The water-injection induced fractures are key factors influencing successful water injection development in the extra-low permeability reservoir. As water flooding, the water-injection induced fractures have the distinctive characteristic of dynamic fracture growth, which fasten the water breakthrough time and decrease the production and final oil recovery rate. However, it is difficult to analyze the distribution of micro-fractures using logging or seismic data, because of the small size. This paper presents the integrated methods to recognize the water-injection induced fractures, and gives the water-flooding policy limit to control the dynamic fracture growth.

The methods integrate well testing, water injection outflow relation performance curve and tracer test. Firstly, the log-log type curve analysis of injector well drawdown test shows the characteristic of dual porous media model with fracture. Secondly, during deliverability test, a series of injection pressure can be obtained as water injection rate changes from small to big. The relationship curve type between different injection pressure points and the injection rates can be considered as a good indicator of ater injection outflow relation performance curve. Finally, the water flow velocity, direction and path is monitored through use of tracer. Once the injection pressure is more than 6-8 MPa, the slope of the OPR curves increases obviously. The three methods evaluate the distribution and growth of the water-injection induced fractures.

The integrated methods are applied to Ansai oilfield in China, with the permeability 5 ×10-3μm2. The reservoir's mid-depth is around 1400m, and formation pressure factor about 0.8. Once the water breakthrough is indicated form field data, the water cut ratio rise sharply, which result in the invalid circulation of water flooding. To evaluate the dynamic fracture, the injector well drawdown test is carried out. Furthermore, the tracer monitoring show the water flow direction and path, which help to design infill wells. The ater injection outflow relation performance curves show that the injection pressure should be less than 6 MPa, which can control injection-induced fractures propagating into the reservoir towards the producer wells, and postpone the water breakthrough time.

This systematic technique provides a reliable method to evaluate the water injection induced fracture and control dynamic fracture growth, which is helpful for significantly improving water management strategies, potentially increasing oil recovery and reducing operational costs (invalid circulation of water and water treatment facilities reduction), thereby enhancing the project economics.

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