Northern China Typical (especially) Low Permeability Sandstone Reservoir Fracture Study

The development tendency of Chinese oil industiy indicates that the large-scale oilfields with good physical properties are fewer and fewer. Hence the exploration and development of low-permeability oilfields are increasingly exerting attention. Since 1990s in China, the ratios of reserves of low-permeability reservoirs to total proven reserves are higher and higher. At present, the reserves of low-permeability reservoirs reach more than 70%. As a result, low- permeability oilfields are thought to be the main bases in Chinese petroleum indusUy. With the development of low-permeability oilfields, it is found that fractures play important roles in waterflooding. On the one hand, natural fractures may increase the permeability of reservoirs, link up pores and improve the production of oil wells, which is profitable to reservoirs. On the other hand, fractures will increase the heterogeneity of reservoirs, result in the water channeling along fractures in the waterflooding and as a result play a negative role in the developmenL In a word, fractures not only decide waterflooding effects, but also control layer division and pattern arrangement. Hence, the study of fractures has been attached more and more attention and it is of significance to get a deeper understanding and study. Chinese petroliferous basins can be classified structurally as extensional, stable and compressional basins. The purpose of the thesis is to develop the different fracture geologic models for different basins. Many typical low-permeability sandstone reservoirs such as Toutai, Yushulin, Chaoyanggou, Hanan, Shanshan, Changqing and Chuankou oilfleld are selected to study the characteristics of fractures. In the Songliao basin, fractures are fanned through the two stages - the extensional and reversal stage. Two or three fracture orientation sets could develop during the early extensional stage. One set is the SN extension fracture and the others are shear fractures. Mineral cement is vety typical in these fracture sets. NE and NW shear fracture sets and EW extension fracture set are found to develop in the late reversal stage, which are controlled by folds. The Erlian Basin is an extensional basin and fractures are considered to be related to extension faults. The fractures extend along the direction of faults, that is NNE fracture set. The fracture nets play a very important role in the exploration and development of the Hanan oilfield. iii 2 In the middle stable basin, although folds and faults are few, fractures can be formed in the brittle rocks. The fracture types are the EW regional fractures, which were formed during Yanslian periods and the NE fractures formed during Xishan periods. The fracture distribution and density are related to lithology and rock thickness. Fractures related to folds are developed in the western compressional basin Sandstone micro-model experiment technique is used and the water-oil displacement experiment is done to simulate the conditions of reservoir flowing. Low-permeability sandstone reservoirs can be divided mainly into two types according to the characteristics, density and the effect of fractures on reservoirs. These two types are fractured sandstone reservoirs (including porous-fractured reservoirs and fractured-porous reservoirs) and porous reservoirs. Different types have different flowing characters duri...