Construction Of Caverns Research Articles

Research on the correction method of the boundary water level of the mountainsides in seepage analysis

The accuracy of the calculation results in seepage analysis is determined largely by the boundary conditions. The construction of underground caverns and reservoir impoundments will inevitably lead to a redistribution of the underground seepage fields and the change in groundwater level. However, commonly used finite-element simulation methods rarely consider the effects of changes in the boundary water level. This study examined the mechanism of the change in groundwater level caused by impoundment. The drainage of underground caverns on the boundary water level of the model on the mountainsides, and the boundary water-level correction method is proposed. By applying simplified engineering examples, the groundwater levels of the boundaries were calculated (impoundment condition). The results show that the distribution of the seepage field before and after the correction is different. Furthermore, the calculation results of the flow rate are exaggerated (drainage condition of the cavern), as the groundwater drawdown at the computation boundary is ignored. The proposed method can correct the water level at the boundary of the model and effectively improve the calculation accuracy of seepage analysis.

Tightness of an underground energy storage salt cavern with adverse geological conditions

A micro-permeable interlayer (MPI) has been found in a salt cavern construction area of the Jintan salt cavern underground gas storage district, Jiangsu, China. The MPI has caused sealing failures of several wells. Tightness is a prerequisite of salt caverns used for energy storage. To quantifiably evaluate the tightness of gas storage salt caverns constructed in formations including an MPI, permeability testing of MPI samples and numerical simulations are carried out. The MPI permeability ranges from 1.10 × 10−16 m2 to 2.12 × 10−14 m2 with an average of 8.16 × 10−15 m2, much larger than that of the rock salt and of other interlayers at the same formation. The effects of the MPI permeability, MPI thickness, width of pillars between caverns, and MPI location on the tightness of the cavern are investigated. The permeability and the location of the MPI are the key factors affecting the cavern tightness. It is suggested to seal the MPI rather than allow it to intersect the caverns untreated in the areas containing the MPI. A rock salt layer between the MPI and the cavern with a thickness of about 5 m is proposed to seal the MPI.

Technological innovations in construction of underground caverns in basaltic rocks at Baihetan Hydropower Station on Jinsha River

The two underground powerhouses on the left and right banks of Baihetan Hydropower Station (BHT-HS) are symmetrically arranged inside basaltic rocks in the alpine gorge reach of the lower Jinsha River. Each of them is equipped with 8 sets of 1,000 MW hydraulic turbine generators, with the largest unit capacity, total installed capacity, span of main powerhouse (i.e. 34 m), diameter of dome-shaped tail-regulating chamber (max diameter of 48 m) and scale of underground caverns around the world. Due to complex layout, interaction between caverns and high hollowing rate, the construction process of the underground cavern group is faced many challenges. For example, the cavern excavation is located in the high ground stress area dominated by horizontal tectonic stress; the underlying basalt fractures are well developed, and the rock is hard and brittle, with low crack initiation strength; columnar jointed basalts develop locally; a number of large and weak gently-sloped shear zones develop diagonally cutting the main powerhouse. Following the construction ideology of “understanding, utilization, protection, monitoring, and feedback analysis on the surrounding rock masses during the construction period”, and with the engineering construction procedure of “the excavation, analysis, prediction and inspection of each rock layer”, this paper proposes a set of innovation construction technologies including “advanced pre-control, thin vertical layer, small planar partition, short excavation progress, fine blasting, strong and quick support, short interval monitoring, quick feedback, precise dynamic optimization” to control the internal cracking of the hard and brittle basaltic rocks, discontinuous deformation of staggered zones and spatio-temporal deformation of underground caverns in high geostress. It has promoted the safe and orderly construction of BHT-HS, the technical progress of the construction of hydropower station projects in China, and may also be applicable to the design and construction of similar projects worldwide.

