Research And Application On Damage And Seepage Characteristics Of Fractured Rock In Underground Water-sealed Oil Storage Caverns

The strategic oil reserve is the first defense line to deal with the oil shortage crisis,and it is an important segment to guarantee the national defense and economic security.Underground oil storage,as a safer and more environmentally friendly way,has been paid much attention by many countries in the world.The mechanical properties of fractured rock subjected to fatigue damage will change,which will affect the rock strength and life.In addition,the expansion of water fracture channel will change the permeability,which will affect the water sealing property.Under the long-term coupling of formation rock,groundwater,oil and gas collection,the seepage characteristics of saturated fractured rock evolve,and then affect the stable seepage field of the whole region.Therefore,the application research of surrounding rock damage and seepage characteri stics is of great significance for the long-term stable operation of underground water-sealed oil storage caverns.In this paper,the osmotic damage constitutive model of fractured rock was derived through theoretical analysis.Performing a series of laboratory experiments,the characteristics of rock specimens fatigue damage under triaxial cyclic loading-unloading were revealed,and the fatigue life of rock materials subjected to cyclic load was explored.By means of extensive finite element numerical simulation and field monitoring,the seepage field under hydro-mechanical coupling during operation was evaluated,and a reasonable prediction model of exponential smoothing method was established.Based on the seepage characteristic evaluation and prediction results,the optimal design scheme of water curtain system for Huangdao underground oil storage caverns was presented.Research results are as follows:(1)Starting from the definition of damage mechanics and Darcy law,the relationship between damage variable and permeability was established by deducing the fluid flow equation of fracture channels,and the osmotic damage constitutive model of fractured rock mass was obtained.A damage variable represented by permeability was obtained by using the fracture uniform distribution model,and the permeability was positively correlated with the square of damage variable.However,in the fracture normal distribution model,the damage variable had a disproportionate relationship with permeability.(2)A series of tests such as triaxial cyclic loading-unloading fatigue damage characteristics test were performed.The results show that the cyclic load magnitude mainly control the fatigue life of the material.When high cycle fatigue occurs,the material may be compacted and the permeability decreases.When low cycle fatigue occurs,the material damage degree is large,the fracture increases and permeability increases.The thermal cycling test can promote the development of microfractures in natural granite specimens,but the development rate of microfractures show a gradually decreasing trend.Under different confining pressures and cyclic axial stresses,different degrees of fatigue damage or fatigue failure occur to rock specimens,and the parameters such as residual strain,deformation modulus and permeability change with different laws.(3)Exponential smoothing method is used to predict the short-term seepage pressure,and the prediction accuracy is evaluated.Based on the concept of minimum seepage pressure,the distribution of seepage field is simulated by using the finite element method.The results show that the minimum seepage pressure point occurs in the range of-20 m～-30 m,which is about the plane of cavern vault.Excessive water curtain pressure leads to the increase of groundwater velocity and water head around the oil storage cavern,especially spandrel.The seepage pressure curve on the middle line of two adjacent storage caverns presents three forms:"V" type,"Λ" type and"∽"type.(4)Based on the evaluation and prediction of seepage characteristics,combined with the first large underground water-sealed oil storage project in China,the optimization of water curtain system is further proposed.In order to ensure the sealing effect,the optimal water curtain hole spacing is 10 m,the height difference between the water curtain hole and the caverns is 25 m,and the optimal water curtain pressure is 70 kPa,which meets the vertical hydraulic gradient criterion.The smaller the height difference between the water curtain hole and the cavern,the greater the groundwater flow into the cavern.Under the influence of topographic factors,excessive water curtain pressure may cause a significant difference of groundwater seepage in each oil storage cavern.