Study On Deformation And Safety Of Rock Mass Surrounding Gas Storage In Bedded Rock Salt

Due to the unique advantages of rock salt, such as low porosity, low permeability, damage recovery, favorable creep, simple hydrologic geology, and abundant distribution, salt cavern is considered as one of the most suitable place to storage oil, natural gas, and radioactive waste, etc. Comparing to abroad rock salt dome mine, there are many mudstones, anhydrites and glauberites in China bedded rock salt mines, and the salt and non-salt layers with small thickness coming alternately, which make the design and safety assessment of gas storage cavern in bedded rock salt is challengeable. The main motive of the paper is to solve several key problems to improve the safety of bedded rock salt cavern gas storage. Indoor experiments, theory analysis, and numerical simulation are adopted in the dissertation. Many detailed problems are contained, such as bedded salt failure criterion, cavern deformation, volume shrinkage rate, pillar and roof stability, ground subsidence, sealing, etc. Basing on the study results, the optimal cavern dimensions and running parameters are proposed to improve the use efficiency and safety of gas storage cavern in bedded rock salt. The detailed studies are depicted as follows.(1) The improved Mohr-Coulomb criterion is proposed considering the remarkable difference of the failure strength of bedded rock salt in triaxial compression and extension stress state. The corresponding formula is regressed by indoor experimental data of rock salt. A comparison of the improved Mohr-Coulomb criterion, two existing criterions and experimental data is made. The advantages of improved Mohr-Coulomb criterion is that the influences of the failure strength change of salt rock when the stress subjected to salt rock convertes from triaxial compression to triaxial extension stress state on the safety of salt cavern gas storage can be reflected. It is more suitable to China bedded rock salt than other existing criterions.(2) The rules of maximum deformations, plastic volumes, volume shrinkage rate, etc., of four different proposed salt cavern gas storages under different conditions are compared with each other. Moreover, the optimized cavern dimensions and running parameters of proposed bedded salt cavern gas storage are given. Influences of different parameters on the stresses and deformations of pillar and cavern roof are studied. The calculated results show cavern with the upper ellipsoid and lower hemisphere shape, and high/diameter of about 3 is good for the control of its force and volume shrinkage. The proposed pillar width is 2-3 times cavern diameters, and the synchrony injection-production mode is suggested in caverns with narrow pillars.(3) Considering the failure of pillars is typical nonlinear catastrophe problems, the cusp catastrophe model is proposed to obtain the stability factors of pillars and cavern roof. It can overcome the shortages of traditional strength reduction finite element method (SR FEM) and greatly improve the accuracy of stability factors obtained by numerical simulations. The influences of different parameters on the pillar and cavern roof stability factors are studied. The pillar stability factors are equidirectional with the increase of gas pressure and pillar width, but reverse to the increase of cavern depth and time. The cavern depth, roof span and gas pressure have significant effects on cavern roof stability.(4) A new model is proposed to predict the dynamic subsidence of ground surface above salt cavern gas storage during the leaching and storage, which takes into account the creep of rock salt. In the model, the extended form of Gaussian curve is adopted to figure out the shape of subsidence areas. The corresponding theoretical formulas are derived. In addition, parameters are studied to investigate the surface subsidence as a function of the salt ejection rate, internal pressure, cavern depth, diameter, height, running time, etc. Through an example, the subsidence of the salt cavern gas storage located at Jiangsu of China obtained by the new model is compared with those by Peter A F formula, Schober & Sroka formula, and FLAC^3D. The results showed the proposed model is precise and correct, and can meet engineering demands. The surface subsidence is equidirectional with the increase of salt ejection rate, depth, diameter, height, and running time, but reverse to the increase of internal pressure.(5) The equivalent permeability model (EPM) is presented in the paper to calculate the equivalent permeability of non-salt layer, which can make the sealing evaluation of bedded salt cavern natural gas storage by numerical simulation easy and sufficient. The influences of non-salt dipping angle, heterogeneity of permeability, permeability, buried depth, and gas pressure, etc. on the time that the natural gas migrates to the ground surface through the non-salt formation are studied. The time natural gas migrating to the surface ground takes is equidirectional with the increase of heterogeneity of permeability, and buried depth, but reverses to the increase of non-salt dipping angle, permeability, and internal pressure.