Defining chemical and mechanical alteration of wellbore cement by CO(2)-rich brines is important for predicting the long-term integrity of wellbores in geologic CO(2) environments. We reacted CO(2)-rich brines along a cement-caprock boundary at 60 °C and pCO(2) = 3 MPa using flow-through experiments. The results show that distinct reaction zones form in response to reactions with the brine over the 8-day experiment. Detailed characterization of the crystalline and amorphous phases, and the solution chemistry show that the zones can be modeled as preferential portlandite dissolution in the depleted layer, concurrent calcium silicate hydrate (CSH) alteration to an amorphous zeolite and Ca-carbonate precipitation in the carbonate layer, and carbonate dissolution in the amorphous layer. Chemical reaction altered the mechanical properties of the core lowering the average Young's moduli in the depleted, carbonate, and amorphous layers to approximately 75, 64, and 34% of the unaltered cement, respectively. The decreased elastic modulus of the altered cement reflects an increase in pore space through mineral dissolution and different moduli of the reaction products.

No abstract

No abstract

No abstract

Nominally anhydrous minerals formed deep in the mantle and transported to the Earth’s surface contain tens to hundreds of ppm wt H2O, providing evidence for the presence of dissolved water in the Earth’s interior. Even at these low concentrations, H2O greatly affects the physico-chemical properties of mantle materials, governing planetary dynamics and evolution. The diffusion of hydrogen (H) controls the transport of H2O in the Earth’s upper mantle, but is not fully understood for olivine ((Mg, Fe)2SiO4) the most abundant mineral in this region. Here we present new hydrogen self-diffusion coefficients in natural olivine single crystals that were determined at upper mantle conditions (2 GPa and 750–900 °C). Hydrogen self-diffusion is highly anisotropic, with values at 900 °C of 10−10.9, 10−12.8 and 10−11.9 m2/s along [100], [010] and [001] directions, respectively. Combined with the Nernst-Einstein relation, these diffusion results constrain the contribution of H to the electrical conductivity of olivine to be σH = 102.12S/m·CH2O·exp−187kJ/mol/(RT). Comparisons between the model presented in this study and magnetotelluric measurements suggest that plausible H2O concentrations in the upper mantle (≤250 ppm wt) can account for high electrical conductivity values (10−2–10−1 S/m) observed in the asthenosphere.

[1] The electrical conductivity (s) was measured for a single crystal of San Carlos olivine (Fo 89.1 ) for all three principal orientations over oxygen fugacities 10 À7 < f O 2 < 10 1 Pa at 1100, 1200, and 1300°C. Fe-doped Pt electrodes were used in conjunction with a conservative range of f O 2 , T, and time to reduce Fe loss resulting in data that is $0.15 log units higher in conductivity than previous studies. At

[1] Knowledge about hydrogen self diffusion (D H ) is critical for determining mantle hydrogen distribution and understanding point defects. Also, chemical diffusion of hydrogen in olivine, such as redox exchange with polarons (D Redox ), depends on D H . In this study deuterium 2 H was exchanged into hydrogen 1 H saturated single crystals of San Carlos olivine between 750 and 900°C at 2 GPa. We measured and fit the resulting 2 H profiles to obtain D H,[100] = 10 (À4.9AE1.4) *e (À140AE30kJ/mol)/(RT) m 2 /s, which is $1 log unit lower than D Redox,[100] , with similar activation enthalpy H a . By comparing these two diffusion coefficients, we estimate the small polaron diffusion coefficient. Additionally, we estimate D H in the [010] and [001] orientations, demonstrating that D H is highly anisotropic in olivine. These D H values were used with the Nernst-Einstein relation to estimate the electrical conductivity by hydrogen in olivine (s H = 10 1.1 *e (À130kJ/mol)/(RT) S/m for 10 À2 wt % H 2 O) that is lower in magnitude than previous measurements. Our results suggest that hydrogen alone cannot account for high electrical conductivity anomalies observed at asthenospheric depths ($10 À2 to $10 À1 S/m). The maximum anisotropic variation of D H and s H in olivine is $2 log units between 750 and 900°C and increases when extrapolated to higher temperature ($3.3 at 1400°C). Anisotropy observed in the mantle may indicate substantial amounts of hydrogen in olivine with lattice-preferred orientation.

[1] Electromagnetic (EM) remote-sensing techniques are demonstrated to be sensitive to gas hydrate concentration and distribution and complement other resource assessment techniques, particularly seismic methods. To fully utilize EM results requires knowledge of the electrical properties of individual phases and mixing relations, yet little is known about the electrical properties of gas hydrates. We developed a pressure cell to synthesize gas hydrate while simultaneously measuring in situ frequency-dependent electrical conductivity (s). Synthesis of methane (CH 4 ) hydrate was verified by thermal monitoring and by post run cryogenic scanning electron microscope imaging. Impedance spectra (20 Hz to 2 MHz) were collected before and after synthesis of polycrystalline CH 4 hydrate from polycrystalline ice and used to calculate s. We determined the s of CH 4 hydrate to be 5 × 10 −5 S/m at 0°C with activation energy (E a ) of 30.6 kJ/mol (−15 to 15°C). After dissociation back into ice, s measurements of samples increased by a factor of ∼4 and E a increased by ∼50%, similar to the starting ice samples.