from the dilution effects on the mud and a change in testing procedure. (Mud fluid loss tests use filter paper and either 100 or 500 psi differential pressure, but cement fluid loss tests use a 325-mesh screen and 1,000 psi differential.)

Tensile strength tests and linear bar expansion tests were attempted, but none of the samples survived the molding process because of excessive cracking, and no data could be collected. Expansion tests with annular expansion rings did not show any appreciable volume changes for any of the slurries tested. Shear bond tests, although indicating the same cracking problems, did show acceptable shear bonds. Typical values for comparable cement slurries range from 150 to 300 psi, depending on slurry density and composition.

The explanations for stress cracking have ranged from rapid release of pressure during testing to dehydration of the samples. It has been speculated that stress cracking seen in these tests may not occur under downhole conditions because the downhole environment is very humid and rapid pressure changes do not occur. 3 Perforating the well, however, is normally performed underbalanced, resulting in a rapid and violent pressure change. These data and the full-scale bond log tests clearly demonstrate that the presence of severe stress cracking is not merely a laboratory phenomenon but will occur downhole.

THINNERS

Caustic and sodium carbonate were added to the mud, or in most cases diluted mud, to prepare the slag cement slurries. These additions increased viscosity in all samples tested. Mud thinners must be used to reduce the viscosity before the slag is added

The thinners were used with varying degrees of difficulty. Starch muds, however, did not respond well to the thinners and were abandoned from further attempts to make a slag slurry.

The thinners used in these tests were all common lignosulfonate mud thinners. The samples were obtained from suppliers and represent materials used commonly in the field. The final rheology of the BFS slurries did not vary, appreciably for any of the samples tested. Plastic viscosity/yield point values were +-2 lb/100 sq ft in all tests,, and all the thinners tested performed precisely as designed.

The remaining wood sugars in the samples were responsible for the retardation, and the concentrations varied 5-25% in the samples. For continued application of the thinners as retarders, closer quality control tests must be performed. Samples of the actual thinner planned for the job must be used in the design because there is too much variance in these materials from lot to lot.

Lab tests showed that the thickening time could vary greatly from one supplier to another, and it varies from lot to lot for a single supplier. Table 3 shows data from a set of tests with one slag slurry mixture, three different suppliers of thinners, and three lot numbers of each.

The thickening time varied 1 hr between lot samples for Suppliers A and C. With Supplier B, the difference between lots was almost 2 hr. The most significant difference was between suppliers. There was a 5-hr difference between the thickening time for Lot 2 from Supplier B and that of the Lot 2 samples from both Suppliers A and C. Obviously in any field use of this slag process, great care must be taken to isolate a particular thinner and to use it in the lab tests as well as the field application.

ECONOMICS

For BFS slurries to compete economically with cement slurries, the engineer must carefully consider the costs associated with the drilling mud. There is no benefit in saving money on cement if those savings (or additional costs) must be spent on the mud. Table 4 shows the per barrel cost data for the three slurries studied. The data were compared to comparable cement slurries with the same density and properties as the final BFS slurries. Note that the mud cost was assumed zero in this comparison, which may not be the case for many muds. The data show that complex cement systems may be more expensive than comparable BFS slurries. The evaluation in Table 4 accounts for a base BFS cost of $0.14/lb and does not include any license fees which can increase the cost of the BFS by more than 40%. 5

Other authors have stated, "The required volume of mud must be isolated and maintained in proper condition for the cementing operation." For most operations, this procedure will require renting additional storage for the mud. This storage cost must be included in the mud cost calculations.

Most muds age over time, and maintaining the same mud properties (proper condition) for this delicate operation may prove difficult. Furthermore, because the mud must be diluted before use, any chemicals added to maintain the properties of the mud and prevent settling increase the cost.

Replacing the mud taken out of the system must be factored into the costs. When a volume of mud is taken out of the active system and isolated as has been recommended, that volume must be replaced by new mud. These costs have not been accounted for in other investigations.

