CO2 injection strategies for enhanced oil recovery and geological sequestration in a tight reservoir: An experimental study

Considering the worldwide development of tight and ultra-tight oil reservoirs, the increasing demand for fossil energy and long-term environmental concerns regarding large CO2 emissions, application of dual-purpose carbon dioxide (CO2) sequestration/enhanced oil recovery in oil reservoirs has gained considerable attention [1,2]. However, unlike its mature application in conventional reservoirs, the effectiveness of CO2 EOR and geological sequestration in tight reservoirs is still under investigation [3]. In this work, we investigated the efficiency of different injection strategies on simultaneous CO2 EOR and storage in ultra-low permeability (< 1 milli-Darcy) [4] core samples from Yanchang Formation in the Ordos Basin, China. Three injection strategies were tested including gas continuous flooding, water alternative gas injection, and cyclic gas injection. Core-flooding experiments under immiscible and miscible conditions were conducted. Furthermore, to investigate underlying mechanisms of oil recovery and carbon storage in tight cores, interfacial tension tests, viscosity measurements, contact angle measurements, and constant composition expansion tests were performed using reservoir rock and fluids. We found that water alternative gas injection was superior to continuous gas injection in achieving the goal of simultaneous high oil recovery and high CO2 storage. However, cyclic CO2 injection provided the most efficient strategy for enhanced oil recovery in the tight rocks studied in this work with a comparatively higher gas storage capacity than the other injection strategies.