Ground realization and characterization of rock mass through core drilling

The increasing demand of power day by day in the country put power sector to Himalayas to harness the power potential. Utilization of underground space for infrastructure development is picking up momentum day by day. Every infrastructure project requires significant geological and geotechnical investigation for timely completion of projects. Lots of survey and Investigation is carried out to ascertain the geological and geotechnical feasibility of the hydro projects and for preparation of DPR. Construction of large caverns for power house presents challenges in terms of geotechnical engineering. Due to large size structure needs to ascertain behaviour of subsurface rock mass. The exploratory core drilling and exploratory drifts provide vital subsurface information. The subsurface exploratory drilling data provide immense help to understand subsurface information and reveals the subsurface behaviour of rock mass. The prior knowledge of weak feature can provide opportunity to prepare proactive solutions to mitigate these issues and structure design.Authors are proposing simplified techniques to be implemented to understand ground realization and spatial reconciliation of weak zones/features with modern approaches in core logging with special core studies of Sunni Dam and Luhri Hydro Electric Projects Stage-II in Higher Himalayas of Himachal Pradesh.

Stability analysis of U-shaped horizontal salt cavern for underground natural gas storage

More and more attentions are being paid to horizontal salt caverns for natural gas storage because of their large working gas capacity and favorable stability. To accelerate the construction of gas storage underground salt caverns, stability analysis of this type of cavern is necessary to assure the safety of such caverns. In this paper, the stability is investigated of a U-shaped horizontal salt cavern under different constant and cyclic internal gas pressures. A 3D geomechanical model is established based on sonar scanning in the field and predicted cavern shape. The stability is analyzed of the rock masses around the cavern under different internal gas pressures and cycle frequencies. Five evaluation criteria are proposed to predict the feasibility and stability of such caverns, including deformation, dilatancy safety factor, volume shrinkage, plastic zone, and equivalent strain. The stability of the rock mass around the cavern under different internal gas pressures is compared to that under different cycle frequencies. The results show that the cavern has a good stability under the constant internal gas pressure of 8 MPa and cyclic internal gas pressures ranging from 8 ~ 18 MPa. The five evaluation indexes of the rock masses around the cavern improve with increasing internal gas pressure. It is proposed that the corner, horizontal-roof center, and external waist positions of the cavern are to be highlighted in the design and construction phases. The cycle frequency has appreciable impact on the stability of the rock mass around the cavern. The difference between the volume shrinkage under cycling compared to constant internal gas pressure is that the cycling simulated results show a rising wave-like curve of creep time. The plastic zone ratio increases with creep time and has a flat peak and a sharp bottom with oscillations. This study provides the design parameters for U-shaped salt cavern in the Huaian salt district and can also be a reference for horizontal salt caverns.

Spatial and Temporal Evolution of Leaching Zones within Potash Seams Reproduced by Reactive Transport Simulations

Leaching zones within potash seams generally represent a significant risk to subsurface mining operations and the construction of technical caverns in salt rocks, but their temporal and spatial formation has been investigated only rudimentarily to date. To the knowledge of the authors, current reactive transport simulation implementations are not capable to address hydraulic-chemical interactions within potash salt. For this reason, a reactive transport model has been developed and complemented by an innovative approach to calculate the interchange of minerals and solution at the water-rock interface. Using this model, a scenario analysis was carried out based on a carnallite-bearing potash seam. The results show that the evolution of leaching zones depends on the mineral composition and dissolution rate of the original salt rock, and that the formation can be classified by the dimensionless parameters of Péclet (Pe) and Damköhler (Da). For Pe > 2 and Da > 1, a funnel-shaped leaching zone is formed, otherwise the dissolution front is planar. Additionally, Da > 1 results in the formation of a sylvinitic zone and a flow barrier. Most scenarios represent hybrid forms of these cases. The simulated shapes and mineralogies are confirmed by literature data and can be used to assess the hazard potential.

