The waterflooding method is the most common practice for reservoir pressure maintenance during oil field development. At an early stage, when planning waterflooding activities, it is especially important to possess a reliable data on water-oil displacement. The conventional way of data acquisition based on laboratory studies is associated with difficulties in the restoration of reservoir wettability, and account of alterations in reservoir and injected fluids properties.
Srednebotuobinskoye filed located in the Western Siberia is characterized by significant variability of geological and physical properties. The laboratory studies show that the oil viscosity over the entire section of the reservoir varies fr om 7 to 28 mPa; water viscosity changes from 1 to 4 mPa; relative permeability to water varies from 0.003 to 0.3. The reservoir properties were re-defined both in the laboratory and in the field conditions, while implementing pilot production and well testing during full field development. At the stage of field studies an approach was applied to the analysis of injection wells that compared productivity and injectivity parameters after conversion to injection (injectivity/productivity). Injectivity/productivity ratio in steady state shows the relative mobility of injected water. To analyze the wells performance it is convenient to use the diagnostic plot, wh ere the productivity index is plotted along the X-axis, and the injectivity index is plotted along the Y-axis. The deviation from the mean to the X-axis indicates the possible well damage. The deviation to the Y-axis indicates possible movement of water to different horizons, or well operation in autofrac mode. For Srednebotuobinskoye field the application of injectivity/productivity diagnostic plot helped determine wells with indications of autofrac or water movement to different horizons, as well as wells with reduced injectivity attributed to damaged wells. Additional well tests confirmed the conclusions on hydraulic fracture overpressure in the group of wells with increased value of injectivity/productivity ratio.
The analysis of injectivity and productivity ratios showed that the mean value of injectivity/productivity parameter varies within a narrow range of 0.5 to 0.8. Possible high levels of relative mobility of injected water in the oil-wet reservoir have not been confirmed. Operation of injection wells with low-salinity water is characterized by low relative mobility values, with the stable displacement front to be expected. The improvement of injected water mobility has confirmed the right choice in favor of selecting the base line-drive water flooding system (with the production-to-injection wells ratio being 1/1).
References
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