For the oil reservoirs with permeability less than 10×10-3μm2, the recovery rate is unsatisfactory because of poor reserve abundance, low pressure, low injection rate and high initial water saturation. This paper discusses the necessity and rationality of injection well fracturing, and optimizes the well spacing and fracture length of the production and injection well in extra low-permeability oilfields. The production wells are usually fractured for higher yield, however, injection wells are rarely fractured.

Mass data from field practice are analyzed in this paper. Changqing, Daqing and Jilin oilfield have all witnessed the journey of square inverted nine-spot well pattern, diamond shaped inverted nine-spot well pattern, square five-spot well pattern, rectangular well pattern, and linear injection pattern. The sweep efficiency and water flow direction are confirmed by core data in lab from 9 infill wells in a square inverted nine-spot well group in Changqing oilfield. Good agreement is obtained with field observations and lab. Using oil reservoir engineering method and numerical simulation with the Non-Darcy flow, this paper describes the optimization well pattern with injection and production wells fracturing.

After water flooding in the conventional areal well pattern, the injection water will flow toward the direction of the maximum principle stress, and finally forms the linear water injection, resulting in the low oil recovery due to the heterogeneous lateral water flooding. After a near 30-year water flooding development, the core data from 9 infill wells have proved the vertical sweep efficiency achieved only 50%, while the plane sweep efficiency 60%. Both water injection rate and oil production decrease with the increase of water cut. However, as water injection channel is formed with injection well fracturing, lateral water flood will displace homogeneously from the row of water wells to the row of oil wells. The injection wells fracturing increase the perforation and the injecting efficiency. Through the optimization of wells and the fracture length with the numerical simulation, an effective displacement pressure system is established for extra low-permeability oilfields. Compared with the adjacent area, Zhaozhou oilfield pilot tests were carried out according to the simulation studies and have achieved encouraging results in Daqing oilfield.

The novelty of injection well fracturing is in the ability to improve the injection efficiency and displace homogeneously toward the channel designed. The higher injecting efficiency is, the less injectors, well spacing density, and cost will be required, which properly serves the situation of low oil price.

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