Nahm, et al., noted that the zero-discharge costs for Mobile Bay approach $2 million per well, or more than $34/bbl for mud disposal. The total figure for zero-discharge costs in this area is appropriate, but the mud cost calculation fails to account for the water discharged from the well operations. Gray water, rainwater, and wash water account for approximately, 160,000 bbl of the total volume on these zero-discharge operations. Added to the 60,000 bbl of mud used on the well, the actual cost for fluid disposal is about $9/bbl.

Data from Mobil Exploration & Producing U.S. Inc.'s operations in Mobile Bay have shown an average mud disposal cost of about $12/bbl. Water disposal costs about $2/bbl. Other costs include the cost of tugs, barges, additional personnel, and related equipment which can add approximately $700,000 in fixed costs per well.

Nahm, et al., also reported an apparent cost and environmental savings from the reduced mud volumes required for cement drill out. Assuming 90 ft between the float collar and shoe for each casing string, the solids tolerance capacity, for each of the muds on the reported wells was less than 1%. From the data presented, mud performance was a severe problem if more than 1,800 bbl were required to drill out 90 ft of cement in a 16-in. casing. 4

The authors in essence reported that the entire mud system of the well was replaced simply because of drilling out the casing shoe. Additionally, the per well average volume of 1,724 bbl reported for casing drill out appears excessive. A review of 30 wells in Mobil's offshore operations showed 5075 bbl per shoe drill out.

Excessive mud contamination during shoe drill out more likely resulted from improper temperature determination or poor cement slurry design rather than a problem with the mud.5 Because of the BFS slurry's ability, to set over a wide temperature range, the problems encountered at shoe drill out were avoided. A similar effect could have been noted simply by changing the cement slurry design.

ENVIRONMENTAL

The environmental impact of solidification of mud with BFS downhole has the potential for saving some mud that would normally be discharged. Although this volume is small, any reduction in total well discharges is important. In most cases, the mud required dilution by 60% or more to accommodate the BFS. This dilution reduces the effective environmental impact further. For a job requiring 1,000 bbl of BFS slurry, less than 400 bbl of mud will be used in the process.

It has been suggested that the incorporation of BFS into the drilling mud during drilling will produce a fluid that will eventually set in the well. 2 4 This procedure reportedly results in better zonal isolation if any mud was bypassed during the actual cementing operation. Although this assumption is valid, 40 lb/bbl BFS added to the mud has the same effect as an inert drill solid added to the mud. The addition reduces the solids tolerance of the mud, thus requiring more mud dilution to drill the well, and in turn increasing the amount of mud used to drill the well. The volume of mud discharged and the costs then increase.

Unless the mud has been specifically formulated to account for the BFS added as a back-round material, this practice should not be attempted.

Additionally, there is a danger that if the rig must be abandoned (as during a hurricane), the entire mud system has the potential for setting up.

CEMENT BOND LOGGING

In this work and the work reported by others, the slag slurries have had cracking and shrinkage. 5 7 8 Tests were run to determine if this shrinkage might occur in a well bore simulation and to determine its effects on cement sheath integrity. 1

An 18-ft model with 5-in. casing in an 8 1/2-in. displacement core was built. The bottom half of the 5-in. x 8 1/2-in. annulus was filled with a 16.4-ppg slurry made from Class H cement, 38% water (by weight of cement), and 0.5% fluid loss additive. The top half of the annulus was filled with a 13.0-ppg slag slurry made from a 9-ppg water-based mud, 213 lb/bbl slag sodium hydroxide, mud thinner, and soda ash. A pressured reservoir of slag slurry was connected to the top of the annulus to keep the model full. The model was maintained at 160 F. with 500-psi nitrogen pressure on the reservoir for the entire test.

Sonic cement bond log (CBL) and ultrasonic pulse-echo logs were conducted daily for 4 days. The omnidirectional CBL tool had a spacing from transmitter to receivers of 3 and 5 ft. The 3-ft receiver recorded pipe amplitude, and the 5-ft receiver gave the variable density log display.

Because the model was short, most of the logs were run by holding the tools stationary adjacent to the model section investigated, and the data were recorded in the tool's time-drive mode. In some runs, the tool was pulled very slowly through the model. Both types of logs and each type of measurement save the same end results.