Prediction method for calculating the porosity of insoluble sediments for salt cavern gas storage applications

Many insoluble sediments accumulate at cavern bottoms during the construction of salt caverns located in highly insoluble salt formations. Before using the void space for gas storage, knowing the porosity is essential for predicting void volume of insoluble sediments. However, most previous studies have focused on construction and operation of salt caverns, and insoluble sediments have been largely ignored. In this study, a series of laboratory tests, including screening and porosity tests, are conducted to obtain the particle size distribution and porosity of samples obtained by dissolving the nonsalt interlayers of target cavern. The results show that the porosity values of the samples decreases with increasing of fractal dimensions and have no relation to the maximum particle size when the fractal dimension is constant. Based on these results, a porosity prediction method is proposed to calculate the porosity of insoluble sediments combined with fractal and packing theories. In this method, the fractal dimension and the compaction index should be controlled within a reasonable range. The accuracy and reliability of this model was studied by calculating the porosity of insoluble sediments in an actual salt cavern. This study can provide a reference for evaluating the void volume in insoluble sediments.

Construction Progress of Deep Underground Salt Cavern Gas Storage and Challenges of its Drilling and Completion Technology

With the rapid development of the national economy and the increasing demand for energy, and the acceleration of natural gas storage strategies, underground gas storage has become more and more important in China’s oil and gas consumption and energy safety. Underground salt cavern gas storage has many advantages and is very suitable for gas storage and peak shaving。However, most of the underground salt rocks in China are layered salt beds, and some of the salt beds are deeply buried. This article introduces the development history of deep underground salt cavern gas storage and the technical challenges faced in the process of drilling and completion of salt cavern construction.

The optimum interwell distance analysis of two-well-horizontal salt cavern construction

ABSTRACT Large-scale energy is often stored in underground salt caverns. Because China has abundant thinly bedded rocksalt, two-well-horizontal (TWH) caverns are considered as alternatives for large-scale energy storage, which possess higher construction rate and larger volume than the vertical caverns. To-date, the analysis of the development of shape and the control methodology with different interwell distances of TWH caverns are not fully understood. In order to eliminate these drawbacks, TWH-cavern construction has been simulated in the current work. A platform was established based on TWHSMC V2.0 simulation code to obtain the theoretical basis for TWH-cavern leaching construction for a more user-friendly visual analysis of the cavern development. Setting cavern construction efficiency as the objective functions for optimization, a series of theoretical simulations were conducted to analyze how the interwell distance affects the TWH-cavern construction. The results obtained illustrate that the interwell distance correlates with a rapid decrease in the leaching time until a tendency to stabilization is observed. The optimum interwell distance is 115 m at a flow rate of 300–500 cubic meters per hour. The interwell distance also influences the longitudinal span of the cavern. When the interwell distance is 115 m, the cavern has the smallest span in the longitudinal span. Therefore, the potential crossing of the interlayer can be minimized, which is good for the safety of cavern. Abbreviations: UGS: Underground gas storage; SPR: strategic petroleum reserves; CAES: Compressed air energy storage; SWV: Single-well-vertical; TWH: Two-well-horizontal; SIMPLE: Semi-Implicit Method for Pressure-Linked Equations; TWHSMC: Two-well-horizontal solution mining cavern; 2D: Two dimensional; VOF: Volume of Fluid; 3D: Three dimensional; API: American Petroleum Institute

Influence of tubing/oil-blanket lifting on construction and geometries of two-well-horizontal salt caverns

Underground TWH (Two-well-horizontal) salt caverns are an ideal storage medium for large-scale energy storage, having large usable volumes and high construction efficiency. Different tubing/oil-blanket lifting techniques relate to different solution mining rates and help define cavern geometries based on the previous experience of SWV (Single-well-vertical)-cavern construction. In this study, we present a complete and comprehensive evaluation of the influence of a particular tubing/oil-blanket lifting method on TWH-cavern leaching and geometric shapes. Based on the solution mining numerical simulation original program TWHSMC V2.0 (Two-well-horizontal solution mining cavern V2.0), we carried out a set of numerical simulations, such that the cavern construction process is real-time display and easily recorded. The results demonstrate that with a matching water injection rate, the outlet brine concentration is not sensitive to tubing/oil-blanket lifting. During construction of a horizontal cavern with a volume of 300,000 m3, lifting tubing on both sides simultaneously yields the shortest leaching time (near 307 days). Only lift the tubing on one side will form an asymmetrical cavern, with the height of the cavity near the side being higher than on the opposite side. The lower the frequency of tubing and oil blanket lifting, the smaller the height/length ratio coefficient is, meaning the cavern is closer to the horizontal. Therefore, when constructing TWH-caverns in thinly-bedded rocksalt formations, simultaneously lifting the both sides of tubing and oil-blanket, in addition to a lower lifting rate, are required. This research provides theoretical basis and engineering guidance for TWH-cavern constructing in thinly bedded rocksalt formations.

