Special Report: Aluminum phosphate cements help with deep, high-temperature wells

May 15, 2006
Drilling Market Focus, which normally appears in the third issue of the month, was published last week.

Arun S. Wagh, Ramkumar Natarajan, Richard L. McDaniel - Energy Technology Division, Argonne National Laboratory, Argonne, Ill.

Drilling Market Focus, which normally appears in the third issue of the month, was published last week.

Aluminum phosphate cements perform better than traditional Portland cement and Ceramicrete cements at well temperatures greater than 250º F. (121.1º C.).

Cementing high temperature and high-pressure wells is difficult with Portland cement because its strength decreases at temperatures above 230º F., accompanied by an increase in its permeability. In addition, rigorous control is required to add weighting agents to increase the cement slurries.

This article discusses a new cement composition based on aluminum phosphate that has sufficient thickening ability at bottomhole circulating temperatures greater than 250º F. The novel cement is a mixture of aluminum oxide and phosphoric acid, in which aluminum hydroxide and boric acid control the setting time. Reported here are the pumping characteristics of this cement as well as its compressive strength, water permeability, and expansion during setting.

Challenges

It’s difficult to cement oil wells at depths greater than 15,000 ft (4,587 m) and bottomhole circulating temperatures (BHCT) higher than 230º F. Portland cement slurries designed for such wells involve retarders, dispersants, silica, and weighting materials.

The principal components of Portland cement, tricalcium silicate (C3S) and dicalcium silicate (C2S), hydrate upon addition of water to form gelatinous calcium silicate hydrate (C-S-H gel). Subsequently, this gel converts to alpha dicalcium silicate hydrate, resulting in a loss in strength.

This conversion is also accompanied by an increase in the permeability of the cement. Though the reduced strength of the cement is still higher than the 500 psi (3.44 MPa) needed to support the casing in the well, the increased permeability (10-100 times the recommended limit of 0.01 md) affects the cement performance.1

Adding silica flour to the dry mix prior to pumping prevents water migration through the cement due to the high permeability. However, working with silica flour presents a health hazard due to undesired airborne particulates.

Moreover the mix is often not homogeneous, making the cement performance unsatisfactory. The logistics of transporting and storing the blended cement also lead to high costs. Novel cementing formulations are needed to overcome these problems with conventional cements.

Argonne National Laboratory has been developing novel phosphate-based cements to address these issues. Reflecting earlier studies, we presented magnesium potassium phosphate (ceramicrete) cements designed for wells in permafrost regions (OGJ, May 9, 2005, p. 53). These cements with suitable modifications can also be used for wells with BHCT as high as 200º F.

But the ceramicrete-based cements are unsuitable for higher temperatures. Alternative materials are needed to formulate cements for hot wells. This article presents formulations based on aluminum phosphate for deep wells with BHCT greater than 250º F.

Cement chemistry

Aluminum phosphate cements form acid-base reactions between aluminum oxide (alumina) and phosphoric acid solution. Details of the kinetics of the cements formation appear in Wagh.2 Alumina dissolves partially in the acidic solution and forms the aqueous ions Al3+ (aq), which react with the phosphate anions H2PO4- to form aluminum hydro phosphate gel. Equation 1 (see accompanying equations box) represents this reaction.

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The gel forms a smooth and low-viscosity slurry. As discussed by Wagh, the dissolution of alumina in the acid is optimal at about 220º F.2 As a result, during pumping in a hot well, slurry forms with maximum dissolution of alumina. Once the slurry reaches a downhole temperature >300º F., further reaction occurs between the hydrophosphate and the residual alumina, given by Equations 2 and 3.

Aluminum phosphate forms an anhydrous ceramic of berlinite if the hydrophosphate gel is heated in a dry atmosphere. When cured in water, however, hydrophosphate will form a hydrated phase of berlinite, AlPO4·nH2O, which is variscite for n = 2 and wavelite for n = 1.

Thus, the cement formed in wet downhole conditions will be rich in these two phases. These phases bond alumina particles together to set the cement in downhole conditions.

Cement characteristics

We used a fine powder of calcined alumina in this study with an average particle size of 2 μm (Fig. 1). This powder was mixed with a small amount of aluminum hydroxide (AC 400) with a particle size of 9 μm (supplied by Aluchem Inc.; Fig. 2). The hydroxide is added to enhance the formation of aluminum hydrophosphate.

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Phosphoric acid (85%) supplied by Mallinckrodt Laboratory Chemicals, a division of Mallinckrodt Baker Inc., was diluted to 50% strength with the addition of water (Table 1). A measured amount of the acid solution was mixed with a predetermined amount of the powder. The mixing was done in a cement mixer.

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The powders were first mixed with the acid solution in a Hobart mixer for 20 min. The slurry had a density of 19.6 lb/gal (2.36 g/ml). This high density is suitable in deep wells to prevent blowout of the slurry.

