Roy, Della M., White, Elizabeth L., Langton, Christine A., and Grutzeck, Michael W., Pennsylvania State University Pennsylvania State University Copyright 1979, American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.

Abstract

Potential new high temperature cementing co positions for geothermal wells have been invest positions for geothermal wells have been invest gated, primarily within the lime-magnesia-silica water and lime-alumina-silica-water systems. The stabilities and phase equilibria of constituent phases have been determined, using reaction times phases have been determined, using reaction times varying from two to 98 days, with temperatures in the range up to 300–400 degrees C, and pressures from 1400–10,000 psi (9.6–68.9 MPa).

Xonotlite, anorthite, boehmite, hydrogarnets, wairakite, x-phase or hexagonal anorthite, and truscottite are among the phases present in the system CaO-Al203-SiO2-H2O- Xonotlite, truscottite, talc, chrysotile, monticellite, and diopside are formed in the CaO-MgO-SiO2-H2O system. In the latter compositions, reaction is slower, and thus attainment of equilibrium is more difficult.

Viscosities of slurries, studies of reaction rates, determination of strength and microhardness, permeability, bond strength to rock, corrosiveness, and the effect of saline solutions generally comprise the major parts of the study. Other characterization tools used are x-ray diffraction, optical microscopy, SEM, microprobe, DTA, TGA, thermal expansion, thermal conductivity, and other analytical techniques.

Introduction

The performance requirements of cements for geothermal wells are rather severe, including the demands of upper temperature limits which are beyond those usually met in deep oil wells. While modified calcium silicate-based cements have performed well at temperatures up to some 160 degrees performed well at temperatures up to some 160 degrees C (320 degrees F) or higher, cements for geothermal wells should be stable up to about 400 degrees C (752 degrees F). This presents problems concerning long range stability, since the usual phases formed are not stable in the upper temperature regime. Recent attempts to extend the range of performance of calcium silicate based cements to higher temperatures have been carried out. While it has been generally accepted that for ordinary deep oil wells, cement compositions would be adjusted so that tobermorite would be formed as the stable binder; this phase decomposes above a maximum temperature which is a function of pressure and composition to yield the phases which are thermodynandcally stable under the new conditions. Higher pressure increases the maximum thermal stability limit as does the substitution of A1 (3+) in the structure. However, the maximum stability temperature does not exceed 320 degrees C at 20,000 psi (138 MPa) pressures, and is significantly lower at lower pressures. Xonotlite is stable to higher temperatures, and with high-SiO2 ratios, truscottite might be stable. Some mechanical problems concerning reactivity and conversion remained to be worked out.

In view of the range of wall rock compositions in geothermal areas, it is important, when determining long term compatibility of cementing materials, to explore a wider range of potential compositions, which could provide the necessary physical and mechanical properties, be technologically feasible for emplacement, and be durable in the chemical and thermal environment. The two major "systems" considered are CaO-Al2O3-SiO2-H2O (using a broader compositional range than for normal oil well cements) and CaO-MgO-SiO2-H2O. Work is in progress also to extend the above systems to progress also to extend the above systems to greater complexities through use of other additives.

Recent studies of high temperature hydrothermal cements investigated the strength, various physical properties, and kinetics of reaction of physical properties, and kinetics of reaction of some conventional as well as special cements, for the most part limited to a maximum temperature of ca 250 degrees C. While Portland cement compositions, pure calcium silicate compositions or high-CaO calcium aluminosilicate compositions have been rather extensively investigated, relatively little study has been made of compositions intermediate between high-alumina cements and silica. One compositional area within the CaO-Al2O3-SiO2-H2O system appeared to have considerable potential.

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