Numerical Analyses on the Rockbolt Behaviour in Rock Wedges Reinforcement Using 2D-DDA

One major concern in the design and construction of caverns in jointed rock mass is the potential failure of rock wedges falling from the roof or sliding out from the sidewalls. The rockbolts are commonly used to improve the stability and to prevent failure in the caverns. In this paper, the two-dimensional discontinuous deformation analysis (2D-DDA) method was used to analyse the stability of the rock wedge supported by rockbolts on the cavern roof. The joint relaxation method was adopted to represent a deformable rock joint under stresses. Parametric studies were conducted to investigate the effects of the confining pressure, the spacing and the angle between rockbolt and direction of wedge falling on the stability of the rock wedge. The numerical simulation results indicate that the wedge is self-supporting after stress redistribution if the confining pressure is larger than the critical confining pressure and the semi-apical angle of the wedge is less than the friction angle of rock joint. The force in rockbolt slightly decreases with reduced rockbolts spacing but increase when the rockbolt is not along the falling direction of rock wedge.

Research on Surrounding Rock Deformation Prediction During Underground Excavation of Hydropower Station Based on Prototype Monitoring

This paper proposes an improved gray model of equal dimensional innovation based on time-varying parameters and the gray characteristics of small sample and poor information of the safety monitoring data during underground cavern construction, so as to predict and judge the deformation of surrounding rocks in underground caverns and to obtain relatively reliable results in the analysis of uncertain systems. Under certain conditions, the improved gray model of equal dimensional innovation based on time-varying parameters can be used to predict the deformation of surrounding rocks during the excavation of underground caverns, thereby providing mathematical analysis tools for safe construction.

Construction modeling and parameter optimization of multi-step horizontal energy storage salt caverns

A multi-step horizontal leaching method can be used to efficiently construct energy storage salt caverns. However, the modeling and control of a multi-step horizontal cavern are complicated and have been investigated little, resulting in many collapses of such caverns. To predict the 3-D development of a horizontal salt cavern during construction, a numerical model is proposed based on a composite structural mesh. The salt dissolution rate is introduced as being related to the brine concentration. The flow/concentration fields are dynamically divided into four subareas. Their governing equations are derived based on mass and volume conservation. The accumulation and re-distribution of the insoluble substances are considered. A C++ program is developed for model implementation. The feasibility and accuracy of the model are verified by comparisons with data in published literature. A series of simulations have been conducted for technological parameter optimization. Results show that using a larger borehole length increases cavern capacity. Using a larger step distance increases the construction control but decreases the cavern capacity. Using a larger flow rate increases construction efficiency but decreases the energy-saving coefficient and construction control. A long borehole length, a medium step distance and a variable flow rate are suggested for horizontal cavern construction.

Oil-water separation device based on super-hydrophobic material with multi-scale surface structure

A buoyancy-adaptive oil-water separation device is developed based on the prepared oil-water separation materials with a micro-nano hierarchical structure. This device can achieve the efficient recovery of diesel oil pads in the process of salt cavern construction without an external power source. The surface structure of superhydrophobic materials and the wettability of droplets on their surfaces are characterized. Then the separation mechanism is theoretically analysed. In addition, the brine breakthrough pressure and the separation efficiency of the device are studied considering the different surface structures of the separation materials. The mesh number and the layer number of separation materials are explored to determine their effects on the breakthrough pressure. It is found that this pressure increases with an increasing mesh number, but the influence of the screen layers is weak. Furthermore, the mesh number and the number of cycle separations that affect the separation efficiency are investigated. The results show that the sieve hole is smaller when the mesh number is larger, and the volume flow rate of diesel oil is thus reduced. Therefore, the screen mesh number should be reasonably selected based on the actual needs. The results of the cyclic separation test indicate that the separation material has good reusability. After 30 cycles of recycling, the separation efficiency of the device can still reach 90.1 ± 1.7%.

