شبيه‌سازي تزريق نيتروژن به مخازن شكاف‌دار جهت افزايش ضريب بازيافت نفت

Abstract Naturally fractured reservoirs present significant challenges to oil recovery due to the complex interplay between the dual porosity of the highly permeable fracture network an‎d the low-permeability rock matrix. In undersaturated oil reservoirs lacking a natural gas cap, initial recovery is often inefficient, leading to rapid pressure decline an‎d reduced final recovery factors. This research eva‎luates the effectiveness of nitrogen (N₂) injection as an enhanced oil recovery strategy to mitigate these challenges, with particular emphasis on the impact of injection timing. Using the CMG-GEM chemical structure simulator, a three-dimensional model of dual porosity based on a five-spot pattern was developed. The study compared three different scenarios: natural depletion (baseline condition), early-stage nitrogen injection (2010), an‎d late-stage nitrogen injection (2044). The equation of state (EOS) was determined an‎d fine-tuned using WinProp software to accurately model the phase behavior an‎d the interaction of nitrogen with the undersaturated reservoir fluid. The results show that nitrogen (N₂) injection significantly improves oil recovery by maintaining strong pressure an‎d displacing oil leaking from the matrix. Simulations revealed that the injected nitrogen exhibits a strong gravity effect, moving vertically to the reservoir top to form a secondary gas cap. This gas cap facilitates gas-assisted gravity drainage (GAGD), where the gas drags the oil from the matrix blocks into the fractures. A key finding of this research is the relationship between injection timing an‎d process efficiency. While early-stage injection achieved the highest final recovery factor (approximately 40% compared to 24.5% for natural depletion), the recovery rate (slope) of late-stage injection was nearly identical over the same injection duration. This demonstrates that the physical efficiency of the nitrogen displacement mechanism remains constant regardless of reservoir maturity. Furthermore, the study demonstrated that although nitrogen injection leads to earlier gas penetration, the formation of a secondary gas cap suggests that optimizing well perforation at shorter intervals can significantly extend production lifetimes. This research provides a technical framework for operators to apply nitrogen-enhanced oil recovery (NEOR) as a secondary an‎d tertiary recovery method in complex fractured systems. Keywords: Naturally fractured reservoirs, nitrogen injection, dual porosity modeling, enhanced oil recovery, gravity drainage, undersaturated oil

محسن محمدي

حميدرضا شاه وردي , روح اله هاشمي