Abstract: In the process of thermal recovery of heavy oil in Bohai Oilfield, cyclic steam stimulated wells often face problems of difficult steam injection due to poor reservoir physical properties or blockage of the circumferential borehole. Hydraulic dilation, an innovative technology,effectively solves the problems of short effective period and high operational costs by applying conventional well stimulation methods.However, there are few experimental studies on the characteristics of reservoir hydraulic dilation, especially those considering the initial cyclic steam stimulation stage or the true triaxial stress field in actual formation conditions. Taking Bohai L heavy oilfield as an example, the grain size of core sample has been analyzed and the microscopic physical properties of heavy oil layer before steam injection development have been observed, including the microscopic scanning and energy spectrum analysis. In addition, the mechanical and high-temperature hydraulic dilation features of core samples have been studied in the true triaxial stress conditions. The experimental results indicate that the average grain size of core sample is 225 μm, which shows fine–medium grain size. The microstructure of core sample is relatively loose, and there are asphalt cements among grains. The silicon and carbon contents are the highest in core sample. The shear dilation rapidly occurs in the uniaxial stress of core sample, but the core volume continues to be compressed due to the restriction of horizontal stress in the true triaxial stress conditions, without any dilation, showing low rock strength. In the true triaxial high-temperature hydraulic dilation experiment, the fluid pressure in core sample showed significant fluctuation, which indicated that the internal fractures continued to initiate, develop, and propagate during the hydraulic dilation stage, leading to the volume dilation of core sample, and the core sample still had strong bearing capacity after reaching the fracture pressure. CT scanning showed that the core volume expanded significantly after hydraulic dilation, significant deformation occurred in the horizontal direction, and secondary complex fracture network appeared inside, indicating superior dilation effect. Finally, a field case is studied to demonstrate the good results of hydraulic dilation construction in heavy oil cyclic steam stimulated wells in Bohai Oilfield. The study results provide feasibility analysis for hydraulic dilation and safe connection and high-efficiency development of heavy oil.

Key words: thermal recovery of heavy oil, hydraulic dilation, true triaxial, core sample, permeability, cyclic steam stimulation