Utilization of Supplementary Cementitious Materials in Geothermal Well Cementing

Abstract

In high temperature geothermal wells commonly conventional cement slurries based on silica blended mixes, are prepared to catch up with the required mechanical properties (compressive strength, thickening time, fluid loss, etc.) for the fresh and hardened cement paste. Typically, 35 to 40 percent silica added to blends to decrease the Ca/Si ratio of cement slurries in order to prevent retrogression in the physical and chemical properties above temperatures of 230 ºF. Ground granulated blast furnace slag (GGBFS) has a Ca/Si ratio lower than 1 and thus silica does not need to be added to cement. The hydration of GGBFS blended cement pastes are improved at elevated temperatures that has vital importance when drilling wells in high temperature conditions. This study presents an experimental study to investigate the applicability of supplementary cementitious materials in cementing of geothermal wells. The materials used in the tests are API Class G cement, silica flour, GGBFS and sodium silicate (water glass). Compressive strength by ultrasonic cement analyzer, fluid loss, and pressurized consistency analyses are conducted. The temperature of analyses and/or the curing temperature of cement pastes conducted are 194 ºF (90 ºC), 248 ºF (120 ºC) and 374 ºF (190 ºC) which correspond to typical low to high temperature geothermal wells. It has been found that adequate amounts of GGBFS in the blend improve physical properties of the cement at high temperatures. Even a replacement of 20 percent GGBFS in the blend improves properties of cement paste. Increase of GGBFS amount in the blend makes the paste progressively more amorphous and foil-like but less fibrillar and crystalline in structure. In addition, utilization of supplementary materials in cement decreases the required amount of chemical additives which also reduces the cost of operation.