Formation damage due to asphaltene precipitation during CO₂ flooding processes with NMR technique

Abstract

In order to quantitatively evaluate the pore-scale formation damage of tight sandstones caused by asphaltene precipitation during CO 2 flooding, the coreflood tests and Nuclear Magnetic Resonance (NMR) relaxometry measurements have been designed and applied. Five CO 2 coreflood tests at immiscible, near-miscible and miscible conditions were conducted and the characteristics of the produced oil and gas were analyzed. For each coreflood test, the T 2 spectrum of the core sample was measured and compared before and after CO 2 flooding to determine the asphaltene precipitation distribution in pores. It is found that, the solubility and extraction effect of the CO 2 plays a more dominant role in the CO 2 -EOR (Enhanced Oil Recovery) process with higher injection pressure. And, more light components are extracted and recovered by the CO 2 and more heavy components including asphaltene are left in the core sample. Thus, the severity of formation damage influenced by asphaltene precipitation increases as the injection pressure increases. In comparison to micro and small pores (0.1–10 ms), the asphaltene precipitation has a greater influence on the medium and large pores (10–1000 ms) due to the sufficient interaction between the CO 2 and crude oil in the medium and large pores. Furthermore, the asphaltene precipitation not only causes pore clogging, but also induces rock wettability to alter towards oil-wet direction.