Study on Productivity Laws and Water Injection Optimization of Low-Mobility and High-Steep Heavy Oil Reservoirs

Abstract

Breaking through the “poor-permeability, sub-marginal, and viscous-oil” conditions is gradually becoming an important research direction for the Bohai Oilfield. Among these, low-fluidity and high-steep viscous oil formations constitute a typical category of complex heavy-crude geological units. Fluid flow resistance in this reservoir type is significantly elevated due to the combined effects of steep stratigraphic dip and low fluid mobility. This further triggers a sharp drop and insufficient output capacity of production wells, thereby hindering high-efficiency oilfield exploitation. According to the characteristics of mobility and high-steep heavy oil reservoirs in the Bohai Sea, a representative conceptual framework is constructed to investigate the production performance laws and injection optimization schemes of such reservoirs. Studies have shown that the main controlling factors for the productivity of this type of reservoir are mobility and dip angle. Under the combined effects of percolation impedance and gravity, peripheral water injection provides slow fluid replenishment to structurally high-positioned oil wells. This will cause the production well to have insufficient energy, resulting in a rapid initial decline. Only after the energy is replenished will the production capacity gradually recover. In the early stage of developing this type of reservoir, enhanced water flooding measures should be adopted to timely recover reservoir pressure to its initial level. Therefore, through the model, the optimization diagrams and empirical fitting equations for the rational injection-output allocation ratio were investigated for high-steep structural viscous oil formations featuring poor fluid mobility at their early development phase. Findings from this work have guided water-flooding intensification and reservoir energy replenishment in the Bohai L Oilfield. Following intensified water-flooding development in the oilfield, its daily crude output maintained steady at over 600 m3 for the whole year, basically realizing zero production decline of the reservoir.