Microbially induced calcite precipitation for sealing anhydrite fractures with gouges

Caprock formation forms a natural barrier for geological storage of CO2, nuclear wastes, and hydrocarbon resources. Fault and natural/artificial fractures that crosscut the storage systems represent potential leakage pathways. Sealing of caprock fractures/faults is of great importance to ensure its long-term sealing integrity. In this study, we conduct microbial-induced-calcite-precipitation (MICP) experiments for sealing anhydrite fractures (artificially cut) with gouges. MICP involves a bio-chemical reaction for calcite precipitation using ureolytic microorganism - Sporosarcina pasteurii. The precipitated calcite, which occurs initially from finer pores to larger pores, induces a 10.7% decrease of porosity inside the fracture after the 1st 12 cycles of MICP treatment. After 18-21 cycles of MICP treatment, the fracture permeability of the two fractured core samples effectively decreases by 2-3 orders of magnitude. Our study also indicates that the MICP sealing efficiency could be improved by lowering the injection rate, optimizing fluid chemistry for a better bacteria retention inside the fracture. The study provides a baseline for using MICP technique to seal anhydrite fractures

Transport and fate of ureolytic Sporosarcina pasteurii in saturated sand columns: experiments and modelling

Despite a broad application of ureolytic bacteria in many bioremediation and biocementation processes, very limited studies have reported their transport and retention behaviors under various physical–chemical–biological conditions. In this study, we report transport and retention of Sporosarcina pasteurii in saturated sand, based on a series of column breakthrough experiments under different conditions including ionic strengths (ISs: 0.5 mM–1 M), flow velocity (50, 100, 200 cm/h), bacteria optical density (OD600 = 1.0, 0.48), column length (280 mm, 150 mm), and changes in IS conditions (0.5 M CaCl2 or deionised water). We use a two-site kinetic model, representing (1) attachment on grain surfaces, and (2) straining at crevices and constrictions, to quantify and predict the bacterial attachment and straining. Model parameters were calibrated by tracer (NaCl) breakthrough curves (BTCs) and bacteria BTCs at different IS/velocity conditions. The model was then applied to successfully predict the bacteria BTCs at lower initial bacteria density (OD600 = 0.48) and for shorter column lengths (150 mm). We demonstrated that higher ionic strength (from 0.5 to 1000 mM) dramatically enhanced the retention efficiency of S. pasteurii through an enhancement of attachment (from 9.4 to 69.6%) and straining (from 8.1 to 34.2%), whilst the bacterial survival and the urease activity were unaffected at high IS conditions (500 and 1000 mM NaCl) within 5 h. Increasing flow velocity (from 50 to 200 cm/h) caused a decrease in attachment (from 39.5 to 22.4%) and decrease in straining (from 40.5 to 19.3%) as a result of the increased hydrodynamic shear forces, which tends to reduce the attachment at the secondary minimum and decrease the extent of flow stagnation regions for straining. Lower initial bacteria OD600 (from 1.0 to 0.48) enhanced the attachment (from 31.8 to 40.9%) and the straining (from 22.9 to 42.2%) as a result of reducing the site-blockage effect. In addition, 0.5 M CaCl2 with a stronger IS increased the retention of in the column, whilst deionised water with a lower IS caused bacterial release. These findings provide useful information for a better understanding of the transport and fate of Sporosarcina pasteurii in saturated soil, and can be used to optimise bioaugmentation strategy and cementation efficiency for soil improvement

Cardiovascular mortality risk attributable to ambient temperature in China.

