Experimental and application research on sweep efficiency for non-condensable gases assisted vertical-horizontal steam drainage in extra-heavy reservoirs

Abstract

The vertical-horizontal steam drainage (VHSD) method, combining vertical and horizontal wells, enhances steam-assisted gravity drainage to improve sweep efficiency and oil-steam ratios in extra-heavy oil reservoirs. This approach has become an effective alternative to steam stimulation in aging oil fields, particularly in China. However, thermal losses in late-stage VHSD can reduce the oil-steam ratio. The use of non-condensable gases like CO₂, CH₄, and N₂ may improve this process. Previous research on non-condensable gases mainly focused on steam-assisted gravity drainage (SAGD) with parallel, closely spaced wells, whereas VHSD uses wells positioned 50–60 m apart, affecting steam chamber dynamics and gas behavior.

This study is the first systematic investigation of non-condensable gases in VHSD, using the Z1 block of Xinjiang Oilfields in China as a case study. It examines the effects of CO₂, CH₄, and N₂ in enhancing VHSD, combining lab experiments with field research. A novel approach emphasizes well arrangement's impact on gas distribution and steam chamber development. The results show that CO₂ outperforms CH₄ and N₂ in solubility and viscosity reduction, leading to the highest sweep efficiency. At 1 MPa, CO₂ and CH₄ increased oil recovery by 4.47 % and 1.63 % respectively. At 2 MPa, the increases were 8.01 % and 5.1 %, while N₂ slightly reduced efficiency. Non-condensable gases accumulated at the steam chamber boundary, impacting heat loss, chamber morphology, and expansion rates. CO₂-assisted development yielded the best results, with a 4.3 % recovery rate increase. While N₂ effectively enhanced the oil-steam ratios, its influence on the recovery rate was relatively modest.