Monitoring on CO2 Migration in a Tight Oil Reservoir during CO2-EOR Process

Monitoring CO₂ migration in the oil reservoir is one of the most important MVA (monitoring, verification and accounting) activities. A systematic monitoring program has been deployed to track the movement of CO₂ in a tight oil reservoir during a CCS-EOR project in the Jilin oilfield, China. CO2 separated from a nearby natural gas reservoir (15-30 mole% CO₂) has been injected into the northern part of H59 oil block with permeability and porosity of 3.5mD and 12.7%, respectively.

A microseismic monitoring program has been implemented to map the CO2 flow anisotropy and estimate its sweeping conformance profile. Gas tracer testing has also been conducted to examine the inter-well connectivity. The composition of produced gas has been analyzed in a real-time mode to monitor the dynamic response of different production wells along with the microseismic.

In applying the microseismic technique, the open and closure of the pre-existing natural fractures were well corresponding with the variation of CO₂ injection pressure. Higher injection pressure can induce the re-open of the pre-existing natural fractures, and the CO₂ flow pattern induced by the fracture network become less anisotropic. And the microseismic events detected were consistent with the orientation of the natural fractures and the stress distribution in the reservoir. However, for the lower injection pressure, the inversed microseismic profiles clearly show the preferential CO₂ flows and their anisotropy in different well groups, which are mainly dominated by the pressure gradient and the heterogeneity of reservoir permeability. And the detected micro-seismicity is caused by stress gradients induced by production and injection gradient, rather than the local natural fractures. The sweeping profiles of CO₂ obtained from the microseismic were in good agreement with the temporal change of the produced CO₂ rate from production wells as well as the permeability characterization of the reservoir.

The mapped preferential flow direction and sweeping profile of CO₂ from microseismic provides a good prediction for the breakthrough of CO₂ and its extent, which is useful for conformance control and injection optimization. Also, the microseismic test provides a very good indicator of CO₂ leakage from the oil reservoir, which is a cost-effective technique in ensuring long and safe storage of CO₂ in the subsurface.