J. William Carey

Analysis and performance of oil well cement with 30 years of CO2 exposure from the SACROC Unit, West Texas, USA

Abstract A core sample including casing, cement, and shale caprock was obtained from a 30-year old CO2-flooding operation at the SACROC Unit, located in West Texas. The core was investigated as part of a program to evaluate the integrity of Portland-cement based wellbore systems in CO2-sequestration environments. The recovered cement had air permeabilities in the tenth of a milliDarcy range and thus retained its capacity to prevent significant flow of CO2. There was evidence, however, for CO2 migration along both the casing–cement and cement–shale interfaces. A 0.1–0.3 cm thick carbonate precipitate occurs adjacent to the casing. The CO2 producing this deposit may have traveled up the casing wall or may have infiltrated through the casing threads or points of corrosion. The cement in contact with the shale (0.1–1 cm thick) was heavily carbonated to an assemblage of calcite, aragonite, vaterite, and amorphous alumino-silica residue and was transformed to a distinctive orange color. The CO2 causing this reaction originated by migration along the cement–shale interface where the presence of shale fragments (filter cake) may have provided a fluid pathway. The integrity of the casing–cement and cement–shale interfaces appears to be the most important issue in the performance of wellbore systems in a CO2 sequestration reservoir.

Magnesium sulphate salts and the history of water on Mars

Recent reports of ∼30 wt% of sulphate within saline sediments on Mars—probably occurring in hydrated form—suggest a role for sulphates in accounting for equatorial H2O observed in a global survey by the Odyssey spacecraft. Among salt hydrates likely to be present, those of the MgSO4·nH2O series have many hydration states. Here we report the exposure of several of these phases to varied temperature, pressure and humidity to constrain their possible H2O contents under martian surface conditions. We found that crystalline structure and H2O content are dependent on temperature–pressure history, that an amorphous hydrated phase with slow dehydration kinetics forms at <1% relative humidity, and that equilibrium calculations may not reflect the true H2O-bearing potential of martian soils. Mg sulphate salts can retain sufficient H2O to explain a portion of the Odyssey observations. Because phases in the MgSO4·nH2O system are sensitive to temperature and humidity, they can reveal much about the history of water on Mars. However, their ease of transformation implies that salt hydrates collected on Mars will not be returned to Earth unmodified, and that accurate in situ analysis is imperative.

Stability of hydrous minerals on the martian surface

The presence of water-bearing minerals on Mars has long been discussed, but little or no data exist showing that minerals such as smectites and zeolites may be present on the surface in a hydrated state (i.e., that they could contain H 2O molecules in their interlayer or extraframework sites, respectively). We have analyzed experimental thermodynamic and X-ray powder diffraction data for smectite and the most common terrestrial zeolite, clinoptilolite, to evaluate the state of hydration of these minerals under martian surface conditions. Thermodynamic data for clinoptilolite show that water molecules in its extra-framework sites are held very strongly, with enthalpies of dehydration for Ca-clinoptilolite up to three times greater than that for liquid water. Using these data, we calculated the Gibbs free energy of hydration of clinoptilolite and smectite as a function of temperature and pressure. The calculations demonstrate that these minerals would indeed be hydrated under the very low-P (H2O) conditions existing on Mars, a reflection of their high affinities for H 2O. These calculations assuming the partial pressure of H2O and the temperature range expected on Mars suggest that, if present on the surface, zeolites and Ca-smectites could also play a role in affecting the diurnal variations in martian atmospheric H 2O because their calculated water contents vary considerably over daily martian temperature ranges. The open crystal structure of clinoptilolite and existing hydration and kinetic data suggest that hydration/dehydration are not kinetically limited. Based on these calculations, it is possible that hydrated zeolites and clay minerals may explain some of the recent observations of significant amounts of hydrogen not attributable to water ice at martian mid-latitudes.

The cross-scale science of CO2 capture and storage: from pore scale to regional scale

We describe state-of-the-art science and technology related to modeling of CO2 capture and storage (CCS) at four different process scales: pore, reservoir, site, and region scale. We present novel research at each scale to demonstrate why each scale is important for a comprehensive understanding of CCS. Further, we illustrate research linking adjacent process scales, such that critical information is passed from one process scale to the next adjacent scale. We demonstrate this cross-scale approach using real world CO2 capture and storage data, including a scenario managing CO2 emissions from a large U.S. electric utility. At the pore scale, we present a new method for incorporating pore-scale surface tension effects into a relative permeability model of CO2-brine multiphase flow at the reservoir scale. We benchmark a reduced complexity model for site-scale analysis against a rigorous physics-based reservoir simulator, and include new system level considerations including local site-scale pipeline routing analysis (i.e., reservoir to site scale). We also include costs associated with brine production and treatment at the site scale, a significant issue often overlooked in CCS studies. All models that comprise our total system include parameter uncertainty which leads to results that have ranges of probability. Results suggest that research at one scale is able to inform models at adjacent process scales, and that these scale connections can inform policy makers and utility managers of overall system behavior including the impacts of uncertainty.

