Xiaying Li

Impact indicators for caprock integrity and induced seismicity in CO2 geosequestration: insights from uncertainty analyses

The geological sequestration (geosequestration) of carbon dioxide (CO2) is a mitigation method for reducing greenhouse gas emission into the atmosphere. The security and safety of CO2 geosequestration are strongly dependent on the mechanical stability of the caprock overlying the reservoir. Underground injection of CO2 increases the pore pressure and thus decreases the effective stress. It may lead to caprock failure, as well as the subsequent leakage of sequestered CO2. In particular, geothermal exploitation and the underground disposal of hazardous liquid wastes have demonstrated a risk of induced seismicity. We performed an uncertainty analysis using a novel response surface methodology and a two-step statistical experimental design, evaluated the statistical significance of operator choices and subsurface uncertainties to caprock integrity, and quantified the moment magnitude of the induced seismicity. Furthermore, the optimal combination (i.e., the worst-case scenario) with the desired properties was forecast. A series of numerical experiments was well designed, and 130 combinations were statistically determined. Based on the results from the analysis of variance for the response surface quadratic model, the impact indicators were presented in histograms according to their significances to the Coulomb failure stress and moment magnitude of the induced seismicity. Lastly, the values of the selected independent impact indicators were predicted to obtain optimal compositions for object function of both Coulomb failure stress and moment magnitude, and the desired properties were being picked out. The optimal combinations had desirability values of 1.000, demonstrating the fitness of the selected statistical models in analyzing the experimental data.

Injection-induced fracturing process in a tight sandstone under different saturation conditions

To investigate the influence of local hydraulic condition on the water injection-induced fracture behavior in tight sandstone, injection tests under triaxial compression were conducted on samples collected from the Sichuan Basin, China. By means of acoustic emission (AE) monitoring, the fracturing behaviors of two samples of different water saturation patterns, partly saturated (the middle part was dry) and fully saturated, are investigated. Experiment results indicate that the local hydraulic condition plays a governing role in the mechanical properties, AE productivity, fracture nucleation and the geometry of the shear fracture zone. During axial loading stage, the partly saturated sample demonstrated ~35% higher elastic modulus than the fully saturated sample. During the injection-induced fracturing stage, progressively increasing AE activity and dilatancy, with increasing injected water volume, were observed preceding the dynamic fracture in the partly saturated sample, demonstrating a positive feedback between damage growing and fluid flow. More AEs were located in the dry or partially saturated regions at water front and thus produced dilatancy. However, the fully saturated sample shows very low AE activity (5% of that in partly saturated sample) which was initiated immediately before the dynamic fracture phase. Due to the very low permeability (~0.001 mD), volume of water injected into the fully saturated sample during the loading and creep stages is very limited, indicating the observed large dilatancy is governed by pore pressure increasing due to stress compaction. AE hypocenters demonstrated that an irregular shear fracture zone was created in the partly saturated sample, while a relatively flat shear plane was formed in the fully saturated sample.

Mechanical Stability of the Potential CO2 Sequestration Sites in Japan

Publisher Summary The generic idea of CO2 geologic sequestration is to inject and sequestrate the gas in deep permeable formations that are confined by impermeable caprocks so that the gas can be almost completely isolated from the atmosphere for a very long period. In Japan, brine aquifers in sedimentary basins have the largest storage capacity and most extensive distribution. In a generic sense, a geologic structure that has confined connate and/or over-pressured water for a geologic period can potentially trap a reasonable volume of CO2 as long as the caprock maintain its hydraulic integrity. The damage of the hydraulic integrity may result from the combining effects of factors such as injection pressure and buoyant pressure that may modify the mechanical conditions of the strata. It is well known that discontinuities such as faults and joints control both hydraulic and mechanical behavior in the subsurface, and that Japan, situated at the juncture of four plates, is a tectonically active region with a high density of faults and remarkable variations in geo-stresses. Therefore, the mechanical stability is one of major concerns in the safety assessment of a sequestration system. In this study, FEM simulations have been conducted that revealed the significant influence of the buoyant pressure on the shear stress of a fault close to CO2 bubble. It is suggested that the preferred sequestration sites should be those with sparse faults and active faults. Finally, based on the historic data of faults and active faults, some mechanically stable sedimentary basins are indicated.

