Geomechanical stability of the caprock during CO2 sequestration in deep saline aquifers

8 páginas, 5 figuras.Sequestration of carbon dioxide (CO2) in deep saline aquifers has emerged as a mitigation strategy for reducing greenhouse gas emissions to the atmosphere. The large amounts of supercritical CO2 that need to be injected into deep saline aquifers may cause large fluid pressure buildup. The resulting overpressure will produce changes in the effective stress field. This will deform the rock and may promote reactivation of sealed fractures or the creation of new ones in the caprock seal, which could lead to escape paths for CO2. To understand these coupled hydromechanical phenomena, we model an axisymmetric horizontal aquifer-caprock system. We study plastic strain propagation patterns using a viscoplastic approach. Simulations illustrate that plastic strain may propagate through the whole thickness of the caprock if horizontal stress is lower than vertical stress. In contrast, plastic strain concentrates in the contact between the aquifer and the caprock if horizontal stress is larger than vertical stress. Aquifers that present a low-permeability boundary experience an additional fluid pressure increase once the pressure buildup cone reaches the outer boundary. However, fluid pressure does not evolve uniformly in the aquifer. While it increases in the low-permeability boundary, it drops in the vicinity of the injection well because of the lower viscosity of CO2. Thus, caprock stability does not get worse in semi-closed aquifers compared to open aquifers. Overall, the caprock acts as a plate that bends because of pressure buildup, producing a horizontal extension of the upper part of the caprock. This implies a vertical compression of this zone, which may produce settlements instead of uplift in low-permeability (k≤10-18 m2) caprocks at early times of injection.V.V. would like to acknowledge the Spanish Ministry of Science and Innovation (MIC) for financial support through the “Formación de Profesorado Universitario” program. V.V. also wishes to acknowledge the “Colegio de Ingenieros de Caminos, Canales y Puertos – Catalunya” for their financial support. This project has been funded by the Spanish Ministry of Science and Innovation through the project CIUDEN (Ref.: 030102080014), and through the MUSTANG project, from the European Community’s Seventh Framework Programme FP7/2007-2013 under grant agreement nº 227286.Peer reviewe

Transition of Cellulose Crystalline Structure and Surface Morphology of Biomass as a Function of Ionic Liquid Pretreatment and Its Relation to Enzymatic Hydrolysis

Cellulose is inherently resistant to breakdown, and the native crystalline structure (cellulose I) of cellulose is considered to be one of themajor factors limiting its potential in terms of cost-competitive lignocellulosic biofuel production. Here we report the impact of ionic liquid pretreatment on the cellulose crystalline structure in different feedstocks, including microcrystalline cellulose (Avicel), switchgrass (Panicum virgatum), pine (Pinus radiata), and eucalyptus (Eucalyptus globulus), and its influence on cellulose hydrolysis kinetics of the resultant biomass. These feedstocks were pretreated using 1-ethyl-3-methyl imidazolium acetate ([C2mim][OAc]) at 120 and 160°C for 1, 3, 6, and 12 h. The influence of the pretreatment conditions on the cellulose crystalline structure was analyzed by X-ray diffraction (XRD).On a larger length scale, the impact of ionic liquid pretreatment on the surface roughness of the biomass was determined by small-angle neutron scattering (SANS). Pretreatment resulted in a loss of native cellulose crystalline structure. However, the transformation processes were distinctly different for Avicel and for the biomass samples. For Avicel, a transformation to cellulose II occurred for all processing conditions. For the biomass samples, the data suggest that pretreatment formost conditions resulted in an expanded cellulose I lattice. For switchgrass, first evidence of cellulose II only occurred after 12 h of pretreatment at 120°C. For eucalyptus, first evidence of cellulose II required more intense pretreatment (3 h at 160°C). For pine, no clear evidence of cellulose II contentwas detected for the most intense pretreatment conditions of this study (12 h at 160°C). Interestingly, the rate of enzymatic hydrolysis of Avicel was slightly lower for pretreatment at 160°C compared with pretreatment at 120°C. For the biomass samples, the hydrolysis rate was much greater for pretreatment at 160°C compared with pretreatment at 120°C. The result for Avicel can be explained by more complete conversion to cellulose II upon precipitation after pretreatment at 160°C. By comparison, the result for the biomass samples suggests that another factor, likely lignincarbohydrate complexes, also impacts the rate of cellulose hydrolysis in addition to cellulose crystallinity

Monitoring phase behavior of sub- and supercritical CO2 confined in porous fractal silica with 85% porosity

Phase behavior of CO2 confined in porous fractal silica with volume fraction of SiO2 φs = 0.15 was investigated using small-angle neutron scattering (SANS) and ultrasmall-angle neutron scattering (USANS) techniques. The range of fluid densities (0<(FCO2)bulk<0.977 g/cm3) and temperatures (T=22 °C, 35 and 60 °C) corresponded to gaseous, liquid, near critical and supercritical conditions of the bulk fluid. The results revealed formation of a dense adsorbed phase in small pores with sizes D<40 A° at all temperatures. At low pressure (P <55 bar, (FCO2)bulk <0.2 g/cm3) the average fluid density in pores may exceed the density of bulk fluid by a factor up to 6.5 at T=22 °C. This “enrichment factor” gradually decreases with temperature, however significant fluid densification in small pores still exists at temperature T=60°C, i.e., far above the liquid-gas critical temperature of bulk CO2 (TC=31.1 °C). Larger pores are only partially filled with liquid-like adsorbed layer which coexists with unadsorbed fluid in the pore core. With increasing pressure, all pores become uniformly filled with the fluid, showing no measurable enrichment or depletion of the porous matrix with CO2