Equilibrium in the clinoptilolite-H2O system

Abstract A thermodynamic formulation for the sorption of H2O in clinoptilolite has been obtained from analysis of equilibrium data collected by thermogravimetry on near end- member Ca-, Na-, and K-exchanged natural clinoptilolite (Fish Creek Mountains, Nevada). Temperature and pressure of the experiments ranged from 25 to 250 °C and 0.2 to 35 mbar H2O vapor pressure. Equilibrium of three clinoptilolite species was successfully formulated with the following expression for the Gibbs free energy of hydration as a function of temperature and pressure: ΔμHy/T = Δμ0Hy/T0 + ΔH̅⁰Hy(1/T - 1/T0) - 3R[ln(T/T0) + (T0/T - 1)] + R ln[θ/(1 - θ)P] +W1/T·θ+W2/T· θ² where R is the gas constant, P is the vapor pressure of H2O, W1 and W2 are the excess mixing parameters, and 6 is the ratio H2O/(maximum H2O) with maximum water contents for the K, Na, and Ca end-members of 13.49, 15.68, and 16.25 wt%, respectively. The molar Gibbs free energy of hydration for calcium, sodium, and potassium clinoptilolite is -36.13 ± 3.02, -29.68 ± 3.77, and -25.53 ± 1.37 kJ/mol H2O, respectively. The molar enthalpy of hydration for these phases is -76.92 ± 2.88, -74.19 ± 3.46, and -67.78 ± 1.25 kJ/mol H2O. The thermodynamic formulation is applied to the occurrence of clinoptilolite at Yucca Mountain, Nevada, where the proposed emplacement of nuclear waste would lead to heating of clinoptilolite-bearing tuffs. Rock units with abundant clinoptilolite (or by analogy other hydrous phases) would remain significantly cooler than units with anhydrous minerals and would evolve a substantial volume of water.

Calorimetric measurement of the enthalpy of hydration of clinoptilolite

The enthalpy of hydration of natural clinoptilolite was determined by isothermal immersion calorimetry on Ca-, Na- and K-exchanged clinoptilolite (Fish Creek Mountains, Nevada). Heats of immersion of clinoptilolite were determined at initial H2O contents ranging from θ = 0.02 to 0.85 (where θ is the ratio [H2O content]/[maximum H2O content]). The heat of immersion (liquid H2O reference state) of Ca-clinoptilolite ranged from -7.5 (θ = 0.87) to -25.7 kJ/mol-H2O (θ = 0.19); values for Na-clinoptilolite ranged from -6.3 (θ = 0.85) to -21.8 kJ/mol-H2O (θ = 0.11); and values for K-clinoptilolite ranged from -7.7 (θ = 0.80) to -24.6 kJ/mol-H2O (θ = 0.02). Linear regression of the calorimetric data provided the following values for the complete heat of immersion (from θ = 0): Ca-clinoptilolite, -30.3 ± 2.0; Na-clinoptilolite, -23.4 ± 0.6; and K-clinoptilolite, -22.4 ± 0.8 kJ/mol-H2O.The heat of immersion measurements were compared with the enthalpy of hydration results of Carey and Bish (1996) determined in a thermogravimetric study of the same samples. The heat of immersion data are similar but of smaller magnitude than the values of enthalpy of hydration and are believed to be more accurate because they represent direct measurements of this thermodynamic property.The effect of dehydration of clinoptilolite on the thermal evolution of the potential high-level radioactive waste repository at Yucca Mountain was considered by comparing the amount of energy consumed by clinoptilolite dehydration with the amount of energy necessary to heat rocks lacking hydrous minerals. The extra energy consumed on heating clinoptilolite from 25 to 200 °C ranges between 70 and 80% in excess of that required for nondehydrating materials (that is, clinoptilolite acts as a heat sink). These results indicate that accurate thermohydrologic modeling of rock units at Yucca Mountain should consider the thermal effect of dehydration/hydration processes in clinoptilolite and other hydrous minerals, in addition to the water produced/adsorbed during heating/cooling.

Quantification of unsaturated-zone alteration and cation exchange in zeolitized tuffs at Yucca Mountain, Nevada, USA

