Microscopic characteristics of ultra-low permeability reservoirs in the Shigang Oilfield of the Subei Basin and strategies for enhancing oil recovery

Abstract

The Shigang Oilfield in the Subei Basin is characterized by low-porosity and ultra-low permeability sandstone reservoirs. The natural productivity of the oil wells is low, but hydraulic fracturing followed by water injection can improve oil production capacity. However, during the development of the oilfield, problems such as low oil recovery rates, low recovery factors, and poor development effectiveness have become apparent, exacerbating development challenges. Therefore, it is necessary to identify the reasons for inefficient development and explore strategies to enhance recovery, providing a theoretical basis for improving the development effectiveness of the Shigang Oilfield. Using methods such as whole-rock mineral composition analysis, gas-measured core porosity and permeability parameters, and core sensitivity evaluation, the micro characteristics of the reservoir were analyzed in terms of rock mineral composition, pore-throat structure, and rock sensitivity. Numerical reservoir simulations and laboratory core experiments were conducted to study the distribution characteristics of residual oil after fracturing water flooding. Strategies to enhance oil recovery were investigated through nuclear magnetic resonance online displacement experiments. The results showed that the reservoir cores exhibited typical low-porosity and ultra-low permeability characteristics, with certain velocity sensitivity and water sensitivity during development. After fracturing water flooding, residual oil in the cores was mainly distributed in pore channels with diameters ranging from 0.01 to 1 μm. The use of surfactant flooding and secondary water flooding increased the recovery rate of residual oil in the cores by 14.81%. The main reasons for inefficient development included ultra-low reservoir permeability, the development of micro pores and micro fractures, and sensitivity to velocity and water. Water injection development could cause rock mineral swelling and migration, increasing flow resistance. Therefore, after fracturing water flooding, only the recovery of residual oil along the main flow paths was significantly improved, with the overall utilization degree remaining low, and a considerable amount of residual oil still concentrated in the reservoir. It is recommended to use chemical flooding and multiple rounds of displacement to enhance the utilization of oil in medium and small pore channels, further improving the development effectiveness of the oilfield.

Keywords

• low-permeability reservoir
• fracturing
• enhanced oil recovery
• technical strategies
• surfactant
• secondary water flooding