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

A Model for the Behavior of Battery Separators in Compression at Different Strain/Charge Rates

During charge and discharge of a lithium battery, intercalation of lithium ions into the electrodes can cause their noticeable expansion, compressing the soft separator between them. To assess the role of these effects on the battery performance, it is necessary to know the response of the battery separator under compressive loading. Here we develop a model for predicting the elastic response of a commercial separator immersed in fluid to compression at different strain rates. We show that the response of the separator is determined by combination of viscoelastic behavior of the polymer skeleton and poroelastic behavior, due to the flow of the fluid in the pores. Poroelastic behavior causes effective stiffening of the separator, which increases with the strain rate. For a sample of ca. 1 cm in diameter these effects become pronounced at strain rates ≥10-3 s-1 and have to be taken into account in coupled mechano-electrochemical models for lithium-ion batteries

Mechanical Properties of a Battery Separator under Compression and Tension

Knowledge of the compressive mechanical properties of battery separator membranes is important for understanding their long term performance in battery cells where they are placed under compression. This paper presents a straightforward procedure for measuring the compressive mechanical properties of battery separator membranes using a universal compression testing machine. The compressive mechanical properties of a microporous polypropylene separator are characterized over a range of strain rates and in different fluid environments. These measurements are then compared to measurements of the rate and fluid-dependent mechanical properties of the separator under tension. High strain rate dependence due to viscoelasticity is observed in both tension and compression. An additional rate dependence due to poroelastic effects is observed in compression at high strain rates. A reduction in mechanical properties is observed in DMC solvent environments for both tension and compression, but is found to be less pronounced in compression. The difference in mechanical properties between compression and tension highlight the anisotropic nature of battery separators and the importance of measuring compressive properties in addition to tensile properties