### Research

 Induced seismicity Poroelastic response to fluid injection Recent increase in earthquake occurrence (M>3) is associated closely with human activities, i.e. fluid injection/extraction for modern energy production. Previous studies have showed that some earthquake of magnitude larger than 4 is observed in a basement rock. Some hydrological modeling studies show that faults in the basement play a significant role in inducing earthquakes by acting as conduits for fluid migration. However, the hydrological modeling cannot capture the perturbation in stresses driven by mechanical interaction between rock and pore-filled fluid. In this study, we use poroelastic coupling that will show changes in stress and pore pressure either within/nearby a target reservoir or in a basement far from the reservoir. Reference Chang, K.W. and P. Segall (201x), Injection induced seismicity on basement faults including poroelastic stressing, Journal of Geophysical Research: Solid Earth, In preparation.Chang, K.W. and P. Segall (201x), Induced seismicity driven by fluid injection-extraction, Environmental Science & Technology, In preparation.Chang, K.W. (201x), Attenuation of injection-induced poroelastic stresses into underlying shaley layers, in preparation. Injection-induced overpressure Role of ambient mudrock Carbon dioxide (CO$_2$) storage in deep geological formations can lead to significant reductions in anthropogenic CO$_2$ emissions if large amounts of CO$_2$ can be stored. Estimates of the storage capacity are therefore essential to the evaluation of individual storage sites as well as the feasibility of the technology. One important limitation on the storage capacity is the radius of review, the lateral extent of the pressure perturbation, of the storage project. We show that pressure dissipation into ambient mudrocks retards lateral pressure propagation significantly and therefore increases the storage capacity. For a three-layer model of a reservoir surrounded by thick mudrocks, the far-field pressure is approximated well by a single-phase model. Through dimensional analysis and numerical simulations, we show that the lateral extent of the pressure front follows a power-law that depends on a single dissipation parameter $M\propto\log_{10}(R_kR_SR_l^2)$, where $R_k$ and $R_S$ are the ratios of mudrock to reservoir permeability and specific storage, and $R_l$ is the aspect ratio of the confined pressure plume. Both the coefficient and the exponent of the power-law are sigmoid decreasing functions of $M$. The $M$-values of typical storage sites are in the region where the power-law changes rapidly. The combination of large uncertainty in mudrock properties and the sigmoid shape leads to wide and strongly skewed probability distributions for the predicted radius of review and storage capacity. Therefore, if the lateral extent of the pressure front limits the storage capacity, the determination of the mudrock properties is an important component of the site characterization. Reference Chang, K.W., M.A. Hesse, and J.-P. Nicot (2013), Reduction of lateral pressure propagation due to dissipation into ambient mudrocks during geological carbon dioxide storage, Water Resources Research, 49(5), 2573-2588, doi:10.1002/wrcr.20197 [link]. Chang, K.W., M.A. Hesse, and J.-P. Nicot (2013), Dissipation of overpressure into ambient rocks during CO2 storage (talk), Energy Procedia (GHGT-11), 37, 4457-4464 [link]. Post-injection overpressure The concept of the radius of review has been used to control the underground injection operation. For geological carbon dioxide (CO$_2$) storage, injection-induced overpressure and displacement of formation brine are the most important constraints. A radius of review based on pressure buildup and propagation will provide a more accurate estimate of the storage capacity. Most of geological storage formations are interbeded by mudrocks with high specific storage and low, but finite, permeability, and thus injection-induced overpressure can be dissipated into mudrocks which displaces formation brine into them. Dissipation of overpressure into mudrocks reduces the lateral propagation of post-injection overpressure within a sandstone reservoir. For a simple layered geometry, a radius of review can be defined by a single dissipation parameter $M$ which is a function of the ratios of mudrock to sandstone permeability and specific storage ($R_k$ and $R_S$) and the aspect ratio of the pressure plume ($R_l$). Previous studies of the capacity estimate and monitoring for the stability of the storage formation have focused on the evolution of overpressure during the injection operation. After the end of the injection period, however, overpressure will continue to diffuse throughout the storage formation, and the maximum radius of review will be obtained at which overpressure is attenuated into mudrock enough to reverse the direction of the lateral pressure propagation. Our results show that the maximum radius of review is approximately 3.5 times the radius of review at the end of injection. For the leakage analysis based on a radius of review, the uncertainties in mudrock