Summary
Foam stability primarily determines the efficiency of foam-induced conformance control, especially when fractures exist in formations. In this work, a well-defined nanocellulose fibril (NCF)-strengthened carbon dioxide foam (NCF-st-CO2 foam) was proposed, aiming to improve the conformance of tight formations with fractures. The bulk characteristics of NCF-st-CO2 foam including foamability, foam stability, and texture were thoroughly investigated in a high-temperature and high-pressure (HT-HP) cell. Afterward, foam flow in fracture models was simulated, intending to correlate the generated differential pressure with the parameters of foam quality, fracture aperture, and fluid velocities, which helped to understand the foam generation, propagation, and mobility in fractures. The capacity of this foam in controlling conformance and improving oil recovery in a fractured core was finally evaluated. Conventional CO2 foam (without NCF) was used as the reference throughout this work. The results indicated that the addition of NCF into CO2 foam considerably retarded the liquid drainage and bubble coalescence, which consequently stabilized the CO2 foam, especially in the presence of crude oil. NCF-st-CO2 foam presented dense bubbles and a thick framework as observed visually from the cell. The flow behaviors of CO2 foam in fractures were largely determined by the bulk foam stability. NCF-st-CO2 foam could be generated in situ (aperture from 0.5 to 5.0 mm) and then properly propagated throughout the fractures at a low mobility without any plugging issues. In coreflooding tests conducted on the model with fracture aperture of 0.5 mm, the NCF-st-CO2 foam injection exhibited a delayed breakthrough, which finally led to an incremental oil recovery of 8.6% original oil in place (OOIP) after conventional CO2 foam treatment. These results demonstrated the promising potential of NCF-st-CO2 foam in conformance control in tight formations.
