| Underground water-sealed cavern has been an important way and development direction of oil and gas storage at home and abroad.Due to its advantages of large storage capacity,small area occupation,low cost,high safety and environmental friendliness,large-scale construction is being carried out rapidly.Underground water-sealed caverns are generally constructed in hard rock areas such as granite with high strength.In international construction,the concept of weak support is followed for the surrounding rock of the caverns.The basic guarantee conditions for its construction and safe operation are the stability of surrounding rock of underground cavern group and the water-sealed property of groundwater.The characteristics and requirements of such engineering must make fully utilize the characteristics of rock mass,among which the most basic characteristics are the geological characteristics,structural characteristics and permeability characteristics of rock mass.Therefore,combining engineering examples to study the structural characteristics and permeability characteristics of rock mass in watersealed caverns has important theoretical significance and engineering value.In view of the fact that the rock mass in the water-sealed caverns has undergone long-term geological processes,and the geological characteristics and mechanical properties of the rock mass are quite different,it is generally classified as fractured rock mass.Therefore,its structural characteristics,conditions and groundwater related characteristics are relatively complex.On the one hand,the structural characteristics and conditions of fractured rock mass affect and control the stability of caverns.On the other hand,the structural characteristics and conditions of fractured rock mass affect and control the groundwater seepage field.Based on this,the characteristics of granite rock mass fracture and the related optimization of water curtain system in water-sealed caverns are determined by combining with the engineering examples involved.According to the permeability characteristics of this kind of fractured rock mass,the permeability of rock mass is very low,and the water in the rock mass mainly flows in the fractures with relatively strong permeability.Therefore,the basis for ensuring the water seal of underground water-sealed caverns is guaranteed lies in the connectivity and permeability of rock fractures,thus achieving the reasonable design of water curtain system and the seepage control of the whole underground water-sealed caverns.In conclusion,it is of great theoretical significance and engineering value to study the fracture connectivity and permeability of granite rock mass in water-sealed cavern and its application in the optimization of water curtain holes.Based on the water sealed cavern project participated in,the thesis takes the granite rock mass of the water sealed cavern as the research object.Firstly,based on the identification data of thin sections of the cavern rock mass and the macroscopic geometric statistical characteristics of fractures,a three-dimensional fracture network model of granite rock mass that conforms to statistical characteristics is constructed;Secondly,a definition of three-dimensional fracture network connectivity was proposed for this type of granite rock mass,and the fracture tensor was introduced to quantitatively characterize the connectivity of the three-dimensional fracture network under background conditions.Through orthogonal experimental design,the influence of geometric characteristic parameters of fractures on the connectivity of the rock mass fracture network was revealed,and the threshold for evaluating the connectivity of the three-dimensional fracture network of this kind of granite was comprehensively determined;Thirdly,based on the pipe network seepage model,the permeability of granite rock mass threedimensional fracture network and its characterization are analyzed.Finally,combined with the characteristics of the water-sealed cavern engineering,the hydrogeological concept model is established.Through numerical experiments,the seepage field characteristics of the cavern reservoir under three different fracture network connectivity and permeability conditions are studied.Based on this,the optimal design of the water curtain hole is carried out,so as to achieve the water sealing property required by the cavern engineering.The research results enrich the research of water sealing theory of underground water sealed caverns and provide a theoretical basis for the optimization design of water curtain system of underground water sealed petroleum caverns.The main research results of this thesis are as follows:(1)Based on the thin section identification data of rock masses,the material composition and rock structure characteristics of the reservoir rock masses were analyzed,and the macroscopic geometric statistical characteristics of fractures in the reservoir rock masses were summarized.The dominant fractures groups are mainly 3 or 4 groups,which are mainly in steep dip angle,and the strike is mainly NE and NW;Based on the brief introduction of the fracture sampling method,the characteristic statistics and parameter estimation methods of fracture information are systematically analyzed,which can realize the conversion of the limited fracture information on the two-dimensional outcrop to the three-dimensional spatial fracture parameters information,laying a foundation for the subsequent research on the connectivity and permeability characteristics of rock fractures.(2)According to the definition of the connectivity state of the three-dimensional fracture network,the connectivity function of the two opposite planes of the threedimensional fracture network was proposed,and on this basis,the connectivity rate of the three-dimensional fracture network was defined.The concept of fracture tensor was introduced to quantitatively characterize the connectivity of three-dimensional fracture network,and the connectivity indexes of fracture network of different sections in the study area were compared and analyzed.The results showed that: the area of fractures per unit volume and the average number of intersection points of each fracture cannot reflect the directional connectivity of the three-dimensional crack network.However,the directional component of the crack tensor has better directional correlation with the directional connectivity of the crack network,and the direction of the maximum value of the crack tensor corresponds to the direction of the weakest connectivity of the fracture network.