Figs. 2 and 3 show the amplitude section from CBL logs of the top and bottom half, respectively, of the model after 24 hr. For brevity only the CBL data are shown.

Both sections of the model appear bonded. The 16.4-ppg slurry seemed better bonded, but the differences could be from the weight and compressive strength of the samples. Figs. 4 and 5 are the amplitudes from the CBL logs of the same sections after 4 days. The 16.4-ppg slurry bond was as strong or stronger than that after 1 day. The slag slurry, however, has lost most of its bond, and the signal approached that of free pipe.

The model was cooled and cut into sections for a visual examination of the cement-pipe contact. Fig. 6 is an end view of the model section showing the BFS slurry. The slag slurry had a small gap developing between the cement and the pipe. The most dramatic effect was the very visible cracks in the slag slurry. A few cracks were in the normal cement, but the slag slurry was separated by a number of cracks in ail directions.

Shrinkage and cracking have been noted by others but have not been reported from a confined well bore simulation. Contrary to other reports, these data clearly demonstrated that the stress cracking seen under laboratory conditions was not a result of the test but can occur under stable temperature and pressure conditions. 5 There was no change in the temperature or pressure during any of the testing, yet the logs and sectioned test cell clearly showed the presence of the shrinkage and cracking problem under stable conditions.

If this problem occurs in a well, maintaining zonal isolation and preventing fluid migration in the annulus will be difficult. Repair of the problem will be virtually impossible.

PERFORATING

The cracking of the samples from the expansion and tensile strength molds caused concern about the effects of perforating the set slag slurries.

Three slag mixtures and two Portland cement slurries were mixed and poured into waxed cardboard molds. The slag mixtures were the same as those in Table 2. One of the Portland slurries was a 16.4-ppg Class H cement, and the other was 14.2-ppg 50:50 mixture of Pozzolan and Class H cement. The samples were cured with water standing on top of the molds.

Each sample was perforated with a 3.2-g charge. This charge is much smaller than those normally used but was chosen to show the anticipated difference in the slurries without totally destroying them. The charge simulates a 1 9/16-in., through-tubing gun with a 1/2-in. standoff. A steel plate represented the casing. The charge was fixed to the top of the molded sample and detonated in an unconstrained test.

Fig. 7 shows the contrast between the two typical cements and the three slag-mix cements. The normal cements have a perforation hole in them and a few cracks across the sample. The slag samples were devastated by the shot.

When the cements were removed from the molds, the normal cements had two or three cracks that ran the length of the sample, but the sample was still intact. The slag samples were rubble on top and on bottom, with a small core of more solid material in the center (Fig. 8).

These unconstrained tests are not typical of the constrained conditions during actual well perforating. Experience has shown the loss of cement sheath integrity after perforating high-strength normal cements in some areas, however. The slag slurries appeared far more brittle and susceptible to destruction by perforating than normal cements. More elaborate tests and evaluation of field experience are required to establish the actual effects of downhole perforating on the integrity of slag slurry sheaths.

CRACKING

The high incidence of cracking and the apparent brittle nature of the slag slurries is radically different from that of normal Portland cement slurries.

Under low magnification with a scanning electron microscope, the set slag/mud slurry and the Portland cement are similar. At higher magnification there is a noticeable, significant difference between the materials.

The slag slurry had no fibers or crystals growing between the grains, whereas the Portland cement showed the typical crystal growth between grains. These crystals give normal cement a portion of its strength and competence. The sharp edges of the slag materials appeared as a pressed agglomeration of grains.

Others have noted this difference and concluded that the cracking tendency of slag cements may be because of the low deformation abilities of the granular material lacking fibrous material. 7 8 This lack of adhesive forces between the grains of slag slurries could explain the brittleness and cracking noted in these experiments.

