12 research outputs found
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