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 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.

Experimental Investigation of Oil Recovery Performance and Permeability Damage in Multilayer Reservoirs after CO2 and Water–Alternating-CO2 (CO2–WAG) Flooding at Miscible Pressures

“…When the pressure reaches a certain value in the CO2 injection process, changes in composition of the crude oil due to the dissolved CO2 lead to asphaltene precipitation. The asphaltene solid particles are captured or adsorbed on the pores' walls, resulting in blocked pores and pore throats [25][26][27][28] . Moreover, variations of ion concentration and the pH of the brine caused by CO2-brine-rock interactions lead to precipitation of metal carbonates.…”

Effect of Pore-Throat Microstructures on Formation Damage during Miscible CO2Flooding of Tight Sandstone Reservoirs

“…One was an ordinary brine which was prepared according to the composition given in Table 3, the other was made in the same way but Mn 2+ was added. The brine containing Mn 2+ was used to screen the water signal during NMR tests in order to obtain only the oil distribution in the core [14] . Uncertainties in the x and y directions are smaller than the symbol size in all cases [17] .…”

“…However, when CO2 is injected in reservoirs and is in contact with crude oil, variations in composition of the crude oil due to the dissolved CO2 can cause asphaltenes to deposit from crude oil and flocculate into asphaltene precipitation particles [14][15][16] . The asphaltene solid particles are captured at pore-throats and adsorbed on the pore surfaces, resulting in blocked pores and pore-throats, and consequent wettability alteration [17][18] .…”

“…Extensive core-flooding experiments have been conducted in order to investigate permeability decline and wettability alteration caused by asphaltene precipitation during CO2 flooding processes [32][33][34] , including the effects of permeability, displacement pressure, displacement methods, crude oil properties and other factors on asphaltene precipitation and reservoir damage [35][36][37][38][39] . In addition, the distributions of blockage by asphaltene precipitation in the pores and throats of sandstone have been evaluated quantitatively using nuclear magnetic resonance (NMR) measurements [14,40] . Nevertheless, the effects of the rock pore-throat structure on the damage to pore-throat structure itself during asphaltene precipitation have been rarely studied directly [35,41] .…”

“…Unfortunately, these prerequisites have not been satisfied in most core-flooding experiments [32,42] . In addition, while the distributions of pore-throat blockage have been studied qualitatively through the distributions of oil and water re-saturation after flooding in previous studies, the influences of wettability alteration have been ignored [14,40] .…”