(3)Based on the orthogonal experimental design,the influences of different geometric parameters on the fracture network connectivity indexes were studied,and the correlation analysis was conducted on the selected fracture network connectivity evaluation indexes to comprehensively determine the threshold of three-dimensional fracture network connectivity evaluation indexes under different geometric parameter combination conditions: the area of fractures per unit volume is 0.4m2/m3,the number of intersections of each fracture is 0.55,the maximum proportion of connected clusters is 0.45,and the proportion of isolated fracture is 0.45,the first invariant of the fracture tensor is 10,and the anisotropy index is 1.0.(4)Based on the pipe network seepage model,the influence of five geometric parameters(including fractures group strike angle,fracture dip angle,length,aperture and density)on the permeability of three-dimensional fracture network was studied.The research results show that: The length and density of fracture are the main factors affecting the permeability of fracture network,and the change of strike angle and dip angle between fracture groups affects the anisotropic characteristics of the permeability of fracture network.(5)Considering the correlation between fracture length and aperture,the correlation function relationship between fracture length and aperture with different distribution laws was established.Research was mainly focused on the fracture length following with negative exponential distribution,aperture following with lognormal distribution.Permeability of three-dimensional fracture network was analyzed with the constant aperture,the lognormal distribution aperture,and related to the fracture length-aperture.The results show that: the permeability of rock mass fracture network considering the correlation between fracture length and aperture is significantly higher than that without considering the correlation,and the permeability of rock mass fracture network with lognormal distribution aperture is the least.The distribution form of the aperture can affect the directivity of the equivalent permeability coefficient of the fracture network.The anisotropy of the equivalent permeability coefficient is the maximum when the length is correlated with the aperture,the minimum when the aperture is lognormal,and the anisotropy of the equivalent permeability coefficient is between the two when the aperture is constant.(6)The typical (?)berg water seal criterion(i.e.no leakage when hydraulic gradient is greater than 1)is deduced and verified according to the force balance principle of the oil and gas bubbles stored in rock mass fractures in underground cavern.It is equivalent and consistent with the criterion of no leakage when the circumlunar water pressure of the main cavern is greater than the oil and gas pressure in the cavern adopted in this study.Based on the water sealing criterion and the water inflow of the main cavern,the optimization of the water curtain hole is demonstrated.(7)Based on the numerical experiments of three types of granite fractures with different development characteristics and anisotropic characteristics,combined with the engineering examples of water sealed caverns,the following understanding has been obtained based on the overall characteristics of the seepage field in the cavern area revealed: under the three working conditions,parallel and vertical arrangement of water curtain holes results in higher total groundwater head and pressure around the main cavern,higher groundwater flow velocity,and larger water inflow;Water curtain holes are arranged at an angle of 45° with the main tunnel.The total groundwater head and pressure around the main tunnel are low,the groundwater flow rate is small,and the water inflow is small;Taking into account the water sealing effect and water inflow,the optimal design choice is to set up water curtain holes at a 45° angle with a spacing of 20 m between the main tunnel.Research has shown that the spacing can be reasonably arranged based on the permeability characteristics of the rock mass in the cave storage area: the smaller the permeability coefficient,the smaller the spacing required for water curtain holes to meet the water sealing criteria;As the permeability coefficient increases,the spacing between water curtain holes can be appropriately increased.Through research,the innovative achievements of the thesis in the connectivity and permeability of granite rock mass fractures in water sealed caverns are as follows:(1)A connectivity function for the two opposite faces of the three-dimensional fracture network is proposed for granite rock mass,and based on this,the connectivity rate of the threedimensional fracture network in granite rock mass is defined.The research suggests that the directional component of the fracture tensor has better directional correlation with the directional connectivity of the fracture network,and the direction where the maximum value of the fracture tensor is located corresponds to the direction where the connectivity of the rock mass fracture network is weakest,and the connectivity index threshold of the three-dimensional fracture network of granite was comprehensively determined;(2)Considering the correlation between fracture length and aperture,the permeability analysis and characterization of three-dimensional fracture network under three conditions of constant fracture aperture,logarithmic normal distribution fracture aperture and correlation with fracture length and fracture aperture are carried out.The research shows that the permeability of rock fracture network considering the correlation between crack length and crack width is significantly higher than that without considering the correlation,and the permeability of rock crack network under the condition of logarithmic normal distribution crack width is the smallest,the distribution of fracture aperture can affect the directionality of the equivalent permeability coefficient of the fracture network;(3)Based on the above research and understanding,the optimal setting parameters for the water curtain holes of the engineering example water curtain system are: a 45° angle with a spacing of 20 m from the main tunnel,and the hole depth remains the original design parameter,which can ensure the water sealing conditions of the cavern. |