In conventional oil well cements, weighting agents such as barite or hematite are dry blended to increase the slurry density. Rigorous controls are followed while adding the weighting agents. Nonetheless, density segregation of the weighting agent in the dry blend during transportation and storage causes problems.3 The segregation results in the uneven ratios of cement to weighting materials. Aluminum phosphate cements may avoid such problems.

After mixing, we tested the slurry according to American Petroleum Institute testing schedules for thickening time in a consistometer (Chandler model No. 7222).4 The ramp time was 83 min at a temperature of 250º F. and pressure of 13,285 psi, and 97 min at 300º F. and 16,650 psi.

When the consistency reached 70 Bearden units (Bc), the slurry was transferred into cylindrical molds with diameter 0.75 in. (1.9 cm) and length of 1.58 in. The consistometer also served as the curing chamber, and the samples were cured overnight in water at 300º F. at 3,000 psi in the consistometer cup. As shown in Fig. 3, the initial Bc was slightly higher than 30. The transition time, i.e., the time required for the slurry to go to a consistency of 70 Bc from 30 Bc under dynamic conditions, was 2.5 hr.

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The long transition time implies that the slurry will take a long time to develop static gel strength of 500 lb-ft/100 sq ft (240 Pa). This conclusion assumes that the cement slurry behaves in the same manner in both the static and dynamic conditions. According to Mueller, the gel strength of the slurries during setting in the dynamic and static states may not be similar.5 Murray also emphasizes this point.6 Thus, the development of static gel strength may be quicker in the case of aluminum phosphate cement.

Table 1 presents data on slurries of different compositions pumped at a BHCT of 250º F. and pressure of 13,285 psi.

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Boric acid is used to lubricate particles and thereby reduce the viscosity during pumping.8 Our earlier research had shown that it also works as a retarder below 200º F. On the other hand, aluminum hydroxide can be used to reduce the transition time. Thus, manipulating both of these components allows development of a slurry with the desired pumping and setting characteristics.

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Fig. 3 shows the X-ray diffraction output of the set cement that was pumped at 250º F. and cured at 300º F. for 24 hr. The peak of variscite (AlPO4∙2H2O) confirms the chemical reaction represented by Equation 3 as one of the setting reactions. Also, peaks of alumina indicate that a significant amount of the binder powder remained unreacted. Thus, one may conclude that the reaction products, such as variscite, bind the unreacted alumina particles and form the set cement.

Properties, set cement

We measured the physical properties of the set cement after it cured for 24 hr at 300º F. and 3,000 psi. After the slurry reached 70 Bc, it was transferred to syringes of diameter 0.75 in. and length of 1.57 in. and placed under water in the consistometer used as the curing chamber. After 24 hr, we removed the syringes and measured the compressive strength with an Instron machine. The results appear in Table 3.

Right-angle-set slurries

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The composition given in Table 1 produces a slurry with initial Bc >30. For smooth pumping purposes, it is necessary to lower the initial Bc. In addition, ideal slurries should exhibit right-angle-set (RAS) where, at the time of setting, the Bc should increase sharply in a short time. Such slurries will show no tendency toward gelation, yet once placed, will set rapidly due to the chemical reaction outlined in Equation 3.

Decreasing the strength of the acid and thereby increasing the water content can achieve RAS. Rapid setting will prevent gas intrusion and produce a matrix with low permeability.

In normal Portland cement systems, the increase in the consistency is often accompanied by an increase in the temperature.1 However, we observed no such temperature increase in the aluminum phosphate cements. Therefore, we used 45% dilute phosphoric acid solution. The exact composition that provides an adequate pumping time of 3 hr, 30 min is given in Table 4.

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Fig. 5 shows the consistency graph of the typical right-angle-set slurry. The consistency remained low for the entire period of the test and increased to 80 from 30 Bc within 10 min. The 1-day compressive strength was 853 psi, which is more than the required value of 500 psi but significantly lower than the strength obtained with 50% concentrated phosphoric acid (Table 3). This discrepancy implies that the best slurry will be the one for which the acid concentration can be adjusted between 45 and 50% to obtain an increased compressive strength, a low initial Bc, and a right angle set.

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Extent of expansion

A number of chemicals are added to cement systems to overcome the shrinkage that occurs during curing and, in some cases, even impart a degree of expansion. A positive cement expansion can snug up the cement against the formation and prevent annular communication via microannuli.8

The two most common expansion additives are calcium sulfate hemihydrate and sodium sulfate. Calcium sulfate hemihydrate, however, causes a viscosity increase in the slurry and makes the slurry thixotropic. It also slows the rate of increase of compressive strength. The maximum temperature of operation in the presence of calcium silicate is 200º F.

