13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Mechanics Mechanism of Re-fracturing in Inclusion Reservoirs Xue Shifeng1,*, Chen Xinmin1, Han Zhongying1, Zhang Weidong2 1 College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong, 266580, China 2 School of Petroleum Engineering, China University of Petroleum, Qingdao, Shandong, 266580, China * Corresponding author: xuesf@126.com Abstract Hydraulic fracturing is an important EOR technology of low permeability reservoirs which occupy a considerable proportion in China’s oil reserves. If hydraulic fracturing fails, re-fracturing will be used in most oil-water wells. However, there are often a large number of inclusions in low-permeability reservoirs. Due to the complexity of the re-fracturing mechanism in inclusion reservoirs, the theoretical researches are far behind the field practices. Theoretical and experimental studies on the mechanical mechanism of re-fracturing are significant for the development of inclusion reservoirs. Ground stress distribution is one of the major factors which affect fracture propagation after re-fracturing. In this paper line inclusion is seen as a thin bar, and the basic solution of elastic mechanics for line inclusion in infinite plane is used to solve the stress field. Stress field around artificial fracture which is influenced by single line inclusion is deduced. The interference problem of single line inclusion and artificial fracture under internal pressure is reduced to a set of Cauchy singular integral equations. In addition, a validation test with a large size (500mm×500mm×500mm) true tri-axial test equipment is carried out. The results show that, the inclusions reduce rock intensity, and induce reservoir to generate more complex fractures. Keywords Mechanical mechanism of re-fracturing, Inclusion reservoir, Evolution of stress, Fracturing physical simulation 1. Introduction The re-fracturing technology, as an enhanced recovery technique, was first developed in the 1950s, and it has become one of the important means to improve the recovery of oil and gas fields. The initiation and propagation of new fracture are greatly influenced by the reservoir structural characteristics during re-fracturing. Present studies mainly focus on the interference problems between artificial fracture and natural ones in natural fractured reservoirs. Many experiments have shown that fracture may present different state by the influence of various factors after the interference of artificial and natural fractures in natural reservoirs [1-6]. Zhou and Xue analyzed the impact of pre-existing fracture conductivity, stress state and injection rate on fracture path complexity [7]. On the basis of laboratory and field evidence, it is conceivable that several events may occur during the time period of the hydraulic fracture propagating toward the natural fracture [8]. However, there are numerous kinds of complex isolated inclusions in the reservoirs. The stress singularity may exist at the tip of the flake inclusions, and under certain conditions, it may also lead to the generation of micro-cracks [9]. Jin Yan et al. [10] analyzed the effects of salutatory barrier on hydraulic fracture propagation, but they have no further theoretical analysis. At present, the inclusion theory is mainly used for composite materials. In this paper, inclusion theory was introduced for the inclusion reservoir. Stress intensity factor is derived at the tip of artificial fractures and inclusions. The direction of fracture propagation after re-fracturing can be judged by the maximum tensile stress criterion in defect reservoirs. In addition, the effects of
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