Abstract:

Edge and bottom water are found in the Jurassic oil reservoirs in the Ordos Basin. In this kind of reservoirs, rapid water cut rise and low recovery by water flooding occur after initital production. In order to explore new methods for enhanced oil recovery in such reservoirs and find new ways to increase production by carbon sequestration in near-abandoned reservoirs, a pilot test was conducted on top CO₂ injection for flow field reconstruction in the Y9 reservoir in the X1 block of Jiyuan oilfield. Through the mechanism analysis of CO₂-assisted gravity drainage, multiphase and multi-component numerical simulation was performed to understand the sensitivity and adaptability of the reservoir's geological parameters, and the reservoir engineering parameters were also optimized for the test area. The results show that the residual oil in the Jurassic bottom water reservoirs after waterflooding mainly exists in three forms: thick oil ring in the zone between injection and production wells after the invasion of bottom water, thin oil ring in the zone from the outer oil-bearing edge to the oil production well due to bottom water coning and edge water intrusion, and residual oil after waterflooding. Injecting CO₂ at the reservoir top is an effective way to inhibit bottom water coning. As an artificial gas cap forms and exaggerates, gas-oil contact moves downwards, and accordingly oil-water contact becomes lower, alleviating bottom water coning. The main factors affecting CO₂-assisted gravity drainage include formation dip, reservoir thickness, permeability, heterogeneity, crude oil properties, and oil saturation, etc. Simulation studies and pilot tests indicate that CO₂-assisted gravity drainage at the reservoir top can effectively reconstruct the flow field in waterflooding reservoirs, thereby enhancing the ultimate recovery.

Key words: Ordos Basin, Jurassic, bottom water reservoir, top CO₂ injection, CO₂-assisted gravity drainage, flow field reconstruction, influencing factor