Performance of a tight reservoir horizontal well induced by gas huff–n–puff integrating fracture geometry, rock stress-sensitivity and molecular diffusion: A case study using CO₂, N₂ and produced gas

This work provides a comprehensive workflow with practical guidelines for integrating fracture-geometry, rock stress-sensitivity, and gas molecular diffusion to evaluate the performance of a tight reservoir horizontal well induced by gas Huff–n–Puff (HnP). The laboratory measurements and field results were integrated in numerical simulation. The stress-sensitivity led the permeabilities of the matrix and fractures to significantly decrease during primary depletion and HnP stages, thereby impairing well productivity. Rigorous characterizations of fracture-geometry and rock stress-sensitivity are crucial for history-matching and oil recovery simulation. During produced gas HnP, the cumulative oil production was reduced by 4.6% original oil in place (OOIP) because of the stress-dependent permeability. The hysteresis in fracture-permeability noticeably decreased the transport of mass and pressure but hardly affected the short-term production owing to the rapid closure of fractures during puff stage. The final oil recoveries induced by gas diffusion were 0.4%, 0.04% and 0.35% OOIP by CO₂, N₂ and produced gas HnP, respectively. The produced gas HnP process produced the highest efficiency of enhanced oil recovery than CO₂ and N₂ owing to its significant pressurizing effect and lower gas-oil ratio. Minimum-miscibility-pressure should not be the primary consideration for gas HnP in tight reservoirs.