OBJECTIVE: To examine cardiovascular disease (CVD) mortality burden attributable to ambient temperature; to estimate effect modification of this burden by gender, age and education level. METHODS: We obtained daily data on temperature and CVD mortality from 15 Chinese megacities during 2007-2013, including 1,936,116 CVD deaths. A quasi-Poisson regression combined with a distributed lag non-linear model was used to estimate the temperature-mortality association for each city. Then, a multivariate meta-analysis was used to derive the overall effect estimates of temperature at the national level. Attributable fraction of deaths were calculated for cold and heat (ie, temperature below and above minimum-mortality temperatures, MMTs), respectively. The MMT was defined as the specific temperature associated to the lowest mortality risk. RESULTS: The MMT varied from the 70th percentile to the 99th percentile of temperature in 15 cities, centring at 78 at the national level. In total, 17.1% (95% empirical CI 14.4% to 19.1%) of CVD mortality (330,352 deaths) was attributable to ambient temperature, with substantial differences among cities, from 10.1% in Shanghai to 23.7% in Guangzhou. Most of the attributable deaths were due to cold, with a fraction of 15.8% (13.1% to 17.9%) corresponding to 305,902 deaths, compared with 1.3% (1.0% to 1.6%) and 24,450 deaths for heat. CONCLUSIONS: This study emphasises how cold weather is responsible for most part of the temperature-related CVD death burden. Our results may have important implications for the development of policies to reduce CVD mortality from extreme temperatures

Carbonate caprock–brine–carbon dioxide interaction : alteration of hydromechanical properties and implications on carbon dioxide leakage

Caprocks play a crucial role in the geological storage of carbon dioxide (CO2) by preventing its escape and thus trapping it into underlying sequestering reservoirs. An evaluation of interaction-induced alteration of the hydromechanical properties of caprocks is essential to better assess the leaking risk and injection-induced rock instability, thus ensuring a long-term viability of geological CO2 storage. We study the changes in minerals, nanopores, elastic velocities, and mechanical responses of a carbonate caprock caused by rock–water/brine–CO2 interaction (CO2 pressure: ≈12 MPa; 50°C). Before the interaction, the total and accessible porosities are 1.6 and 0.6%, respectively, as characterized by the small-angle neutron scattering (SANS) technique. SANS results show that the total porosity of the carbonate caprock increases, apparently because of rock–brine–CO2 interaction, and the increasing rate rises as brine concentration increases (2.2% for 0 M NaCl, 2.6% for 1 M NaCl, and 2.7% for 4 M NaCl). The increase in total porosity is due to the dissolution of calcite, which tends to enlarge accessible pores (by 0.8 to 1.2%) while slightly decreasing the inaccessible pores (by 0.1–0.2%). Under a CO2–acidified water environment, the compressional-wave (P-wave) and shear-wave (S-wave) velocities (5536.7 and 2699.7 m/s) of a core sample containing natural fractures decrease by 8.5 and 8.1%, respectively, whereas both P- and S-wave velocities (6074.1  and 3858.8 m/s) for an intact sample show only ≈0.5% decreases. The interaction also causes more than 50% degradation of the uniaxial compressive strength for the core sample with natural fractures. X-ray microcomputed tomography experiments on three tiny cores (diameter: 1 mm) after 5-day treatment with CO2 (12 MPa) also show that matrix erosion occurs under CO2–acidified water environment but barely occurs without a direct contact with liquid water. Our study suggests that the hydromechanical properties of carbonate caprocks could evolve over the long-term CO2–brine invasion, and it is critical to monitor the CO2–acidified brine interface for a better and long-term evaluation of the caprock integrity

Meter-scale MICP improvement of medium graded very gravelly sands : lab measurement, transport modelling, mechanical and microstructural analysis

