Oil & Gas Geology ›› 2018, Vol. 39 ›› Issue (4): 696-705.doi: 10.11743/ogg20180407

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Characteristics and formation mechanisms of waterflood induced fractures in low-permeability reservoirs: A case study from Chang 6 reservoir in Ansai oilfield,Ordos Basin

Zhao Xiangyuan1,2, Zeng Lianbo1, Jin Baoguang3, Wang Ce4, Li Shujun4   

  1. 1. College of Geosciences, China University of Petroleum(Beijing), Beijing 102249, China;
    2. Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China;
    3. CNPC Greatwall Drilling Engineering Company Ltd., Beijing 100101, China;
    4. Changqing Oilfield Company, PetroChina, Xi'an, Shaanxi 710018, China
  • Received:2017-08-29 Revised:2018-02-21 Online:2018-08-28 Published:2018-07-23

Abstract: The study took Chang 6 reservoir in Ansai oilfield of Ordos Basin as an object to demostrate the essential ch-aracteristics and the formation mechanisms of waterflood induced fractures,based on the analysis of dynamic production features of the water-injecting lowpermeability reservoir.And the main formation mechanisms of waterflood induced fractures in the study area were numerically simulated in the end.Waterflood induced fracture is an injector-centered open fracture or a fast flow channel,which is formed when the water injection pressure exceeds the opening pressure of various fractures or the formation fracture pressure during the long-period waterflooding of low-permeability reservoirs.The fractures are an artificial geological attribute and the main cause of heterogeneity for waterflood development of low-permeability reservoirs,and they are widely occurring and inevitable in these reservoirs.Three formation mechanisms of waterflood induced fractures were summarized as follows:(1) when the water injection pressure exceeds the opening pressure of natural fractures,the natural fractures become open and are extended; (2) When the water injection pressure exceeds the formation fracture pressure,new fractures are developed throughout the injection life; (3) Other types of artificial fractures are generated by production or stimulation measures such as perforating and fracturing around the injector.In Chang 6 reservoir,high angle structural shearing fractures,in en echelon arrangement,are widespread approximately along the direction of major principal stress,and the dominate fractures are the first type of waterflood induced fractures.According to numerical simulation results,the bottom hole pressure of the injeciton well exhibits a continuous irregular periodic change with the continuous expansion of the waterflood induced fractures.

Key words: waterflood induced fracture, waterflooding development, low-permeability reservoir, Ansai oilfield, Ordos Basin

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