• Energy Research

Abstract High temperature geothermal systems in China are mainly located along the Himalayan belt and one of the main problems during production is calcite scaling. This paper presents a quantitative assessment of calcite scaling and possible removal and prevention methods for the Kangding geothermal field in the Western Sichuan Plateau, as an example. Reservoir fluid composition is reconstructed based on geochemical processes that may take place from reservoir to surface. Results show that the fluid is HCO3⋅Cl-Na or Cl⋅HCO3-Na type with a temperature of 259−283 °C. It’s over-saturated with respect to calcite in both surface and reservoir conditions but under-saturated to quartz and amorphous silica, indicating that the calcite scaling will be a problem. For well BH6, the fluid pH is 5.63 at reservoir conditions and the steam fraction at the wellhead is about 6.0 %. Adiabatic boiling calculation indicates that from reservoir to surface conditions, CO32− and CaCO3 concentrations in the fluid keep increasing and the fluid evolves to become over-saturated with respect to calcite and the saturation index is higher than 0.5 and calcite precipitates in the pipeline. The boiling depth is estimated to be about 150 m from the wellhead which can provide a guide for scaling depth determination. The calcite scale quantity is calculated to be 151−300 kg or a thickness of 0–2.94 cm according to the pumping test, consistent with what has been observed. Calcite scale can be removed mechanically or prevented by injecting chemical inhibitor as well as thermodynamic methods (including injecting acid, CO2, cold water and putting the feeding pump below the boiling depth). Counter-measures should be chosen based on the mode of utilization and its cost. For well BH6 that is planned to be used for power generation, chemical inhibition may be the choice.

Abstract Recent publications on the successful mineralisation of carbon dioxide in basalts in Iceland and Washington State, USA, have shown that mineral storage can be a serious alternative to more mainstream geologic carbon storage efforts to lock away permanently carbon dioxide. In this study we look at the pore solution chemistry and mineralogy of basaltic glass and crystalline basalt under post-injection conditions, i.e. after rise of the pH via matrix dissolution and the first phase of carbonate formation. Experimental findings indicate that further precipitation of carbonates under more alkaline conditions is highly dependent on the availability of divalent cations. If the pore water is deficient in divalent cations, smectites and/or zeolites will dominate the secondary mineralogy of the pore space, depending on the basalt matrix. At low carbonate alkalinity no additional secondary carbonates are expected to form meaning the remaining pore space is lost to secondary silicates, irrespective of the basalt matrix. At high carbonate alkalinity, some of this limited storage volume may additionally be occupied by dawsonite −if the Na concentration in the percolating groundwater (brine) is high. Using synthetic seawater as a proxy for the groundwater composition and thus furnishing considerable amounts of divalent cations to the carbonated solution, results in massive precipitation of calcite, magnesite, and other Ca/Mg-carbonates under already moderate carbonate alkalinity. More efficient use of the basaltic storage volume can thus be attained by promoting formation of secondary carbonates compared to the inevitable formation of secondary silicate phases at higher pH. This can be done by ensuring that the pore water does not become depleted in divalent cations, even after carbonate formation. Using seawater as carbonating fluid or injection of CO2 into the basaltic oceanic crust, where saline fluids percolate, can reach this goal. However, such an approach needs sophisticated reactive transport modelling to adjust CO2 injection rates in order to avoid too rapid carbonate deposition and clogging of the pore space too close to the injection well.

The injection of water dissolved CO2 and H2S into basalts into the Nesjavellir geothermal system (Iceland) is to begin in 2022. This study is a pre-injection investigation assessing the likely response of the fluid-rock system to the gas charged water injection. The target aquifer has a temperature of < 200 °C at the injection well, but the temperature increases to ∼300 °C towards the center of the geothermal field where the production wells are located. The aquifer has current in-situ pH values of 6.7–7.7 and CO2 and H2S concentrations of 30.1–1079 and 60.4–505 ppm, respectively. These pre-injection aquifer fluids are saturated with respect to numerous sulfide minerals but undersaturated with respect to the major carbonate minerals. The fluid during the anticipated pilot carbon and sulfur charged water injection is expected to have a temperature of ∼84 °C, a pH of ∼4.9 and dissolved CO2 and H2S concentrations of 1223 and 480 ppm, respectively. Geochemical modelling confirms that the injection of CO2 and H2S charged fluids will dissolve the altered basaltic host rock near the injection well followed by the precipitation of secondary minerals including sulfides and carbonates further from the well. Calculations suggest about 70% and 100%, respectively, of this injected CO2 and H2S will be mineralized between the injection and production wells. The increasing of the CO2 and H2S content of the injection fluid will increase mineralization efficiency if the increased acidity of this fluid increases the mass of basalt dissolution in the subsurface. Carbon, sulfur and helium isotope systematics and abundances imply that large part of CO2 and H2S emitted from the Nesjavellir powerplant, as well as those released from the geothermal fluids naturally originate from magmatic sources. Mass balance considerations suggest that the currently planned dissolved gas injection into the Nesjavellir system will negligibly affect the CO2 and H2S budget of the aquifer. However, efforts to maximize the mineralization efficiency when upscaling this carbon storage system should be made to limit possible increase in reservoir fluid CO2 concentration.