Microbially induced carbonate precipitation (MICP) is a promising biogrouting method for ground improvement. Most studies to date have focused on MICP treatment of uniform clean sands, with few studies having been conducted at large-scale on well-graded field soils more representative of in situ deposits. This study presents a laboratory meter-scale MICP test on medium-graded very gravelly sands consisting of 3.9% fines collected from a quarry. The MICP treatment was conducted in a radial flow cell (diameter: ~1m; thickness: ~0.15 m) with an injection well located at the centre and a constant hydraulic head at the outer boundary to replicate field conditions. Aqueous chemistry of the effluent samples inside the flow cell was continuously monitored, and transport of tracer and bacteria breakthrough in the flow cell, and in separate 1-dimensional columns, was modelled and simulated for a better understanding of the MICP process. The MICP-treated soil was subjected to a series of hydraulic and mechanical tests and microstructural analysis. Transport modelling and effluent sampling monitoring of the electrical conductivity and pH show that there was an overall good delivery and reaction of the bacteria and chemicals in the radial flow cell, but there also existed preferential flow paths due to soil heterogeneity and fines migration, which caused variations in permeability. Interestingly, compared to previous studies, the biocemented core samples with medium-graded angular particles in this study had higher strengths (2.6-7.4 MPa) for a given calcite content (9.2-15.1%) than those in comparable studies treating uniform soils. Scanning Electron Microscopy (SEM) and X-ray computed tomography scans show that this can be attributed to the higher initial density of grain-to-grain contact points in medium-graded sands, and a high grain angularity which resulted in particle interlocking and longer grain-to-grain contact surfaces. Consolidated-drained triaxial compression tests on two samples cored from near the injection well showed peak deviatoric strengths of ~5 MPa under an effective confining stress of 500 kPa, with the clear formation of shear bands during loading. Comparison with the untreated soil showed that MICP treatment tripled the peak deviatoric strength achieved, as well as increasing the stiffness of the material. The study highlights that the formation of preferential flow paths may be a challenge for producing uniform biocementation in field applications of MICP. We propose that successful MICP treatment in heterogeneous soils will require a well-designed and well-executed site investigation programme that can identify, a priori, the geometry of any significant high or low permeability features within the soil body to inform the final MICP treatment strategy

Unraveling high-pressure gas storage mechanisms in shale nanopores through SANS

As storage rocks rather than source rocks, shale reservoirs can potentially serve as energy storehouses for energy security and sequester CO 2in the long term to mitigate climate change. Despite extensive studies investigating the geochemical and geophysical properties of shale and gas adsorption and transport in the shale matrix, limited studies have been devoted to characterizing nanoscale gas storage mechanisms in shale at elevated high pressure. In this study, contrast-matching small-angle neutron scattering (SANS) has been conducted to quantify the gas storage mechanisms and capacity in three shale samples up to elevated high pressure using deuterated methane. The three-phase Porod invariant method is uniquely used to estimate the average scattering length density (SLD) in open pores over the measured pore range, in which open pores, closed pores, and rock matrix are the three phases. The estimated average SLD in open pores is smaller than the SLD of the bulk phase at the pressure between 10 MPa and the contrast-matched point (∼60-70 MPa, which is sample-dependent), while it is higher than that of the bulk at the pressure below 10 MPa and above the contrast-matched pressure, indicating a variation of average adsorbed phase density over the measured pressure range for the measured shale samples. The average adsorbed phase volume could first increase and then decrease with increasing pressure until the high-pressure region. Three essential factors, including the final injection pressure, total organic carbon (TOC), and accessible porosity, could be used to screen a potential targeted shale reservoir and maximize methane storage and long-term CO 2 sequestration

Cardiovascular mortality risk attributable to ambient temperature in China

To examine cardiovascular disease (CVD) mortality burden attributable to ambient temperature; to estimate effect modification of this burden by gender, age and education level.We obtained daily data on temperature and CVD mortality from 15 Chinese megacities during 2007-2013, including 1,936,116 CVD deaths. A quasi-Poisson regression combined with a distributed lag non-linear model was used to estimate the temperature-mortality association for each city. Then, a multivariate meta-analysis was used to derive the overall effect estimates of temperature at the national level. Attributable fraction of deaths were calculated for cold and heat (ie, temperature below and above minimum-mortality temperatures, MMTs), respectively. The MMT was defined as the specific temperature associated to the lowest mortality risk.The MMT varied from the 70th percentile to the 99th percentile of temperature in 15 cities, centring at 78 at the national level. In total, 17.1% (95% empirical CI 14.4% to 19.1%) of CVD mortality (330,352 deaths) was attributable to ambient temperature, with substantial differences among cities, from 10.1% in Shanghai to 23.7% in Guangzhou. Most of the attributable deaths were due to cold, with a fraction of 15.8% (13.1% to 17.9%) corresponding to 305,902 deaths, compared with 1.3% (1.0% to 1.6%) and 24,450 deaths for heat.This study emphasises how cold weather is responsible for most part of the temperature-related CVD death burden. Our results may have important implications for the development of policies to reduce CVD mortality from extreme temperatures