Waterflooding Optimization Using Improved Streamline Simulation for the Biggest Fractured Low-Permeability Reservoir in China
Authors: Lei Zhengdong , Zhu Zhouyuan , Liu Shuangshuang , Yu Tao , Yuan Jiangru , Li Jiahong , Zhang Jing , Li Lei , Zhao Hang 
Published: October 2018

Waterflooding has been the main development method for fractured low-permeability reservoirs, which is often associated with poor sweep and high water cut due to natural, hydraulic, and water injection induced fractures in these reservoirs. Streamline simulation is a potential solution for optimization of waterflooding in this scenario. In this work, we present use of streamline simulation and its inter-well flux information to improve overall waterflooding performance in fractured low- permeability reservoir.

One of the main advantages of streamline simulation is to visualize fluid flow pathways, obtain inter- well flux information, and to adjust well rates accordingly to maximize recovery. Describing fluid transport in fractured low-permeability reservoirs brings additional challenges due to complicated matrix-fracture interactions. We first analyze the impact of hydraulic fracture growth and fracture-well pattern orientations on waterflooding sweep. A judging criterion of injection sweep factor is proposed for flood optimizations accordingly. Streamline simulation model is established for the reservoir sector under study. Then, we make injection/production rate adjustments based on streamline-derived injection efficiencies. Finally, reallocation of injection fluid from low efficiency to high efficiency injectors is conducted in order to improve volumetric sweep efficiency and improve project economics correspondingly.

The application of this workflow is demonstrated with a real field example of a fractured low- permeability sandstone reservoir in Changqing oilfield in China. A comprehensive streamline simulation reservoir model is built for this reservoir with fine geological details. To avoid further growth of injection-induced fractures, we conduct thorough geomechanical studies together with field observations, which show the limit of preventing further fracture propagation and rapid water breakthrough being injection rate of 30 m3/day. With such practical limit honored, streamline simulation is conducted, with streamline-based inter-well flux information processed to identify well pair interactions and calculate metrics such as injection efficiencies and well allocation factors. By using our new judging criterion to determine target sub-region based on sweep efficiency, we have successfully conducted flood optimizations using both empirical correlations and also linear programming method. New water injection strategy is implemented and a decrease of water cut in this challenging sector is achieved for the target producers. The flood management has resulted in positive response with production rate increase of 20% in this pilot area with twenty-eight producers. Furthermore, this optimized development plan achieved more than 6% incremental oil recovery compared with the base case.

This work provides a comprehensive workflow for using streamline simulation for better understanding of the impact of fractures on waterflooding performance and also for optimizing production to maximize oil recovery in fractured low-permeability reservoir.