Abstract Mineralization of water dissolved carbon dioxide injected into basaltic rocks occurs within two years in field-scale settings. Here we present the results from a CO2-water-basaltic glass laboratory experiment conducted at 50 °C and 80 bar pressure in a Ti high-pressure column flow reactor. We explore the possible sequence of saturation with Fe-Mg-Ca-carbonate minerals versus Fe-Mg-clay and Ca-zeolite saturation states, which all compete for divalent cations and pore space during injection of CO2 into basaltic rocks. Pure water (initially with atmospheric CO2) – basaltic glass reactions resulted in high pH (9–10) water saturated with respect to Mg-Fe-clays (saponites), Ca-zeolites, and Ca-carbonate. As CO2-charged water (˜20 mM) entered the column and mixed with the high pH water, all the Fe-Mg-Ca-carbonates became temporarily supersaturated along with clays and zeolites. Injected waters with dissolved CO2 reached carbonate mineral saturation within 12 h of fluid-rock interaction. Once the pH of the outflow water stabilized below 6, siderite was the only thermodynamically stable carbonate throughout the injection period, although no physical evidence of its precipitation was found. When CO2 injection stopped while continuing to inject pure water, pH rose rapidly in the outflow and all carbonates became undersaturated, whereas zeolites became more saturated and Mg-Fe-saponites supersaturated. Resuming CO2 injection lowered the pH from >8 to about 6, resulting in an undersaturation of the clays and Na-zeolites. These results along with geochemical modelling underscore the importance of initial pCO2 and pH values to obtain a balance between the formation of carbonates versus clays and zeolites. Moreover, modelling indicates that pauses in CO2 injection while still injecting water can result in enhanced large molar volume Ca-Na-zeolite and Mg-Fe-clay formation that consumes pore space within the rocks.

AbstractThe long-term security of geologic carbon storage is critical to its success and public acceptance. Much of the security risk associated with geological carbon storage stems from its buoyancy. Gaseous and supercritical CO2 are less dense than formation waters, providing a driving force for it to escape back to the surface. This buoyancy can be eliminated by the dissolution of CO2 into water prior to, or during its injection into the subsurface. The dissolution makes it possible to inject into fractured rocks and further enhance mineral storage of CO2 especially if injected into silicate rocks rich in divalent metal cations such as basalts and ultra-mafic rocks. We have demonstrated the dissolution of CO2 into water during its injection into basalt leading to its geologic solubility storage in less than five minutes and potential geologic mineral storage within few years after injection [1–3]. The storage potential of CO2 within basaltic rocks is enormous. All the carbon released from burning of all fossil fuel on Earth, 5000 GtC, can theoretically be stored in basaltic rocks [4].

AbstractHere, we report on the mobility of metals at the early stage of CO2 injection into basalt, before significant precipitation of secondary minerals. Short-lived pulses (50-100hours) of CO2-charged water were injected into a high pressure column flow reactor filled with basaltic glass grains at 22°C, 8MPa of total pressure and a velocity of 0.4cm/min. The residence time of the water within the column ranged from 8 to 10hours. The column was conditioned with pure water, resulting in alkaline outflow (pH ∼9). The pH of the inlet CO2-charged water was ∼3.2, and the lowest pH measured in the column was 4.5, after less than 10hours of water/rock interaction. The dissolved metal concentrations and metals relative mobility increased dramatically during the CO2-pulses; more than 100 times for Sr, Fe, Al, Ca, Ba, Mn, and Mg. Of these elements, all but Al can bind with CO2 to form carbonate minerals. Only the dissolved Al, Fe, Mn and Cr concentrations exceeded allowable drinking water limits. After the CO2-pulses, all of the elemental concentrations decreased close-to or even below what was measured during the conditioning of the column. The pH never reached ∼9 which was the initial pH before CO2-pulses.

Abstract Chemical denudation rates during the 2014–15 Barðarbunga eruption, calculated using river chemical fluxes, increased substantially confirming that volcanic activity and its products such as fresh lava, and acidic volatiles accelerates these rates. Although the long-term net effect of the combined input of volcanic gases and basalt from the eruption appears to be the overall net drawdown of CO 2 , it is found that the rapid release of acid gases to surface waters once the basaltic lava comes in contact with surface waters will lead to a short-term release of CO 2 from these waters.