properties as well as in information of potential leakage pathway are major factors to determine the leakage propability. Therefore, more precise characterization of mudrock properties is required for the accurate estimate of the storage capacity as well as the leakage flux. Reference Chang, K.W. and M.A. Hesse (201x), Radius of review for geological carbon storage in a layered formation, Environmental Science & Technology, In preparation. Multiphase flow and solute transport in heterogeneous geological formations Numerical study of solute-driven exchange flow: Mechanical dispersion and Flow barriers In a layered reservoir intersected by a fault, quasi-steady exchange flow along the fault develops if the upper aquifer contains denser fluid. If the fault permeability is homogeneous, the average number of the quasi-steady plume fingers, $\langle\nu\rangle$, scales with the square root of the Rayleigh number $Ra$ and the exchange rate measured by dimensionless convective flux, the Sherwood number $Sh$, is a linear function of $Ra$. The presence of flow barriers triggers unsteady exchange flow and subsequently controls the growth of the plume fingers. If the barriers dominate the flow system, they create preferential pathways for exchange flow that determine the distribution of the steady fingers, and $\langle\nu\rangle$ converges to a constant value. Wider barriers induce substantial lateral spreading and enhance the efficiency of structural trapping, and reduce the exchange rate that follows a power-law $Sh\propto Ra^n$, where $n<1$ and decreases with increasing barrier length. Reference Chang, K.W. and M.A. Hesse (201x), Scaling regimes in solute driven convection in porous media with dispersion, Journal of Fluid Mechanics, In preparation. Chang, K.W. and M.A. Hesse (201x), Solute driven exchange flow encountering geological barriers in a fault, Advances in Water Resources, In preparation. Experimental study of solute-driven exchange flow Geological formations are crossed by multi-scale fractures and/or faults, and conductive faults may mainly control reservoir performance. Conductive faults are modeled using small grids in a vertical two-dimensional domain to see the multiphase flow exchanges between neighboring medium across the fault or the vertical fluid migration through the fault. A major limitation of this modeling approach is that faults appear as one-dimensional structures in which fluid migration occurs only by counter-current flows. This simplified model cannot capture unstable exchange flows within the fault that will determine the rate of leakage. In three-dimensional models, the fluids can bypass each other and the exchange will be much faster. The larger reservoir volume relative to the fault will allow a quasi-steady exchange flow across the fault before the fluid densities are equalized. We aim to quantify the exchange rate as a function of the fault properties and geometry, the fluid properties, and the type of fluid bypassing. Limitations of geophysical imaging and uncertainty in the fault properties make numerical models difficult to constrain the dynamics of the exchange flow through the fault. Therefore, our experimental study complements the numerical model to understand the dynamics of the unstable exchange flow. This study is motivated by geological CO$_2$ storage in brine-saturated aquifer, but the unstable exchange of multiphase fluids through conductive faults is also important in many other geological and engineering applications, in particular the migration of hydrocarbons in tectonic-driven faulting system or hydraulically developed fractures in unconventional reservoirs. Better understanding of the fluid flow in a faulting system will allow more precise estimate of the reservoir capacity as well as more efficient operation of injection or production wells. Reference Woods, A.W., M.A. Hesse, R. Berkowitz, and K.W. Chang (2015), Multiple steady states in exchange flows across faults and the dissolution of CO2, Journal of Fluid Mechanics, 769, 229-241, doi:10.1017/jfm.2015.100 [link]. Numerical study of multiphase flow near a fault zone The injected CO$_2$ into target formation can continue to migrate through permeable pathways due to geological heterogeneity as well as buoyancy. This movement drives a countercurrent flow of brine leading to increased residual phase trapping. The purpose of this simulation study is to understand the effects of geological structures, especially faults, on the dynamic behavior of the buoyancy-driven CO$_2$ plume and the amount of residual trapping. We studied the behavior of CO$_2$ plumes (speed, direction, saturation at displacement front, residual phase trapping) in 2D and 3D formations with a range of fault properties (conductive vs. sealing, angle relative to dip, distance from initial plume location). Reference Chang, K.W. and S.L. Bryant (2009), The effect of faults on dynamics of CO2 plumes, Energy Procedia (GHGT-9), 1(1), l839-1846 [link]. Chang, K.W., S.E. Minkoff, and S.L. Bryant (2009), Simplified model for CO2 leakage and its attenuation due to geological structures, Energy Procedia (GHGT-9), 1(1), 3453-3460 [link].