Identification And Modeling Of Tight Sandstone Natural Fractures In Chang 7 Section Of Triassic Yanchang Formation In Jiyuan Oilfield,Ordos Basin

With the exploration and development go forward year by year,unconventional oil and gas resources have become the main growth of China's oil and gas reserves.As a major unconventional oil resource,tight oil is known as the "black gold" of the oil industry and is regarded as a new hot spot in the global unconventional oil and gas exploration and development after shale gas.The development of tight oil is often limited by the seepage capacity of the reservoir.However,tectonic fractures in such reservoirs is prevalent.Fractures change the mechanical characteristics of reservoirs,and control the seepage system of reservoirs furthermore,so it seriously affect the fracturing construction and oil or gas development efficiency.Therefore,fractures are of great value for the development of tight oil,and the effective characterization and evaluation of the development degree of fracture in tight reservoir become the key to the rational use and effective development of this resources.The Ordos basin is rich in tight oil.The chang7 section,which is developed in the semi-deep and deep lake,is the main source rock in Mesozoic,and is also the typical tight reservoir in the basin,but the fracture study of the Chang7 section is very weak.This research selected Chang7 section of the Jiyuan oil field in the Ordos basin as the target layer,and took fractures as the target.Based on the theories of reservoir sedimentology,sedimentary petrology,tectonics geology and logging geology,identified and modeled the fractures by combining the non-linear theories and phased modeling techniques and other techniques.The characterization of fracture is a multi-dimensional comprehensive system,the single identification technology and prediction method can not meet the needs of fracture evaluation due to the small size and strong anisotropism of spatial development characteristics.The validity of fracture model is questionable on the basis of limited imaging logging data.This research discussed the validity of fractal dimension of conventional logging(AC,GR)in fracture detection,and established discrete fracture network model and fracture equivalent attribute model by using imaging logs and conventional logging data,based on the comprehensive analysis of fracture development characteristics obtained from outcrops,cores,imaging logs and other data.The main achievements and understanding are as follows:1.The main controlling factors of fracture development are lithology,stratum thickness and tectonism in the study area.The fractures with high angle in the study area presents typical structural genesis characteristics.There are two groups of conjugate fracture systems with the trend of NEE and NW,NNW and NE.Due to the heterogeneity of the rocks,one of each conjugate fractures is more developed and the other is less developed,in which the NEE and NNW fractures are more developed.2.Conventional logging fractal dimension curves under lithologic control can constrain fracture development probability in vertical direction.The method can not identify the number and orientation of fractures,but it can predict favorable zones of fracture development.Compared with the interpretation results of imaging logging,the accuracy of the method to detect fractures is 76%;3.The spatial resolution of DFN model is low under the independent imaging logging constraints.The conventional fractal dimension curve is used to conduct interborehole infilling in imaging logging Wells.The DFN model established is of high resolution and the range of fracture development zone is more concentrated.As the two modeling methods adopt the same fracture parameters(density,size,occurrence,etc.),the occurrence and density of the fracture in the two models are the same4.The fractures has little effect on the porosity of the reservoir,but has great effect on the permeability,local permeability reach dozens of millidarcy.The model shows that there are significant differences in permeability between different sublayers and horizontal direction of fracture(i),vertical direction of fracture(j)and parallel direction of fracture(k)of each sublayer;5.The equivalent fracture property model of each small layer and each group of fractures in the target layer shows that the permeability value parallel to the fracture direction(k)is the largest,and the vertical permeability improvement of the reservoir is more obvious.In a specific geological and historical period,it can be used as the dominant channel for oil and gas migration and the channel for oil and gas dispersal.The study of "three histories" of the basin in the study area shows that the common development of fractures is the necessary condition for the multilevel composite reservoir in Jiyuan oilfield.