Induced seismicityRecent increase in earthquake occurrence (M>3) is associated closely with human activities, i.e. fluid injection/extraction for modern energy production, wasterwater disposal or geological carbon dioxide (CO2) sequestration. 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. Poroelastic response to fluid injection
The hydrological modeling cannot capture the perturbation in stresses driven by mechanical interaction between rock and pore-filled fluid, which is called as "poroelastic coupling." 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. Assuming the same properties of each sequence in a layered model and the same amount of fluids is injected, we can see the difference in the pore pressure change between uncoupled and coupled models. The coupling increases the magnitude of the pore pressure increase, which is compensated by negative stresses (compression). Seismicity rate estimates including poroelastic coupling
Fluid injection induces seismicity on the basement fault, and the spatiotemporal distribution of the sesimicity rate strongly depends on the fault properties, i.e hydraulic connectivity to the target reservoir and/or fault permeability. Based on Dieterich [1994] a seismicity rate model that relates changes in Coulomb stress to changes in seismicity rate, we estimate the seismicity rate on the basement fault. For hydraulically connected/conductive faults, direct diffusion of pore pressure into them increases the seismicity rate.
For isolated and sealing faults, poroelastic stresses are transmitted to deeper faults, triggering earthquakes, even without elevated pore pressure.
The physical mechanism of induced seismicity on basement faults is a function of many parameters:
Chang, K.W. and P. Segall (2017), Reduction of injection-induced pore-pressure and stress in basement rocks due to basal sealing layers, , 174(7), 2649-2661, doi:Pure and Applied Geophysics10.1007/s00024-017-1567-1 [link]
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Chang, K.W. and P. Segall (2016), Seismicity on basement faults induced by simultaneous fluid injection-extraction, , 173(8), 2621-2636,Pure and Applied Geophysics
and P. Segall (2016), Injection induced seismicity on basement faults including poroelastic stressing, , Journal of Geophysical Research: Solid Earth121(4), 2708-2726, doi:10.1002/2015JB012561 [link].Pore pressure perturbation due to fluid injectionRole of ambient mudrock Carbon dioxide (CO2) storage in deep geological formations can lead to significant reductions in anthropogenic CO2 emissions if large amounts of CO2 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.
49(5), 2573-2588, doi:10.1002/wrcr.20197 [link].
37, 4457-4464 [link].
Chang, K.W. and
M.A. Hesse (201x), Radius of review for geological carbon storage in a layered formation, In preparation.Multiphase flow and solute transport in heterogeneous geological formationsNumerical study of solute-driven exchange flow: Mechanical dispersion and Flow barriersIn 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.
Chang, K.W. and M.A. Hesse (201x), Solute driven exchange flow encountering geological barriers in a fault, , under review.Hydrogeology JournalChang, K.W. and
M.A. Hesse (201x), Scaling regimes in solute driven convection in porous media with dispersion, in preparation.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 CO2 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.
Experimental study of solute-driven exchange flow
K.W. Chang (2015), Multiple steady states in exchange flows across faults and the dissolution of CO_{2}, , Journal of Fluid Mechanics769, 229-241, doi:10.1017/jfm.2015.100 [link].
Numerical study of multiphase flow near a fault zone
The injected CO2 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 CO2 plume and the amount of residual trapping. We studied the
behavior of CO2 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).
1(1), l839-1846 [link].
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