Experimental Study on the Characteristics of Nitrogen-Assisted VHSD Development in Shallow Heavy Oil Reservoirs

In the development of heavy oil reservoirs employing vertical-horizontal well steam driving (VHSD), there are various issues like a significant reservoir deficit and a poor oil-steam ratio. A novel 3D physical experimental model was developed to carry out the entire process simulation experiment of nitrogen-assisted VHSD to better understand the characteristics and mechanism of nitrogen-assisted VHSD. The production characteristics, oil displacement efficiency, and steam chamber evolution of nitrogen-assisted VHSD were analyzed. The findings indicate that the vertical and horizontal wells' oil recovery declines as the number of cycles rises and that steam sweep efficiency is low. Based on the liquid production dynamics and the propagation of the steam chamber, the nitrogen-assisted VHSD process is divided into three stages: expansion laterally, downward expansion after fusion, and steam chamber breakthrough. In the first stage, the injected nitrogen and steam accumulated at the top of the reservoir and expanded laterally, with a stage recovery of 27.53%. The steam chamber gradually merged and grew downward in the second stage. At this stage, the oil production rate was stable, and the stage recovery was 24.68%. When the steam chamber entered the horizontal well in the third stage, the rate of oil production gradually declined, and the stage recovery was only 2.11%. The research provides important references for nitrogen-assisted VHSD field tests in heavy oil reservoirs.