Regular Article

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

¹ State Key Lab of Oil and Gas Resources and Engineering, China University of Petroleum-Beijing, Beijing 102249, People’s Republic of China
² Research Institute of Petroleum Exploration and Development, CNPC, Beijing 100083, People’s Republic of China

^* Corresponding author: yangshenglai_cupb@126.com

Received: 17 July 2018
Accepted: 5 November 2018

Abstract

In order to quantitatively evaluate the pore-scale formation damage of tight sandstones caused by asphaltene precipitation during CO₂ flooding, the coreflood tests and Nuclear Magnetic Resonance (NMR) relaxometry measurements have been designed and applied. Five CO₂ 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 T₂ spectrum of the core sample was measured and compared before and after CO₂ flooding to determine the asphaltene precipitation distribution in pores. It is found that, the solubility and extraction effect of the CO₂ plays a more dominant role in the CO₂-EOR (Enhanced Oil Recovery) process with higher injection pressure. And, more light components are extracted and recovered by the CO₂ 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 CO₂ 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.