Fracture Propagation Driven by Fluid Outflow from a Low-permeability Aquifer

Deep saline aquifers are promising geological reservoirs for CO2 sequestration if they do not leak. The absence of leakage is provided by the caprock integrity. However, CO2 injection operations may change the geomechanical stresses and cause fracturing of the caprock. We present a model for the propagation of a fracture in the caprock driven by the outflow of fluid from a low-permeability aquifer. We show that to describe the fracture propagation, it is necessary to solve the pressure diffusion problem in the aquifer. We solve the problem numerically for the two-dimensional domain and show that, after a relatively short time, the solution is close to that of one-dimensional problem, which can be solved analytically. We use the relations derived in the hydraulic fracture literature to relate the the width of the fracture to its length and the flux into it, which allows us to obtain an analytical expression for the fracture length as a function of time. Using these results we predict the propagation of a hypothetical fracture at the In Salah CO2 injection site to be as fast as a typical hydraulic fracture. We also show that the hydrostatic and geostatic effects cause the increase of the driving force for the fracture propagation and, therefore, our solution serves as an estimate from below. Numerical estimates show that if a fracture appears, it is likely that it will become a pathway for CO2 leakage.Comment: 21 page

Effect of CO2 Injection Temperature on Caprock Stability

AbstractDeep saline aquifers are promising candidates for long-term CO2 storage, provided they don’t leak. However, injection of CO2 causes pressure buildup and affects the geomechanical stresses in the caprock. If CO2 is injected at a temperature different from the temperature in the aquifer, additional stresses develop due to thermal expansion/contraction. Our work addresses the question whether these stresses are capable of fracturing the caprock and causing leakage. Using a fully coupled thermo-poromechanical model we simulate 10 years of continuous injection of CO2 at different temperatures. We use the geomechanical parameters for aquifer on Krechba (In Salah, Algeria) including recently published data on initial in situ stresses. We found that when CO2 is injected at temperature 40-50°C the stresses in the caprock become tensile and even overcome the tensile strength causing fracturing of the caprock. After initiation the fractures begin to propagate, driven by high fluid pressure in the reservoir. We estimate the fracture length to be 50 m within the first 10 years of propagation

Interactions of Sarin with Polyelectrolyte Membranes: A Molecular Dynamics Simulation Study

Nanostructured polyelectrolyte membranes (PEMs), which are widely used as permselective diffusion barriers in fuel cell technologies and electrochemical processing, are considered as protective membranes suitable for blocking warfare toxins, including water-soluble nerve agents such as sarin. In this article, we examine the mechanisms of sorption and diffusion of sarin in hydrated PEMs by means of atomistic molecular dynamics simulations. Three PEMs are considered: Nafion, sulfonated polystyrene (sPS) that forms the hydrophilic subphase of segregated sPS–polyolefin block copolymers, and random sPS–polyethylene copolymer. We found that sarin concentrates at the interface between the hydrophilic and hydrophobic subphases of hydrated Nafion acting as a surfactant. In hydrated sPS, where the scale of water–polymer segregation is much smaller (1–2 nm), sarin also interacts favorably with hydrophobic and hydrophilic components. Water diffusion slows as the sarin content increases despite the overall increase in solvent content, which suggests that sarin and water have somewhat different pathways through the segregated membrane. Upon replacement of counterions of monovalent potassium with those of divalent calcium, sarin diffusion slows but remains substantial in all ionomers considered, especially at high sarin concentrations. The behavior of sarin is similar to that of its common simulant, dimethyl methylphosphonate

Role of Liquid vs Vapor Water in the Hydrothermal Degradation of SBA-15

The hydrothermal stability of mesoporous silica is critical for applications including catalytic processing of biofuels due to the presence of significant amounts of water. We have combined neutron diffraction intensity analysis with NLDFT analysis of nitrogen sorption isotherms to characterize the spatial distribution of the secondary pore network in SBA-15 following postcalcination hydrothermal treatment in both liquid and vapor phase water at temperatures from 115 to 155 °C under autogenous pressure. The results are consistent with a degradation mechanism in which silica dissolves from regions of small positive curvature, e.g., near the entrance to the secondary pores, and is redeposited deeper into the framework. Pore volumes decrease fastest for the micropores and more slowly for larger secondary mesopores. Under water treatment at 115 °C, the mesopore diameter increases and the intrawall void fraction decreases significantly. The behavior is similar for steam treatment but occurs more slowly. Differences in the chemical environment and transport limitations are discussed. At higher temperatures of 155 °C, pores in the region surrounding the mesopore are nearly eliminated, trapping water deeper in the matrix, which can be seen with neutron scattering but is inaccessible to nitrogen isotherm measurements