OTHER CONCERNS

Several other areas of concern arose from these lab studies and additional literature searches. The following points should be addressed before slag slurries are used routinely:

• Safety. In the preparation of mud/slag slurries, 414 lb of sodium hydroxide are added per barrel of slurry. The mix has a very high pH and is a serious eve and skin hazard. Extreme care must be exercised at all times in handling this fluid. Personnel unfamiliar with highly caustic and corrosive fluids must be cautioned in their use.

  If a mud with a high partially hydrolyzed polyacrylamide (PHPA) content is used, significant amounts of ammonia will be released, creating a hazard in confined spaces.

  Some of the mud/slag samples emitted enough H2S to set off the H2S monitors in the lab. There is a definite potential for an H2S problem in the field if large quantities of the mix are brought to the surface by drill out of the shoe or circulating out excess slurry. Batch mixing the slurries in enclosed areas (as in a mud pit room) poses the potential for serious health hazards.

• C02 resistance. Set Portland cement will react with C02 to form acid-soluble calcium carbonate. Some operating areas inject or produce C02 and periodically use acid to clean wells. This process can remove cement that was behind the pipe or in perforations for injection profile control.

  The slag slurries are reported to be C02 resistant. Samples of the three different slag slurries were placed in C02-injection wells. The first samples crumbled and disappeared from the sample holder during the test. Most of each sample was recovered from a second set of tests. The samples were then exposed to 7 1/2% hydrochloric acid, and then ranged 45-70% soluble. This degree of acid solubility does not meet Mobil's criteria for CO2 resistance.

• Shear effects. There is some evidence that shear will affect the final properties of the slag slurry. A PHPA mud and a bentonite mud were each used to make slag slurries to study shear.

  When the PHPA system was mixed at API rates, the slurry had 3 1/2 hr thickening time and no settling. When mixed at less shear, the slurry had over 6 hr thickening time, and some settling occurred.

  Shear did not affect the bentonite mud s%,stem as much. The API test gave a slurry with just over 4 hr thickening time, while the low shear sample was 3 hr 13 min.

  One of the proposed benefits of the mud/slag slurry is that it can be mixed with the mud equipment rather than the cementing unit.

  Unless materials from the actual location and a duplication of the shear of the field equipment are used, reproducing lab properties in the field may be difficult.

• Corrosion. One author has studied the reducing nature of slag cements, and he concluded that slag cement could reduce the normal passive protection of steel-9 This conclusion does not mean that the material would be corrosive, but it does mean that the steel would not be as protected if oxidizers penetrated the matrix. This potential reduction of protection coupled with the high occurrence of cracking could lead to an increased corrosion potential, compared to more traditional cementing methods.

• BFS consistency. The American Society of Testing & Materials (ASTM) specifications for BFS addresses its use in concrete and mortars.10 The chemical tests and test methods evaluate long-term compressive strength development but do not address combinations with mud.

  Initial tests with lab muds indicated variations in thickening times of more than 6 hr simple, by changing BFS sources. Tests have shown performance changes both between suppliers of BFS and within the same supplier. These tests indicated that chemical consistency does not mean performance consistency.

  There is a clear indication of the need to test the actual BFS source.

RESULTS

The study included rheological compatibility of the blast furnace slag (BFS) slurries with the base muds. Although there were some viscosity increases from adding cause to the BFS, the finished BFS slurries were compatible with the base muds for all fluids tested. Compatibility between fluids, however, does not mean that there will be no channeling in the well.

This common misconception is illustrated in simple mud terms: Often a sweep (either thick or thin) of fluid is pumped into the well during drilling. With no channeling and intermixing of fluids, the sweep should come back to the surface as a slug. Field observations have shown that even though these sweeps are totally compatible with the mud, rarely do they come back to the surface as a slug and often are never seen at all.

To assume improved displacement based solely on compatibility, ignores the entire body of work on fluid rheology and displacement mechanics. Compatibility is important but is not the sole controlling factor in fluid displacement. Good cementing practices must be used and cannot be ignored.

