Fundamental Study On Construction And Stability Of Oil And Gas Storage Cavern In Bedded Salt Rock Deposit

For West-East Natural Gas Pipeline Project, it should build the underground oil and gas storage cavern in the bedded salt rock deposit in eastern China. As the salt deposits have geologic characteristic of monolayer thickness and flabbiness interlayer in China, it is difficult to build the storage cavern in the salt rock deposit. It is also different from building the storage cavern by the salt dome in the foreign countries. Meanwhile, the interlayer has great influence on the building and stability of the storage cavern. Therefore, it needs fundamental data and building theory on the stability of the storage cavern in the bedded salt rock deposit to provide technical support for the practice.Based on building the storage cavern in the bedded salt rock deposit by the water-soluble building cavity, the paper combines the experiment with the theory analysis, the numerical analysis with the similar simulation to study the soluble property of the rock salt which is related to the building of the storage cavern and the mechanical property of the rock salt which is related to the stability of the storage cavern with interlayer, including the influence of the repeated stress and the strain rate on the interlayer and the mechanical property of the salt rock, and on the failure style of the rock salt with interlayer. Furthermore, it also uses FLUENT software to study the flow field running style of two kinds of the shape salt cavern and simulate the flow field style of the column salt cavern in the laboratory.The following are the main research contents and results:(1) The dissolved angle has great effect on the dissolution rate of the rock salt. The dissolution rate is the fastest if the inclined solution angel is -45°upwards. The dissolution rate in -45°is 3 g /cm2*h, which is 6 times faster than that in 90°(0.5g /cm2*h). The glauberite dissolution rate is three grades lower than that of 90°, which belongs to the difficult dissolution salt.(2) The average peak strength of gypsum dry specimens is 13.3MPa. After saturated in brine and the repeated loading and unloading, the strength of gypsum does not fall. However, the ability of deformation of the specimens after saturated in brine is enhanced. The strength of salt-mudstone is the same as that of mudstone, either dipping in water or brine. The experiment shows that strength and modulus of elasticity have no change in twenty-day dipping.(3) Due to the different Poisson's ratio in the rock salt with interlayer, it results in inconsistencies of the horizontal deformation. The additional stress in the horizontal direction near the interface of rock mass makes the uniaxial stress into the triaxial state of stress. The rock mass with larger elastic modulus and smaller Poisson's ratio becomes into the state of three-dimensional pressure-tensile stress while the rock mass with smaller elastic modulus and bigger Poisson's ratio becomes into the state of large compressive stress. The failure style of rock salt with interlayer shows tensile cracks in the part of the rock salt, i.e. the columnar fracture; the part of the interlayer shows pressure pulling failure, i.e. cone-shaped ring from outside to inside shear rupture. It shows stress repetition on the uniaxial stress-strain curves.(4) Influenced by the cyclic loading, the strength of Glauber's salt is greatly reduced. The Young's modulus in the unloading process is a little bit higher than that in the loading process. The Young's modulus of the rock salt and the rock salt with interlayer change little with the stress levels and the loading and unloading times in the loading and unloading process. The initial cycle of loading and unloading curve is linear and overlapping. With the stress levels and the cycle times improving, the hysteresis loop can slowly show, but the hysteresis loop area is very small.(5) The loading strain rate has few effects on the strength of the rock salt. The elastic modulus increases slightly with the increasing strain rate, but it is comparatively smaller increase. The Poisson's ratio of the rock salt decreases with the increasing strain rate, which indicates that the ability of the lateral deformation diminishes. With the loading strain rate increasing, the strain decreases when the stress reaches the peak. The deformation modulus has logarithmic relation with the strain rate loading: E 0 = 0.2Ln(ε?)+3.2(6) In the single-layer rock salt deposits, the directional control connectivity dissolution mining techniques is used to build cavern. The shape of cavern is similar to oval or quasi-circular. The diameter is about 40-50m (less than the deposit thickness of single layer) and the horizontal length ranges from 500m to 1000m according to the geological conditions. The main idea of building cavern through interlayer is that circulation way should be reasonably selected, and mixed-built cavern methods are combined to real-timely adjust the position between the central tube and the intermediate tube.(7) The circle melting cavity is made as an example to do numerical simulation. It is studied that when cavity radius is less than 30m and the oil pad location is nearer than the distance from the injection port cavity radius (i <1), the moving rate of the fluid flow near the wall increases with the distance between two mouth of the tube which is ideally 20 ~ 30m.When the distance between the oil pad location and the injection hole is bigger than that of the cavity radius (i≥1), the ideal distance of two mouth can more than 30m.(8) After the analysis of combining the physical modeling with the numerical simulation, it is known that in order to speed up the dissolution of the salt near the cavern wall or the building of the whole cavern, the distance between two mouth of the tube should not be large when the cavern radius is smaller; the distance between two mouth of the tube can be increased (>40m) when the cavern radius is more than 30m. In addition, the out water mouth can be adjusted to vertical small-caliber porous jet to greatly improve the parietal near the solute convection and speed up the dissolution rate of the rock salt and the building of the cavern.