Tightness Analysis of Underground Natural Gas and Oil Storage Caverns With Limit Pillar Widths in Bedded Rock Salt

To fully utilize the abandoned salt cavern resources and to increase the total amount of the fossil energy reserve of China, reconstructing some of these salt caverns for underground gas storage (UGS) or strategic petroleum reserve (SPR) would be an effective method. The salt resources in China mainly are bedded salt, which brings great challenges for the cavern construction and safety evaluation. In this paper, the investigations are presented to evaluate the tightness of the UGS and SPR salt cavern facilities, located in the bedded rock of Jintan, China. Microcosmic analysis, and permeability and porosity tests of the surrounding rock are carried out to determine their properties, which provide the basic data for the tightness assessment. A 3-D numerical model is developed based on the test results and the geological features of the target formation. The numerical simulation results show that the seepage velocity, seepage range and loss rate of leakage of the SPR salt caverns are much smaller than those of UGS salt caverns. The cavern's pillar width with a pillar to diameter ratio (P/D) of 1.5 can satisfy the tightness requirement of SPR salt caverns, but it cannot meet the requirement of UGS caverns. This indicates that some existing abandoned salt caverns in Jintan which are unsuitable for UGS due to their small pillar width have the potential to be rebuilt for SPR. This would help to increase the storage capacity of crude oil in China. The results can also provide a reference for the implementation of similar projects in other bedded salt districts.

Determination of actual dissolution rates from some rock properties in construction of deep salt cavern for natural gas storage

Available energy resources should be efficiently used to meet the global energy demand. The storage of energy resources, especially as natural gas, is important to balance extreme fluctuations in demand and supply of energy. Underground caverns in deep salt domes are used as storage for natural gas due to the low permeability and appropriate creep conditions of salt. Several projects are being carried out in Turkey to store imported natural gas. The design and construction parameters of these deep salt caverns should be determined to provide reliable and durable storage. Dissolution rate, which is an important parameter affecting the design and construction of underground salt caverns, is generally measured from core samples in laboratory settings. However, results do not match actual underground dissolution rates. The aim of this study was to predict actual dissolution rates from some rock properties. The physical and mechanical salt properties were determined from core samples obtained from different depths of drilled wells opened for salt caverns. The relationships between salt properties and actual dissolution rates were statistically analyzed. A statistical model was developed to predict actual dissolution rates using uniaxial compressive and tensile strengths, Poisson's ratio, and cohesion and experimental vertical dissolution rates. The model is reliable for predicting actual dissolution rates in construction of underground gas storage caverns in salt domes. Predicted dissolution rates can be used in the design of underground gas storage caverns in salt domes.

Comprehensive safety evaluation method of surrounding rock during underground cavern construction

The deformation instability of surrounding rock and the collapse of rock blocks are two common failure modes observed during the construction of underground caverns. Therefore, a comprehensive safety evaluation method (CSEM) of surrounding rock for application during underground cavern construction is presented in this paper. The method can be used to evaluate the deformation stability of surrounding rock and predict the collapse of rock blocks, rapidly. First, a deformation stability prediction and evaluation method (DSPEM) for rock mass is established. It combines the safety evaluation method based on a deformation statistical analysis and an inverse deformation prediction method using measured deformation data of surrounding rock. This approach possesses the unique characteristics of complementary and mutual verification. In addition, a comprehensive early-warning index system (CWIS) for assessing the deformation stability of surrounding rock is presented. It consists of three types of warning indicators and three levels of warning values. The early-warning values can be adjusted reasonably by the prediction correction method according to the measured displacements. For the collapse of rock blocks controlled by structural surfaces, a rapid prediction method of rock block collapse (RPMBC) based on the block theory is then presented. Finally, combining the DSPEM, RPMBC, and CWIS, the CSEM of surrounding rock for assessing the safety of cavern construction is established. It can provide technical support for decision making on the safety of surrounding rock during the construction of underground caverns.

