Study On Morphological Variation And Safety Control Technology Of Cavity In Underground Gas Storage

In recent years, global energy crisis has enormously aggravated and become more serious than ever before. Salt rock caverns buried deep in the ground have been wildly used when it comes to strategic safe storage facilities for natural resources, which has been one of the top priorities in countries all over the world. With quite a few of mechanical properties well-suited for energy storage or highly radioactive/polluting waste embedment, underground salt rock is the ideal storage media. Creep property, as one of the most important properties when it comes to the long-term stability and security issues of salt rock for underground gas storage. Therefore, it is imperative to study the mechanism and the corresponding failure of salt rock, in order to put forward a realtively optimal proposal for underground gas storage operation in our country.In this paper, the mechanism and transformation features of salt rock is systematically researched and analyzed. Additionally, researches carried out include a combination of theoretical analyses and numerical simulation. Through summary of creep models successfully used in assessment of stability of salt caverns in other countries, one specific creep model has been chosen as the base of numerical calculation of the transformation features of salt caverns caused by creep, with consideration of the properties of layered salt rock with multi-layer mudstone in between, which differs a lot from the salt rock in other places. Main research and calculation are as followed:(1) Theoretical analyses of according mechanism of salt rock creep are carried out in detail, through summary of underground as storage in salt rock caverns in foreign countries.(2) Based on the research of short-term mechanic intensity and creep experiments conducted exclusively for creep properties of salt rock; analyses on deformation features, time-related creep velocity are fully conducted. After comparing several classic creep models, Norton Power-Law model, which is ideal for description of salt creep behaviour, is chosen to evaluate the safety operation of storage caverns in the long term.(3) In combination of intensity laws of salt rock, the according mechanical failures involved in cavern deformation during the operation of underground natural gas storage, and evaluation criteria recommended in geotechnical engineering, specific stability evaluation parameters are chosen in this paper, which are rheolytic displacement of the surrounding rock, volumetric shrinkage of salt rock caverns, safety coefficient of the surrounding rock and secure width between two separate caverns in one UNGS(short for underground natural gas storage).(4) FLAC3D is used for simulation of the deformation of salt rock caverns caused by creep after a mechanically-reasonable model has been built in ANSYS. The simulation follows below: 1) Volumetric shrinkage of salt rock caverns under different operation conditions (different internal gas pressure, different operation periods) is calculated to determine the maximum and minimal internal gas pressure in salt rock caverns.2) Optimal height-diameter ratio is determined by calculating the rheolytic displacement of the surrounding rock and volumetric shrinkage of caverns. And the relatively ideal cavern shape in this particular case is calculated through simulation of volumetric shrinkage of salt caverns in different operation periods, which is a combination of part of a ellipsoid and a sphere connected to each other. This shape assures stability in mechanism, and more importantly, when it comes to a long and secure operation of underground natural gas storage, effective suppression of volumetric shrinkage due to creep property of salt rock.3) Research targeted in the salt rock in our country, which is thin layered salt rock embedded with multi-layer mudstone, is conducted to confirm that mudstone in salt rock plays a crucial part in suppress the volumetric shrinkage of underground caverns.4) The safety coefficient of the surrounding rock of a gas storage cavern is calculated in different operation conditions to evaluate the mechanic stability of salt rock caverns.5) In reference of empirical formula, secure width between two separate caverns in one UNGS is calculated accordingly.(5) Correspondent suggestions on safety operation are given based on the outcome of the simulative calculation and practical engineering practice in UNGS.