Geothermal well cementing

Sugama, T., and Garciello, N. (1996) Sodium metasilicate-modified lightweight high alumina cements for use as geothermal well-cementing materials. Advanced Cement Based Materials 3,45-53. 

L. E. Kukacka and T. Sugama. Lightweight C02-resistant cements for geothermal well completions. Brookhaven Nat Lab Rep BNL-60326, Brookhaven National Lab, 1994. 

The latter reaction can form long chain phosphates, where n is theoretically infinite. Being formed by heat treatments, these phosphates are excellent candidates for high-temperature ceramics and glasses. Because the subject of this volume is low-temperature ceramics, we will not discuss the condensed phosphates in detail, except in one case in Chapter 15, where cements for geothermal wells are discussed with sodium metaphosphate. However, bear in mind that CBPCs can be precursors to high temperature phosphates and glasses. For this reason, as we have seen in the literamre survey presented in Chapter 3, early interest in CBPCs was the formation of refractory shapes at room temperature, which were then fired to produce the final refractory components. 

Sugama and Allan [5] used calcium aluminates (tricalcium aluminate, C -A, monocalcium aluminate, C A, or calcium dialuminate, C A2) as the cation donors and reacted them with an ammonium polyphosphate fertilizer solution and formed quicksetting cements. The purpose of this study was to develop cements that are not affected by the CO2 environment and are useful as downhole cements in geothermal wells (see Chapter 15). The composition of the fertilizer was 11.1 wt% N as ammonia, 37.0 wt% P2O3, 50.79 wt% water, and the rest trace elements. Differential scanning calorimetry (DSC) showed that the reaction rates of the three minerals are in the decreasing order ... 

T. Sugama, N.R. Carciello, T.M. Nayberg, and L.E. Brothers, Mullite microsphere-filled lightweight calcium phosphate cement slurries for geothermal wells setting and properties, Cem. Cone. Res., 25 [6] (1995) 1305-1310. 

BNL, Halliburton Energy Services, and Unocal Corp. have developed cements suitable for geothermal wells with this formulation [11]. The brand name of this cement is ThermaLock. This cement was tested successfully in Unocal s geothermal well in Sumatra, Indonesia, and its first use has been reported by Japan Petroleum Exploration Company for completion of geothermal wells in Kyushu, Japan. 

The density of such cements is in the range of 15-17 lb per gallon. Such high-density cements tend to fracture the formations in geothermal wells in which the cement gets lost. 

Oil well cements must usually perform at elevated temperatures and pressures, both of which increase with increasing depth. The maximum temperature encountered at the bottom of deep wells may reach 250°C, and may even exceed 300°C in geothermal wells. Under these conditions the temperature of the sluny during pumping may reach 180°C (bottom hole static temperature). The pressure to which the cement sluny is exposed is equal to the hydrostatic load plus the pumping pressure, and may reach 150 MPa. 

A sodium metasilicate modified high-alumina cement may be used in geothermal wells at temperatures up to 300°C (Sugama and Garciello, 19%). Sodium calcium silicate hydrate and boehmite are formed as products of reaction in the hydrothermal reaction. 

Phosphate-bonded high-alumina cement is reported to be useful for special purposes [28]. Increase in strength and heat resistance is claimed with P2O5 addition [29]. Special cements based on calcium aluminate and sodium polyphosphate, (NaP03> , which set and harden at 50 300°C, have recently been developed for use in geothermal wells [30]. 

Geothermal cements are also employed to fix the steel wellbore casing in place and tie it to the surrounding rock (8). These are prepared as slurries of Portland cement (qv) in water and pumped into place. Additional components such as silica flour, perlite, and bentonite clay are often added to modify the flow properties and stability of the cement, and a retarder is usually added to the mixture to assure that the cement does not set up prematurely. Cements must bond well to both steel and rock, be noncorrosive, and water impermeable after setting. In hydrothermal applications, temperature stability is critical. Temperature cycling of wellbores as a result of an intermittent production schedule can cause rupture of the cement, leading to movement and, ultimately, failure of the wellbore casing. 

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