In contrast, phosphate-bonded sealants expand during setting, and hence, are ideal. We measured the extent of expansion using the density of the slurry before setting and that of the set ceramic.

The slurry density (Table 1) was 2.36 g/ml, while that of the ceramic was 2.28 g/cc, thus the extent of expansion was 3.3%. The expansion may be the result of the low density of the reaction product, variscite (2.53 g/cc, as compared to that of alumina, which is 3.7 g/cc).

Pumping time

The pumping time is very sensitive to the amount of Al(OH)3 that is added to increase the rate of the reaction. As shown in Fig. 6, a reduction of 15 g of the hydroxide for every 1,000 g of calcined alumina increases the pumping time at BHCT of 250º F. to 4 hr from 2.

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Similarly, Table 5 shows that a reduction of 23 g increases the pumping time to 6 hr from 2 at BHCT of 300º F. The surface area of the aluminum hydroxide, 1.75 sq m/g, is large when compared to the surface area of 0.3 sq m/g for the alumina particles. Also, the solubility of the aluminum hydroxide is relatively high when compared with that of alumina.7

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This higher surface area and solubility make the aluminum hydroxide an accelerator, which increases the rate of the reaction of aluminum hydrophosphate.

Learnings

Aluminum phosphate cement is ideal for deep well sealing where the BHCT is greater than 250º F. It has an adequate pumping time of 3 hr and exhibits a compressive strength of more than 500 psi within 24 hr. Its pumping time can be adjusted by varying the amount of aluminum hydroxide in the blend.

The study was conducted on the cement itself. It may be possible to reduce the cost of this cement by adding non-reactive extenders, such as sand. Even without an extender, since alumina and phosphoric acid are not very expensive, it should be possible to use the cement formula provided in this study directly.

Though the study reported here is for alumina, other oxide minerals of aluminum, such as kaolinite (Al2Si2O5(OH)4) or boehmite (AlO(OH), a major constituent of most bauxite ores), may also reduce the cost of this cement. Detailed studies using these materials will be reported in the future.

Acknowledgment

This work was supported by the US Department of Energy, National Petroleum Technology Office, and National Energy Technology Laboratory, under Contract W-31-109-38.

References

1. Nelson, E.B., Well Cementing, Schlumberger Educational Services, Houston, 1990.

2. Wagh, A.S., Susan, G., and Jeong, S.Y., “Chemically Bonded Phosphate Ceramics: II. Warm-Temperature Process for Alumina Ceramics,” J. Ceram. Soc. 86 (11), 2003, pp. 1845-1849.

3. Pace, R.S., McEitlesh, P.M., Cobb, J.A, and Olsberg, M.A., “Improved Bulk Blending Techniques for Accurate and Uniform Cement Blends,” SPE paper 13041, SPE annual technical conference, Houston, Sept. 16-19, 1984.

4. American Petroleum Institute Standards, Specification 10A, 22nd Ed., 1995.

5. Mueller, D.T., “An Investigation of the Static State Properties of Right-Angle-Set Cements,” prepared for the Southwestern Petroleum short course, Lubbock, Tex., Apr. 21-22, 1993.

6. Murray, J.R., Dillenbeck, R.L., and Ramy, N.E., “Transition Time of Cement Slurries, Definitions and Misconceptions, Related to Annular Fluid Migration,” SPE paper 90829, SPE Annual Technical Conference and Exhibition, Houston, Sept. 26-29, 2004.

7. Wagh, A.S., “Chemically Bonded Phosphate Ceramics,” New York: Elsevier, 2004, 300 pp.

8. Moran, L.K., Murray, T.R., and Moyer, W.R., “Cement Expansion: A Laboratory Investigation,” SPE paper 21685, presented at the Production Operations Symposium, Apr. 7-9, 1991.

The authors

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Arun S. Wagh ( [email protected]) is a staff ceramist at Argonne National Laboratory, Argonne, Ill. He has also served as senior lecturer at the University of the West Indies in Jamaica and reader of physics in Bombay University, India. Wagh holds a PhD (1972) in physics from the State University of New York, Buffalo, and BS and MS degrees from Bombay University. He is a member of the American Ceramic Society.

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Ramkumar Natarajan ([email protected]) is a post-doctoral fellow working on chemically bonded phosphate cements at Argonne National Laboratory. He has served as a materials science engineer at Brakes India Ltd., Chennai, India. Natarajan holds a BS (1998) in metallurgy from Regional Engineering College, Trichy, an MS (2002) in materials engineering from the University of Illinois, Chicago, and a PhD (2005) in materials engineering from the University of Illinois, Chicago.

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Richard L. McDaniel ([email protected]) works as a senior electronics technician at Argonne National Laboratory, Argonne, Ill. He holds an AA (1977) in electronics occupations from Wilbur Wright City College of Chicago.