Small Angle Neutron Scattering Study of Conformation of Oligo(ethylene glycol)-Grafted Polystyrene in Dilute Solutions: Effect of the Backbone Length

The conformation and clusterization of comblike polymers of polystyrene densely grafted with oligo(ethylene glycol) (OEG) side chains in 1.0 wt % solutions of D2O, toluene-d8, and methanol-d4 was investigated as a function of the degree of polymerization (DP) of the backbone by small angle neutron scattering (SANS). Each side chain had four EG repeat units, and the DP of the polystyrene backbone varied from 8 to 85. The global conformation of the polymers in toluene and methanol was shown to assume ellipsoidal, rigid cylindrical, or wormlike morphologies with increasing DP of the polystyrene backbone. At the same time, in D2O, the polymer conformation was described by the form factor of rigid cylinders. The second viral coefficient A2 was measured for the polymer with a DP of 85 in all three solvents, and the solvent quality of toluene, methanol, and D2O was identified to be good, marginal, and poor, respectively, for this polymer. Because of a poor solvent quality, the PS backbone (DP ) 85) is partially collapsed in D2O, whereas it is moderately expanded in toluene and methanol. Polymers with a DP of 8 were found to form clusters in all three solvents, with the characteristic size between 100 and 200 Å and a fractal dimension of 2. With the increase in the DP, the clusters diminished in D2O and completely disappeared in toluene and methanol. This observation suggests that the clusterization of these short side-chain polymers is caused by end-group and hydrogen bonding interactions between different chains

When using small samples to evaluate hydrocarbon reservoirs, proceed with caution

Small-angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) with contrast matching techniques (Melnichenko and others, 2012) were used to investigate size distribution and gas accessibility in pores in an approximately 10.6 cm long Mississippian \ud Barnett Shale butt core from the Fort Worth Basin, \ud Texas, USA. SANS and USANS measurements record scattering from all pores, both open and closed, in the size range 10nm - ~10 μ. The techniques can also be used to determine the material that contains pores and the number of pores as a function of size. By injecting deuterated methane gas (CD4) at contrast matching pressure it is possible to distinguish which pores are accessible, or open, to fluids and which ones are not

Association and Structure of Thermosensitive Comblike Block Copolymers in Aqueous Solutions

The structures and association properties of thermosensitive block copolymers of poly(methoxyoligo( ethylene glycol) norbornenyl esters) in D2O were investigated by small angle neutron scattering (SANS). Each block is a comblike polymer with a polynorbornene (PNB) backbone and oligo ethylene glycol (OEG) side chains (one side chain per NB repeat unit). The chemical formula of the block copolymer is (OEG3NB) 79- (OEG6.6NB) 67, where subscripts represent the degree of polymerization (DP) of OEG and NB in each block. The polymer concentration was fixed at 2.0 wt % and the structural changes were investigated over a temperature range between 25 and 68°C. It was found that at room temperature polymers associate to form micelles with a spherical core formed by the block (OEG3NB) 79 and corona formed by the block (OEG6.6NB) 67 and that the shape of the polymer in the corona could be described by the form factor of rigid cylinders. At elevated temperatures, the aggregation number increased and the micelles became more compact. At temperatures around the cloud point temperature (CPT) T ) 60 °C a correlation peak started to appear and became pronounced at 68 °C due to the formation of a partially ordered structure with a correlation length ∼349 Å

Transition of Cellulose Crystalline Structure and Surface Morphology of Biomass as a Function of Ionic Liquid Pretreatment and Its Relation to Enzymatic Hydrolysis

Cellulose is inherently resistant to breakdown, and the native crystalline structure (cellulose I) of cellulose is considered to be one of themajor factors limiting its potential in terms of cost-competitive lignocellulosic biofuel production. Here we report the impact of ionic liquid pretreatment on the cellulose crystalline structure in different feedstocks, including microcrystalline cellulose (Avicel), switchgrass (Panicum virgatum), pine (Pinus radiata), and eucalyptus (Eucalyptus globulus), and its influence on cellulose hydrolysis kinetics of the resultant biomass. These feedstocks were pretreated using 1-ethyl-3-methyl imidazolium acetate ([C2mim][OAc]) at 120 and 160°C for 1, 3, 6, and 12 h. The influence of the pretreatment conditions on the cellulose crystalline structure was analyzed by X-ray diffraction (XRD).On a larger length scale, the impact of ionic liquid pretreatment on the surface roughness of the biomass was determined by small-angle neutron scattering (SANS). Pretreatment resulted in a loss of native cellulose crystalline structure. However, the transformation processes were distinctly different for Avicel and for the biomass samples. For Avicel, a transformation to cellulose II occurred for all processing conditions. For the biomass samples, the data suggest that pretreatment formost conditions resulted in an expanded cellulose I lattice. For switchgrass, first evidence of cellulose II only occurred after 12 h of pretreatment at 120°C. For eucalyptus, first evidence of cellulose II required more intense pretreatment (3 h at 160°C). For pine, no clear evidence of cellulose II contentwas detected for the most intense pretreatment conditions of this study (12 h at 160°C). Interestingly, the rate of enzymatic hydrolysis of Avicel was slightly lower for pretreatment at 160°C compared with pretreatment at 120°C. For the biomass samples, the hydrolysis rate was much greater for pretreatment at 160°C compared with pretreatment at 120°C. The result for Avicel can be explained by more complete conversion to cellulose II upon precipitation after pretreatment at 160°C. By comparison, the result for the biomass samples suggests that another factor, likely lignincarbohydrate complexes, also impacts the rate of cellulose hydrolysis in addition to cellulose crystallinity