Numerical investigation of slippage characteristics of normal and reverse faults under fluid injection and production

Nowadays, a great deal of petroleum geology and engineering projects associated with underground fluid injection and production (FIP) are widely conducted around the world. The FIP engineering may cause complex stress perturbation and trigger seismicity, which have been extensively reported and studied. In this paper, we investigated the fault slippage characteristics influenced by FIP. It reveals that for a fault (normal or reverse) that penetrating through the reservoir into the caprock and underburden, the footwall reservoir is the relatively stable one for fluid injection in a fluid FIP engineering. No matter it is a normal or a reverse fault, injecting fluid into footwall reservoir and producing fluid from the hanging wall reservoir can induce smaller fault slippage. After having determined the better fluid injection–production pattern, we studied the influence of three key factors of fault, i.e., dip, offset and depth, on fault stability. We found that, in our range of study, the influence of single action of fault dip, offset or depth on fault slippage in a FIP engineering was small. However, the influence of the combined effects of the factors may be large. Finally, we studied the effect of different pressure management scenarios on fault responses based on the specific fluid injection–production pattern. The results revealed that appropriate pressure management could effectively reduce fault slippage in a FIP engineering. However, inappropriate pressure management may cause much larger fault slippage. Given these concerns, it is therefore vital that the effect of pressure management scenario to be modeled prior to FIP.

Mixed Region Simulation on Subsurface Gas Storage of CO2 and CH4 in a Power-to-Gas System

Power-to-Gas (PtG) is a chemical energy storage technology that converts electrical energy into a high-energy density combustible gas. This technology alleviates the contradiction between power supply and demand due to the intermittent electricity production from environment-friendly renewable energy. Hydrogen (H2), produced by electrolysis, can be used to produce synthetic methane (CH4) by reacting with carbon dioxide (CO2) that is captured from carbon emission sources. Subsurface gas storage is one of the most important processes in a PtG system. However, nearly a half of the total stored gas is used as cushion gas to maintain a suitable reservoir pressure, indicating large amounts of CH4 might be wasted and trapped in geological formations. Based on a PtG system, CO2 can be a promising choice as a cushion gas due to its high effective compressibility near its critical conditions. In this paper, a numerical model is established based on the theory of the fluid flow and molecular diffusion to study the role of CO2 as a cushion gas in increasing the gas storage capacity. The accuracy of this model is verified by comparing with that of Curtis M. Oldenburg. However, because of the declining purity of recovered gas induced by the convection and diffusion of two kinds of gases in the same reservoir. The influences of reservoir thickness on the distribution of mixed region are discussed. The results show that thicker reservoir would be a better choice for the geological storage of CH4 with CO2 as a cushion gas.

Dynamic Optical Fiber Monitoring of Water-Saturated Sandstone During Supercritical CO2 Injection at Different Sequestration Pressures

In this paper, two intact cylindrical sandstones were bonded with the fiber Bragg grating sensors for laboratory experimental studying the injection and migration process of carbon dioxide (CO2) under varying temperature and sequestration pressures. A series of core flooding experiments were conducted under undrained conditions with different confining and pore pressures without changing the effective confining pressure. As a result, the strain responses of CO2 in three different states after injecting into specimen rose with the increase in the pore pressure. And the dynamic strain responses of supercritical CO2 (scCO2) was slightly higher than that of liquid CO2 due to the character of sCO2 and the effect of temperature. The initial time differences of axial strain measurement along the three gratings on a single fiber can precisely indicate the migration fronts of scCO2 plume. The difference in strain response time of the three gratings is in a descending order of liquid CO2, scCO2, and gaseous CO2.

Response of Velocity Anisotropy of Shale Under Isotropic and Anisotropic Stress Fields

We investigated the responses of P-wave velocity and associated anisotropy in terms of Thomsen’s parameters to isotropic and anisotropic stress fields on Longmaxi shales cored along different directions. An array of piezoelectric ceramic transducers allows us to measure P-wave velocities along numerous different propagation directions. Anisotropic parameters, including the P-wave velocity α along a symmetry axis, Thomsen’s parameters ε and δ, and the orientation of the symmetry axis, could then be extracted by fitting Thomsen’s weak anisotropy model to the experimental data. The results indicate that Longmaxi shale displays weakly intrinsic velocity anisotropy with Thomsen’s parameters ε and δ being approximately 0.05 and 0.15, respectively. The isotropic stress field has only a slight effect on velocity and associated anisotropy in terms of Thomsen’s parameters. In contrast, both the magnitude and orientation of the anisotropic stress field with respect to the shale fabric are important in controlling the evolution of velocity and associated anisotropy in a changing stress field. For shale with bedding-parallel loading, velocity anisotropy is enhanced because velocities with smaller angles relative to the maximum stress increase significantly during the entire loading process, whereas those with larger angles increase slightly before the yield stress and afterwards decrease with the increasing differential stress. For shale with bedding-normal loading, anisotropy reversal is observed, and the anisotropy is progressively modified by the applied differential stress. Before reaching the yield stress, velocities with smaller angles relative to the maximum stress increase more significantly and even exceed the level of those with larger angles. After reaching the yield stress, velocities with larger angles decrease more significantly. Microstructural features such as the closure and generation of microcracks can explain the modification of the velocity anisotropy due to the applied stress anisotropy.