| Summary: | Based on the dynamic characteristics of water-flooding in ultra-low permeability oilfields, the concept of dynamic fracture is proposed and its formation mechanism and impacts on oilfield development are discussed. Dynamic fractures refer to the new-generated fracture channels when bottom hole pressure exceeds rock breakdown pressure and propagation pressure, or the effective fracture channels generated when the originally closed or filled natural fractures are reactivated, due to high pressure in near wellbore area of injection well in a long term water injection of the ultra-low permeability reservoir. Dynamic fractures, controlled by the current stress field, constantly extend toward the direction of the oil well with the rise of the injected water volume and bottom hole pressure, until connecting with the oil well hydraulic fractures. The analysis of the sealed coring well groups L76-60 and L88-40 located in the same block in the JA Oilfield, Changqing Oilfield, NW China shows that the dynamic fractures changed the seepage characteristics of water displacing oil in the ultra-low permeability reservoir, greatly aggravated the reservoir heterogeneity, which led to the reduction of profile producing degree and the distribution of remaining oil on both sides of the fractures in continuous or discontinuous belts. The generation and propagation of dynamic fractures are closely related to water injection pressure, injection-production ratio, oil and water well stimulation measures. When the porosity-permeability condition of the matrix allows effective oil displacement, high pressure water injection or oil and water well stimulation measures should be avoided as far as possible. Key words: dynamic fracture, ultra-low permeability reservoir, development geological attribute, formation mechanism, water-flooding swept volume
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