The Study Of Fractured Rock Permeability In Huangdao Water Sealed Underground Oil Tank

Huangdao water sealed underground oil tank lies in Huangdao district, ShanDong province,which is one of strategic oil storages being build recently and is the first underground oil storage with the capacity of 300×104m3 in our country. In this paper,fracture causes and distribution,fracture permeability,discrete fracture network simulation and discrete fracture connectivity are analysed based on the projects of Huangdao water sealed underground oil tank.The Proterozoic granite gneiss is the major lithology in the study area as Low-permeability fractured media is the ideal location for the underground oil tank. The research of spatial structure and basic hydraulic parameters can provide critical hydrogeological data for the following analysis of fracture flow simulation and water seal effect evaluation.During the long process of geologic evolution, different quantity of tectonic stress displayed in size and directions have had so significant impact on the rock mass so that which shows the characteristics of heterogeneity,randomness,diversity and complexity.All of these characteristics makes the research of rock mass structure becomes more complex.However, it was found that the fracture of rock mass are regulared statistically and be controlled by regional tectonic movement.So,the identification of tectonic movement in study areas is basis for fracture network simulation and fracture flow model establishing.With the development of computer technology,network simulation of rock mass discontinuities which is based on statistics and probability theory is improved and brought into practice gradually.Network geometric graphics are simulated with the probability distribution of the discontinuities geometric parameters that are Measured in-situ.The whole proeess will be completed by the Monte-Carlo method.Seepage in fractured rock research is important for scientific research and application of engineering. With the development of engineering disciplines and the showing of actual engineering problems to be solved urgently,it is more and more concerned by researchers.The study involved a number of areas, such as geotechnical engineering, water conservancy and hydropower engineering, petroleum engineering, and construction of rock structures in underground projects. In this paper, flow characteristics of the study area are discussed in two sides:fracture permeability tensor and the fracture network connectivity.1. Fracture permeability tensor which is the basic parameters in fissured area is expressed through the matrix in mathematically. three-dimensional fracture permeability tensor of study area was caculated based on the statistics of field surface cracks and drilling image fracture results.Meanwhile,the result was rectified by water pressure test to Reflect the horizontal and vertical variation of the fracture.2. Fissure fracture network connectivity analysis based on fracture simulation.It is very significant to clarify fracture aperture, trace length, hydraulic conductivity coefficient and so on before the analysis of fracture connectivity.Components of the fracture network is based on the simulation results which to determine an effective impervious boundary, all the connected region share the same hydraulic fracture link within it. Connected component algorithm is also based on the logic cutting mutual relations.According to the above theories and analysis, this paper did following three researches for Huangdao water sealed underground oil tank:1. Law of fracture distribution analysisIn this paper, the causes of faults and tectonic theory was used to analyse the fracture distribution of the region which is characterized by the Yanshan Pacific Plate subduction to the W or to the NW direction.Complicated tectonic movements of early Yanshan,mid Yanshan and late Yanshan stage contribute to fracture formation.In the early-Yanshan stage, main fault zone (including the F3, F4)formate in the area, while secondary to the NE and NW extension of the fracture; during the mid-Yanshan stage,in the pulling stress environment, dykes was forming while magma gushing up; during the late-Yanshan stage,study area was compressed W by the Pacific plate,while F3 show the nature of the right-transtension, and F4 are presented the left-transtensional,the fracture gap became larger in the west part of the study area in tensile stress.2. Variation of permeability tensor(1) A preliminary analysis of the study area on the plane and the vertical variation are sdudied from different scales to assess on the permeability.The hydrogeological and geophysical test results are used in this paper.(2) Permeability coefficient is very importance for fracture rock mass,but it is also difficult to confirm. A new technicalness was exertioned in this paper to get permeability variety not only horizontally but also vertically.For much date and complicated calculation in field project, VB program in Excel was developed to caculate principal value and directions of hydraulic conductivity.The result is principal directions of hydraulic conductivity is in accord with dip direction and dip angle in horizontal and the principal value of hydraulic conductivity is in accord with changes of aperture in vertical.(3)Permeability tensor derived from drilling imaging which could reasonably reflect the variation of the main value penetration,but human error is still exist because of the effect of recognition by the software. For this point, minimum pixel relatively estimation was used to make the correct estimation more accurate. And Also it is suggested that correct result through the water pressure test methods in different depths. The correction factor of this area is between 0.07 and 0.89. Through the correction in different depth, principal directions of hydraulic conductivity is stable and the principal value of hydraulic conductivity is in accord with water pressure test result quantitatively. 3. Fracture network simulation and seepage flow(1)This paper made a two-dimensional structure network simulation for the reservoir area based on the probability of fracture parameter statistical distribution types and distribution parameters. Fracture density map and RQD map in different directions was used to analyse the fracture around the tank.the result is:fracture density is less than 1 piece per meter and RQD distribution is between 70～90% in sectionⅠ; fracture density is less than 0.5 piece per meter and RQD is more than 90% in sectionⅡ.(2)Three-dimensional network simulation was used in the entire study area to analysis fracture network distribution,aperture distribution,transmissivity,coefficient of storage,fracture component and drilling connected zones.lt is find that connected direction of the area is in southeast base on the fracture seepage characteritics,meanwhile. Connectivity of ZK13 and ZK15 was simulated to make sure the range of infection area of water levels in boreholes.