Abstract Zeolitized horizons in the unsaturated zone (UZ) at Yucca Mountain, Nevada, USA, are an important component in concepts for a high-level nuclear waste repository at this site. The use of combined quantitative X-ray diffraction and geochemical analysis allows measurement of the chemical changes that accompanied open-system zeolitization at Yucca Mountain. This approach also provides measures of the extent of chemical migration that has occurred in these horizons as a result of subsequent cation exchange. Mass-balance analysis of zeolitized horizons with extensive cation exchange (drill hole UZ-16) and with only minimal cation exchange (drill hole SD-9) shows that Al is essentially immobile. Although zeolitization occurred in an open system, the mass transfer of constituents other than water is relatively small in initial zeolitization, in contrast to the larger scales of cation exchange that can occur after zeolites have formed. Cation exchange in the clinoptilolite ± mordenite zeolitized horizons is seen in downward-diminishing concentration gradients of Ca, Mg, and Sr exchanged for Na and (to lesser extent) K. Comparison with data from drill hole SD-7, which has multiple zeolitized horizons above the water table, shows that the upper horizons accumulate Ca, Mg, and Sr to such an extent that transport of these elements to the deepest UZ zeolitized horizon can be blocked. Quantitative analysis of zeolite formation yields insight into processes that are implied from laboratory studies and modeling efforts but are otherwise unverified at the site. Such analysis also yields information not provided by or contradicted by some models of flow and transport. The results include the following: (1) evidence of effective downward flow through zeolitic horizons despite the low permeability of these horizons, (2) evidence that alkaline-earth elements accumulated by zeolites are mostly derived from eolian materials in surface soils, (3) validation of the very effective operation of unsaturated zeolitic horizons as cation-exchange barriers, (4) independent support of models that indicate average water infiltration rates of ∼5 mm/yr over the past 10 Ma, and (5) evidence that the presence or absence of cation-exchange profiles can be used to identify those portions of the site where transport through the UZ is concentrated. This last point is relevant to repository design because a knowledge of where transport takes place can be used to advantage in defining the boundaries of a repository such that high-flux portions of the site can be avoided.

A simple environmentally friendly, and chemically specific method for the identification and evaluation of the alkali-silica reaction

Abstract A rapid, dual staining method is described whereby reaction products associated with the alkali-silica reaction (ASR) are readily identified by their pink or yellow color following treatment in the laboratory or field. The method is based on both the compositional and physical characteristics of the ASR gel; hence, it provides greater information than non-chemical-specific techniques (such as the uranyl acetate method). In addition, the chemicals used in the staining method pose minimal health risks and are environmentally benign.

Hydrogen-bonded water in laumontite II: Experimental determination of site-specific thermodynamic properties of hydration of the W1 and W5 sites

Abstract Isothermal vapor sorption experiments under controlled partial pressures of H2O (between 0.1 and 30 mbar, at 23.4, 30.1, 49.5, 64.5, and 79.3 °C) and liquid water immersion calorimetry experiments at 25.0 °C were conducted to determine standard molar thermodynamic properties of hydration of the W1 and W5 sites in laumontite that host hydrogen-bonded water. A Langmuir adsorption model was used in a thermodynamic analysis of the isothermal adsorption data for the W5 site together with a symmetrical regular solution model. Resulting values for the standard molar Gibbs energy and entropy of hydration of the W5 site relative to liquid water are -8430 ± 113 J/mol and -16.7 ± 2.1 J/(mol·K), respectively, and the Margules parameter, WG, is 1590 ± 63 J/mol. The standard enthalpy of hydration of the W1 site was determined by liquid-water immersion calorimetry experiments on laumontite containing vacant W1 and fully occupied W5, W2, and W8 sites. Discontinuous hydration and dehydration of W1 at 23.4 ± 0.7 °C and 24 ± 1 mbar PH₂O was used to constrain the molar Gibbs energy of hydration of this site. Resulting values for standard molar Gibbs energy of hydration and enthalpy of W1 relative to liquid water are -380 ± 170 and -8800 ± 1150 J/ mol, respectively. Isothermal adsorption at 23.4 °C and isobaric thermogravimetric experiments indicate that during dehydration of W1, only 0.83 moles of water are released from the crystal structure and 0.17 moles are relocated to a disordered site that has energetic properties similar to the W8 site. Calculations using the thermodynamic data determined in this study indicate that the water content of laumontite in equilibrium with liquid water ranges from ~4.5 H2O per 12 framework O atoms at room temperature and one bar pressure to ~3.5 H2O at 250 °C and at liquid-vapor saturation pressure for water.

High-stress triaxial direct-shear fracturing of Utica shale and in situ X-ray microtomography with permeability measurement: SHALE FRACTURE, μCT, AND PERMEABILITY

The challenge of characterizing subsurface fluid flow has motivated extensive laboratory studies, yet fluid-flow through rock specimens in which fractures are created and maintained at high-stress conditions remains under-investigated at this time. The studies of this type that do exist do not include in situ fracture geometry measurements acquired at stressed conditions, which would be beneficial for interpreting the flow behavior. Therefore, this study investigates the apparent permeability induced by direct-shear fracture stimulation through Utica shale (a shale gas resource and potential caprock material) at high triaxial-stress confinement and for the first time relates these values to simultaneously acquired in situ X-ray radiography and microtomography images. Change in fracture geometry and apparent permeability was also investigated at additional reduced stress states. Finite element and combined finite discrete element modeling were used to evaluate the in situ observed fracturing process. Results from this study indicate that the increase in apparent permeability through fractures created at high-stress (22.2 MPa) was minimal relative to the intact rock (less than 1 order of magnitude increase) while fractures created at low stress (3.4 MPa) were significantly more permeable (2 to 4 orders of magnitude increase). This study demonstrates the benefit of in situ X-ray observation coupled with apparent permeability measurement to analyze fracture creation in the subsurface. Our results show that the permeability induced by fractures through shale at high stress can be minor and therefore favorable in application to CO2 sequestration caprock integrity but unfavorable for hydrocarbon recovery from unconventional reservoirs.