Hyung-Mok Kim

Multi-Region Boundary Element Analysis for Coupled Thermal-Fracturing Processes in Geomaterials

This paper describes a boundary element code development on coupled thermal–mechanical processes of rock fracture propagation. The code development was based on the fracture mechanics code FRACOD that has previously been developed by Shen and Stephansson (Int J Eng Fracture Mech 47:177–189, 1993) and FRACOM (A fracture propagation code—FRACOD, User’s manual. FRACOM Ltd. 2002) and simulates complex fracture propagation in rocks governed by both tensile and shear mechanisms. For the coupled thermal-fracturing analysis, an indirect boundary element method, namely the fictitious heat source method, was implemented in FRACOD to simulate the temperature change and thermal stresses in rocks. This indirect method is particularly suitable for the thermal-fracturing coupling in FRACOD where the displacement discontinuity method is used for mechanical simulation. The coupled code was also extended to simulate multiple region problems in which rock mass, concrete linings and insulation layers with different thermal and mechanical properties were present. Both verification and application cases were presented where a point heat source in a 2D infinite medium and a pilot LNG underground cavern were solved and studied using the coupled code. Good agreement was observed between the simulation results, analytical solutions and in situ measurements which validates an applicability of the developed coupled code.

Characterizing Excavation Damaged Zone and Stability of Pressurized Lined Rock Caverns for Underground Compressed Air Energy Storage

In this paper, we investigate the influence of the excavation damaged zone (EDZ) on the geomechanical performance of compressed air energy storage (CAES) in lined rock caverns. We conducted a detailed characterization of the EDZ in rock caverns that have been excavated for a Korean pilot test program on CAES in (concrete) lined rock caverns at shallow depth. The EDZ was characterized by measurements of P- and S-wave velocities and permeability across the EDZ and into undisturbed host rock. Moreover, we constructed an in situ concrete lining model and conducted permeability measurements in boreholes penetrating the concrete, through the EDZ and into the undisturbed host rock. Using the site-specific conditions and the results of the EDZ characterization, we carried out a model simulation to investigate the influence of the EDZ on the CAES performance, in particular related to geomechanical responses and stability. We used a modeling approach including coupled thermodynamic multiphase flow and geomechanics, which was proven to be useful in previous generic CAES studies. Our modeling results showed that the potential for inducing tensile fractures and air leakage through the concrete lining could be substantially reduced if the EDZ around the cavern could be minimized. Moreover, the results showed that the most favorable design for reducing the potential for tensile failure in the lining would be a relatively compliant concrete lining with a tight inner seal, and a relatively stiff (uncompliant) host rock with a minimized EDZ. Because EDZ compliance depends on its compressibility (or modulus) and thickness, care should be taken during drill and blast operations to minimize the damage to the cavern walls.

Failure Monitoring and Leakage Detection for Underground Storage of Compressed Air Energy in Lined Rock Caverns

Underground compressed air energy storage (CAES) in lined rock caverns (LRCs) provides a promising solution for storing energy on a large scale. One of the essential issues facing underground CAES implementation is the risk of air leakage from the storage caverns. Compressed air may leak through an initial defect in the inner containment liner, such as imperfect welds and construction joints, or through structurally damaged points of the liner during CAES operation for repeated compression and decompression cycles. Detection of the air leakage and identification of the leakage location around the underground storage cavern are required. In this study, we analyzed the displacement (or strain) monitoring method to detect the mechanical failure of liners that provides major pathways of air leakage using a previously developed numerical technique simulating the coupled thermodynamic and geomechanical behavior of underground CAES in LRCs. We analyzed the use of pressure monitoring to detect air leakage and characterize the leakage location. From the simulation results, we demonstrated that tangential strain monitoring at the inner face of sealing liners could enable one to detect failure. We also demonstrated that the use of the cross-correlation method between pressure history data measured at various sensors could identify the air leak location. These results may help in the overall design of a monitoring and alarm system for the successful implementation and operation of CAES in LRCs.

Sensitivity analysis for fault reactivation in potential CO2-EOR site with multi-layers of permeable and impermeable formations

CO2-EOR is considered as a promising solution for enhanced oil recovery (EOR) and is attracting attention as being a more economical CO2 geological sequestration solution along with oil recovery enhancement. However, injecting CO2 at high pressure may cause many geomechanical changes and potential instabilities in surrounding formation such as ground uplift, caprock fracturing, and nearby fault reactivation. Such instabilities could significantly influence the stability of both surface facilities and subsurface structures. Especially, miscible CO2-EOR, by which recovers more oil than immiscible one but, uses less CO2, requires an injection pressure exceeding the minimum miscible pressure (MMP), which is determined by characteristics of reservoir conditions and oil compositions. Thus, for successful and safe CO2-EOR operation, injection pressure interval between MMP and the maximum pressure that could be tolerated from geomechanics safety concerns should be appropriately designed considering site-specific reservoir conditions. In this study, we perform a numerical simulation of coupled multiphase fluid flow and geomechanical analysis using TOUGH-FLAC simulator for the potential CO2-EOR site in Indonesian oil field, and demonstrate how much fault reactivation is sensitive to fault structure, slip-weakening property of faults, reservoir permeability, and in situ stress conditions. The model site consists of impermeable shale and permeable sandstone reservoir units so that the potential for fault slip through this multilayered formation is highlighted in the simulations. Our simulation results showed that fault slip initiation can be reached earlier period when in situ stress is anisotropic and reservoir is more permeable, because the stress state at the faults is near the frictional strength limit and the pore pressure buildup reaches to the fault much faster. The analysis shows that multilayered formations with high- and low-permeability layers are advantageous in CO2-EOR since intense pore pressure buildup and subsequent fault reactivation could be impeded by pressure dissipation in high-permeability layers. However, we noted that fault reactivation may become substantial when the fault has a slip-weakening property and the residual frictional coefficient of the site-specific fault is very low.

