Research on Cements for Geothermal and Deep Oil Wells
Authors: Chow S.Y. , Kalousek G.L. 
Published: December 2007

Abstract
This paper describes research relating some of the indicated low strengths of hot-well cements to the CaO/SiO ratio and the amount of A120. An optimum composition was 5 CaO: 6 Si02 plus 7-percent A1203. The A1203 stabilized tobermorite to about 250 deg. C (482 deg. F), without which serious strength loss occurred. Another favorable composition was 2 CaO:3 (SiO2+ A1203), provided at least 5-percent Al O was available in the cement. Six hydrated calcium silicates were found as potential binders in the study. The effect of A1203, ranging from beneficial to deleterious, was determined.

Introduction
Problems in cementing hot wells increase with increasing well temperature, approximately over the range of 125 to 400 deg. C (257 to 752 deg. F), and increasing bottom-hole pressure. Cement placed at about 125 deg. C may undergo several transformations as the temperature rises to the operating level, including reactions with the additions to cement such as fly ash, residual drilling mud, and the wallrock.
This research on calcium silicate cements at temperatures of 150 to 345 deg. C (302 to 653 deg. F) followed two independent but related approaches:measurement of strength as a criterion of quality anddevelopment of the chemistry of the binders and how these related to strength. The results of the chemical studies will be treated in another paper.
The time of autoclaving of the strength cubes was necessarily limited to 7 days because of the large number of tests in the program. It was recognized that the equilibrium binders, the ultimate end products in contact with the wallrock for long periods of time at high temperatures, are not always periods of time at high temperatures, are not always attained in 7 days. The slowness of approach to equilibrium of some cements was recently reported by Eilers and Root.
As the chemical studies progressed, it became evident that at least six different binders and their Al O-containing counterparts would have to be evaluated for strength. If strength was to be the principal criterion of quality, a massive empirical principal criterion of quality, a massive empirical program of tests would be required. This was program of tests would be required. This was beyond the resources of the project. The most promising approach, particularly in screening out promising approach, particularly in screening out undersirable cements, appeared to be the determination of the potential equilibrium binders in the chemical studies. The starting materials were highly reactive reagent quality CaO, microcrystalline quartz (Tripoli of 8,000-sq cm/gm Blaine fineness), or silicic acid. The water-to-solids ratio was 5.0, and the autoclaving was conducted up to a temperature of 345 deg. (653 deg. F) and for periods up to 35 days. These combinations of materials and conditions generally permitted attainment of equilibrium products in 7 to 10 days.
The chemical tests were made on 5-gm samples in small autoclaves. Obviously, such specimens were inadequate for quantitative strength measurements. However, the accumulated actual strengths and relative hardness of the small samples showed a definite correlation. Without exception, pastes that were hard and free from drying shrinkage showed high strengths in the counterpart cubes. It is not intended that such qualitative results serve as a criterion of quality. The relative results are intended as guides for further studies.

CEMENTS AND PROCEDURES
Portland cement slurries generally lose pumpability rapidly at the higher temperatures of hot pumpability rapidly at the higher temperatures of hot wells. The dicalcium silicates react more slowly than portland cement and were selected in combination with ground quartz for this study. A proprietary cement made from beta 2 CaO SiO and quartz in a CaO/SiO2 ratio (C/S) of 0.85 was used in many tests. The 0.85 C/S composition was modified by adding Ca(OH) for the higher C/S mixes and adding quarts for lower C/S compositions. Gibbsite, A1203 3 H2O, served as the source of A1203.
SPEJ
P. 307