Fracture Prediction Of Chang 6 And Chang 8 Low-permeability Reservoirs In Yanchang Formation,Fanxue Oil Region,Ordos Basin

Fractures are an important channel for oil and gas transport in low-permeability reservoirs,and also have a large impact on the development of oilfield waterflooding development.Accurately predicting reservoir fractures,forming an objective understanding of their planar distribution characteristics and analysing the main control factors affecting reservoir fracture development are the keys to effective and efficient waterflooding in low porosity,super-low permeability reservoir.The Chang 6 and Chang 8 oil formations in the Fanxue oil region have shown nonhomogeneous water absorption in injection wells,decreased drastically oil production after water breakthrough,and directional watered out during long-term waterflooding development.It is imperative to conduct systematic studies on the identification and prediction of sandstone fractures and their distribution characteristics,and to clarify the extent of fracture development in the study area.In this study,a fracture response sensitivity curve was selected based on the conventional logging data of sandstone fracture segments identified in core and acoustic-electric imaging logging data,and a comprehensive model for sandstone fracture prediction was constructed using fracture indication parameters such as curve change amplitude,curve change rate and curve fractal dimension.The model was applied to identify and predict sandstone fractures in the Chang 6 and Chang 8 oil formations in the Fanxue oil area,and the fracture spreading characteristics and main control factors were studied.The following key insights were obtained:(1)Based on the fracture scale methods of cores,acoustic and electrical imaging logs and conventional logs,the fracture indication parameters were constructed using the characteristics of curve change magnitude,change rate and fractal dimension,and a comprehensive sandstone fracture prediction model was established to achieve effective prediction of sandstone fractures in the Chang 6 and Chang 8 oil formations in the study area.(2)The predicted results from conventional logging data are in good agreement with the imaging logging interpretation and core features,but there are still some fracture misclassifications that need to be removed by manual intervention.(3)The sandstone fracture planes of the Chang 6 and Chang 8 oil formations in the Fanxue oil area are predominantly NE-SW oriented,followed by NW-SE and near EW.The fractures in the study area are controlled by basement fractures and regional tectonic activity since the Middle Cenozoic,mainly as a result of the combined effect of tectonic movements during the Yanshan and Xishan phases.The degree of sandstone fracture development is closely related to the lithology and its combination,as well as the sedimentary micro-facies.