Enhanced Oil Recovery (EOR) Technology Coupled with Underground Carbon Dioxide Sequestration

Abstract Enhanced oil recovery (EOR) technology coupled with underground carbon dioxide sequestration is introduced. CO 2 can be injected into an oil reservoir in order to enhance oil production rate and CO 2 EOR can be turned into CCS in a long term sense. Coupling CO 2 EOR with CCS may secure a large scale and consistent CO 2 source for EOR, and the CO 2 EOR can bring an additional economic benefit for CCS, since the benefit from enhanced oil production by CO 2 EOR will compensate costs for CCS implementation. In this paper, we introduced the characteristics of CO 2 EOR technology and its market prospect, and reviewed the Weyburn CO 2 EOR project which is the first large-scale CO 2 EOR case utilizing an anthropogenic CO 2 source. We also introduced geotechnical elements for a successful and economical implementation of CO 2 EOR with CCS and they were a miscroseismic monitoring during and after injection of CO 2 , and determination of minimum miscible pressure (MMP) and maximum injection pressure (MIP) of CO

Coupled Viscous Fluid Flow and Joint Deformation Analysis for Grout Injection in a Rock Joint

Fluid flow modeling is a major area of interest within the field of rock mechanics. The main objective of this study is to gain insight into the performance of grout injection inside jointed rock masses by numerical modeling of grout flow through a single rock joint. Grout flow has been widely simulated using non-Newtonian Bingham fluid characterized by two main parameters of dynamic viscosity and shear yield strength both of which are time dependent. The increasing value of these properties with injection time will apparently affect the parameters representing the grouting performance including grout penetration length and volumetric injection rate. In addition, through hydromechanical coupling a mutual influence between the injection pressure from the one side and the joint opening/closing behavior and the aperture profile variation on the other side is anticipated. This is capable of producing a considerable impact on grout spread within the rock joints. In this study based on the Bingham fluid model, a series of numerical analysis has been conducted using UDEC to simulate the flow of viscous grout in a single rock joint with smooth parallel surfaces. In these analyses, the time-dependent evolution of the grout fluid properties and the hydromechanical coupling have been considered to investigate their impact on grouting performance. In order to verify the validity of these simulations, the results of analyses including the grout penetration length and the injection flow rate were compared with a well-known analytical solution which is available for the simple case of constant grout properties and non-coupled hydraulic analysis. The comparison demonstrated that the grout penetration length can be overestimated when the time-dependent hardening of grout material is not considered. Moreover, due to the HM coupling, it was shown that the joint opening induced by injection pressure may have a considerable increasing impression on the values of penetration length and injected grout volume.

Stability Analysis for Ground Uplift in Underground Storage Caverns for High Pressurized Gas using Hoek-Brown Strength Criterion and Geological Strength Index (GSI)

Abstract A simple analytical approach for stability assessment of underground storage caverns against ground uplift of overburden rock above the rock caverns for high pressurized fluid such as compressed air energy storage (CAES) and compressed natural gas (CNG) was developed. In the developed approach, we assumed that failure plane of the overburden is straight upward to ground surface, and factor of safety can be calculated from a limit equilibrium analysis in terms of this cylindrical shape failure model. The frictional resisting force on the failure plane was estimated by Hoek-Brown strength criterion which replaces with Mohr-Coulomb criterion such that both intact rock strength and rock mass conditions can be considered in the current approach. We carried out a parametric sensitivity analysis of strength parameters under various rock mass conditions and demonstrated that the factor of safety againt ground uplift was more sensitive to Mohr-Coulomb strength criterion rather than Hoek-Brown criterion.

Temperature Dependent Rock Fracturing in Boreholes

The paper describes a recent numerical code development and laboratory investigations on coupled thermal-mechanical processes of rock fracture propagation. A series of laboratory tests were conducted on rock strength and fracture toughness within a temperature range from -80C to 400C, and key temperature dependent parameters were obtained on granite specimens. The numerical development is based on a fracture mechanics code FRACOD that has previously been developed by some of the authors of this paper. The code simulates complex fracture propagation in rocks governed by both tensile and shear mechanisms. For the latest development an indirect boundary element method, namely the fictitious heat source method, is implemented in FRACOD to simulate the temperature change and thermal stresses in rocks. This method is particularly suitable for the thermal-mechanical coupling in FRACOD where the displacement discontinuity method is used for mechanical simulation. An example case is presented where a borehole drilled into the rock formation. Depending on the initial reservoir rock temperature, cooling fractures may or may not occur in the borehole wall from the same differential temperature between the borehole wall and the reservoir rock.