Study On Multi-scale Structural Features And Characterization Method Of Shale Reservoirs

The shale pores and fractures are multi-scale discontinuous structure,the extensive development of which makes the shale become natural gas reservoirs.The pores and fractures in different scales are reservoir space and seepage channel of shale gas.The accurate characterization of the shale multi-scale structural features is the necessary part of shale gas reservoir evaluation and fracturing transformation,which is of great significance to the exploration and development of shale gas.The identification and description of the discontinuous structure of shale in different scales and space has become the key problem of the research on high-efficiency development technology.However,there is still no characterization method of discontinuous structure in shale from microscopic to macroscopic.So the systematical reasearch on the multi-scale structural feature of shale in Longmaxi Formation is conducted,and the characteristization method of discontinuous structure in shale from microscopic to macroscopic is established.The main contents and knowledge are as follows:1.The study on characteristics of shale reservoir is carried out,which lays the foundation for the study of multi-scale structural feature and control factors of shale.The mineral composition of rock,the mineral composition of clay,the contentn of TOC,rock mechanics parameters and porosity parameters of shale in region of interest were tested,then the distribution law and the correlation was analyzed.2.The microscopic structure of shale,studied by field emission scanning electron microscopy,can be divided into three cathegories: organic matter hole,inorganic pore and microfracture,and the basic feature of microstructure in different types are described.3.The micro-pore structure of shale was characterized by high pressure mercury intrusion method,N2 adsorption method and CO2 adsorption method.The development control factors of micro-pore were analyzed according to the features of shale reservoir.The results show that the mesopores are the main contributors to the pore volume,followed by micropores,and the megalospores contribute least.The contribution of micropores to the specific surface area of shale dominates,and the contribution rate is 80%.4.Based on SEM image observation,the characterization method of shale pore structure is established by using the integral geometry method,and the intuitive description is conducted.The results show that the pore diameter of the shale is mainly 1 ~ 50 nm and the pore peak is 13 nm.The mesopores is the main contributors to the porosity,followed by micropores,the megalospores contribute least.5.Regard the contribution of pore diameter with size more than 500 nm to pore volume of shale in high pressure mercury test as a quantitative indicator for microfracture development.Based on the analysis of control factors of shale microfracture development and the observation results under microfracture microscope of shale,the prediction model of microfracture development degree of shale is established,and the development of shale microfracture in a well of Longmaxi Formation is predicted.6.Based on the direct observation and statistical method,the basic features of shale macroscopic outcrop fractures,fractal,fracture spacing,fracture inclination,fracture length,fracture opening and filling state of core fractures are studied.Combined with the geomechanics of the shale reservoir,the main control factors of the shale core fractures development are analyzed.7.The characterization model of the development degree of the shale network is established,and the calculation model of characterization parameter is established by using integral geometry method.Monte Carlo simulation method is used to verify the parameter calculation model.The outcrops of fracture network are fully characterized by using the established model of the shale fracture network.8.The basic parameters of the network topology of shale fractures are defined,and a characterization method of the fractured network connectivity based on the topological structure is established,which is used to characterize the connectivity of core fracture network and outcrop fracture network of shale.Based on the systematial model of fractured network,the concept of comprehensive evaluation index of fracture network is put forward,and a characterization method of rock fracturing network based on fracture connectivity is established.Then compare and analyze the shale fracture network in Longmaxi Formation and Wufeng Formation.9.The two-dimensional fractal network model of shale was established by MATLAB and FRACA software.The relevant parameters of each model were calculated.The influence factors of fracture connectivity and permeability were studied by numerical analysis.The quantitative relationships between the parameters of shale fracture networks is given.The connectivity of the shale network is controlled by the dimensionless density of the fracture network,and the fracture connectivity index is linearly related to the dimensionless density.The equivalent permeability of the shale network is in a definite power relationship with the fracture connectivity index.Then the quantitative characterization and prediction of the network connectivity and permeability of the shale fractures can be achieved.10.A network connectivity estimation model of fractured rock mass based on core fracture information is established by using integral geometry method.The fracture network is classified according to the connectivity index of fracture and a method for characterization and evaluation of shale fracture network based on connectivity of core fracture is presented.