The influence of the water injection method on two-well-horizontal salt cavern construction

With high construction efficiency and large usable volumes, underground TWH caverns in rock salt formations provide optimal storage for large-scale compressed natural gas. Previous studies of SWV cavern construction indicate that different water injection methods will result in different leaching rates and will therefore form different cavern geometries. Therefore the influence of the water injection method on the TWH cavern construction process and geometries were thoroughly evaluated in this study. Based on the leaching numerical simulation code TWHSMC V2.0, we conducted a series of simulation experiments. The results illustrate that under the same water injection rate, the cavern construction speed of the alternate injection method is faster than the unidirectional injection method. Using the unidirectional method will form asymmetrical triangular pyramidal caverns. In contrast, using the alternate method will form symmetrical elliptical cone caverns. Under repeated alternating “Injection” and “Withdrawal” operations, the larger the flow rate, the shorter the cavern construction time, and the smaller the H/L ratio coefficient, as well as the closer the cavern geometry is to the horizontal cavern. Thus, during construction of a TWH-cavern in thinly-bedded rock salt (halite) deposits, repeated and alternating injections and withdrawals with high flow rates are necessary.

Research on Selection Method for the Types and Sites of Underground Repository for Compressed Air Storage

[Introduction] The selection of types and sites of underground repository for compressed air storage is one of the most important issues of large scale compressed air energy storage (CAES) plant planning. [Method] The advantages and disadvantages of 4 types of underground repository for compressed air storage were concluded based on comparison of analyzing literatures published domestic and oversea. The geology characteristics and distribution region of the suitable hard stratum for construction of underground rock cavern were also analyzed. The technology feasibility of rock cavern for compressed air storage was validated by the in-situ experiment results obtained from the first pilot repository for CAES in China. Sites planning method of underground repository for large scale CAES plant was studied taking Guangdong province for example. [Result] Rock caverns built in salt and hard rock stratum are the most feasible types of the underground cavern for CAES plant and hard rock stratum widely exists in most windy and solar energy abundant aera in China. [Conclusion] Wide distribution of the suitiable stratum for underground cavern construction in intermittent energy generation area solves the difficult problem of site selection for large scale CAES plant. Experiment data indicates that good quality of the rock mass is helpful for the solution of key problems such stability and sealing of underground cavern. All in all, it is feasiblity in technology at the present stage in China.

Modeling the construction of energy storage salt caverns in bedded salt

During the construction of energy storage salt caverns in bedded salt, the unpredictable failure of interlayers results in irregularly shaped caverns with lower storage capacity and with potential safety hazards. A model is established of the coupled salt dissolution and interlayer failure during the construction of a salt cavern. Equations of the salt dissolution rate and of the brine concentration-flow field of the dynamic cavern development are introduced. Failure of the interlayers with a soluble skeleton is described in three steps as disintegration, fall and accumulation. Failure of the interlayers with an insoluble skeleton is modeled by location determination, strength calculation and collapse implementation. A C++ program is developed for the implementation of the model. A cavern, JT86, in Jintan Gas Storage, China, is simulated by the proposed model and by a traditional model. The disintegration, collapse, fall and accumulation of the insoluble interlayers can be simulated by the proposed model, which results in a more accurate cavern shape prediction. The simulation shows a dissolution-collapse-dissolution cycle and a self-locking condition, which might be the reason for the uneven development of caverns in inclined bedded salt. To increase the cavern stability, an extra leaching stage is suggested around the overhanging interlayers to bring the collapse under control.