BFS slurries can set over a wide temperature range, giving them a distinct advantage in specific applications. This ability probably led to the reduction in contaminated mud during drill out. BFS slurry use in kickoff plugs appears to be a good application for this technology, as strength development is the primary concern in this applications The ability to set over a wide temperature range and the rapid development of compressive strength have definite applications in setting some types of plugs. The ability to solidify the mud may have promise in lost circulation situations. One must remember, however, that this material is not cement but is a mud solidification processed

Furthermore, this technology is not the same as "slag cement," which is a blast furnace slag/Portland cement slurry. The addition of Portland cement to the BFS slurry stabilizes the reactions. Slag cement cannot be considered comparable to these BFS slurries in any property because the two are completely different processes. 7-10

As currently tested, BFS slurries do not offer the same final properties as cements and cannot be considered as a replacement for conventional cement slurries. Although a BFS slurry can be designed with similar thickening time and compressive strength development as a cement slurry the remaining problems with these slurries compromise many critical cement properties.

The testing performed demonstrated that BFS slurries exhibit severe stress cracking under all conditions, excessive brittleness that- can affect perforation performance, a lack of chemical resistance, and a high sensitivity to additives and BFS sources.

Until these problems are corrected, the "blanket" use of BFS slurries for most cementing applications cannot be recommended. Considerable research will be critical for its use in oil field applications.

REFERENCES

1. Benge, O.G., and Webster, W.W., "Evaluation of Blast Furnace Slag Slurries for Oilfieid Application," IADC/SPE paper 27449, presented at the 1994 IADC/SPE Annual Drilling Conference, Dallas, Feb. 15-18.

2. Cowan, K.M., Hale, A.H., and Nahm, J.J., "Conversion of Drilling Fluids to Cements With Blast Furnace Slag: Performance Properties and Applications for Well Cementing," SPE paper 24575, presented it the Society of Petroleum Engineers 67th Annual Technical Conference and Exhibition, Washington D,C., Oct. 47, 1992.

3. Javanmardi, K., Flodberg, K.D., and Nahm, J.J., "Mud to Cement Technology Proven in Offshore Drilling Project," OGJ, Feb. 15, 1993, pp. 49-57.

4. Nahm, J.J. , Javanmardi, K., Cowan, K.M., and Hale, A.H., fix Mud Conversion Cementing Technology: Reduction of Mud Disposal Volumes and Management of Rig-Site Drilling Wastes," SPE paper 25988, presented Lit the Society of Petroleum Engineers/Environmental Protection Agency Exploration & Production Environmental Conference, San Antonio, Mar. 7-10, 1993.

5. Schlemmer, R.P,, Branam, N.E., Edwards, T.M., and Valenziano, R.C., "Drilling Fluid Conversion: Mud Selection and Conversion Techniques." SPE paper 26324, presented at the SPE 68th Annual Technical Conference and Exhibition, Houston, Oct. 3-6, 1993.

6. Cowan, K.M., "Solidify Mud to Save Cementing Time and Reduce Waste," World Oil, October 1993.

7. Hakkien, T., "The Permeability of High Strength Blast Furnace Slag Concrete," Technical Research Centre of Finland, Building Materials Laboratory Nordic Concrete Research Publication No. 10, 1992.

8. Hakkien, T., "The Microstructure of High Strength Blast Furnace Slag Concrete," Technical Research Centre of Finland, Building Materials Laboratory Nordic Concrete Research Publication No. 10, 1992.

9. Macohee, D.E., "Theoretical Description of the Impact of Blast Furnace Slag (BFS) of Steel Passivation in Concrete," Magazine of Concrete Research, Vol. 45, No. 162, 1993.

10. ASTM C989-89, Standard Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortars.

11. Cowan, M., "Mud-Based Cements Refined," Petroleum Engineer International, April 1993.

12. Bell, S., "Mud-To-Cement Technology Converts Industry Practices," Petroleum Engineer International, September 1993.

13. Belong, A.B., Fardig "Hydration Products of Alkai Activated Slab," 9th International Congress on the Chemistry of Cement.

14. Koch Minerals Co., "Introducing Well-Cem Cement for Oil Well Applications," Technical Bulletin 10002, (316/832-6922).

15. Blue Circle Cement Corp., "NewCem (WC)," Product Bulletin, (410/721-4904).

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