Experimental Study on the Enhancement Mechanism of Multi-Component Thermal Fluid Huff-n-Puff in Deep Extra-Heavy Oil Reservoirs

Due to high formation pressure and extremely poor crude oil mobility, the synergistic enhancement mechanism of heat–gas–chemical interactions and the governing laws of key factors during multi-component thermal fluid (MCTF) huff-n-puff in deep extra-heavy oil reservoirs remain unclear. In this study, a one-dimensional sand pack model was used to systematically conduct MCTF huff-n-puff depletion production experiments under a high pressure of 30 MPa, complemented by viscosity–temperature tests, PVT dissolution viscosity reduction tests, and viscosity reducer concentration–viscosity tests. A single-factor analysis approach was employed to investigate the effects of injection temperature, gas medium type, gas–oil ratio (GOR), and viscosity reducer concentration on the huff-n-puff recovery factor. Based on the single-factor tests and the nine-run L9 orthogonal experiments, the results show that injection temperature is the primary controlling factor: the low-pressure stage (2 MPa) recovery factor of CO2 rises from 15.9% to 18.7% as the temperature increases from 90 °C to 120 °C, and further to 19.6% at 150 °C; 120 °C is identified as the preferred injection temperature under the studied conditions. Owing to its higher solubility (114.9 m3/m3) and solution gas drive energy, CO2 outperforms CH4 and flue gas, delivering a recovery factor 3 percentage points higher than that of CH4 and 18 percentage points higher than that of flue gas. The recovery factor increased monotonically with GOR over the tested range of 10–30, and no gas channeling was observed. The recovery benefit of the viscosity reducer increases markedly as the concentration rises from 1 to 5 wt%, whereas the incremental gain beyond 5 wt% becomes marginal. Run 4 (120 °C + CO2 + GOR 20 + 5 wt% viscosity reducer) achieved the highest total recovery factor of 44.6% among the nine orthogonal combinations, representing the best-performing combination within the L9 design rather than a verified global optimum. The grey relational ranking of the four factors is injection temperature > viscosity reducer concentration > gas type > GOR. The findings provide an experimental basis, under the tested Shengli extra-heavy oil system and the present 30 MPa one-dimensional sand pack conditions, for optimizing MCTF huff-n-puff process parameters in deep extra-heavy oil reservoirs.