Abstract:

CO₂ flooding, as a widely applied enhanced oil recovery technique, has been extensively used in many oilfields. However, solid deposition that occurs during the flooding process can inflict severe damage on reservoir physical properties. To thoroughly investigate this phenomenon, we conducted a series of physical simulation experiments that mimic actual reservoir conditions. Both inorganic ion precipitation and heavy component precipitation were generated in situ within the core samples. The influence of solid deposits on reservoir porosity and permeability was systematically characterized by measuring the changes before and after deposition. The plugging conditions in different pores were quantitatively evaluated using nuclear magnetic resonance (NMR) technology based on transverse relaxation time (T₂) distribution that directly reflects pore size distribution and the reduction in T₂ amplitude indicates the degree of pore blockage. Both inorganic ion precipitates and heavy organic precipitates can occlude pore throats, thereby reducing effective flow area and causing a marked decrease in both permeability and porosity. The pressure gradient across the core drives fluids to bypass blocked regions and enter smaller pores, which further complicates fluid flow and may induce additional trapping. An elevation in salinity accelerates inorganic ion precipitation rate, as higher ionic strength promotes nucleation and crystal growth, thus permeability and porosity decline faster under high salinity. Notably, pressure effect is not monotonic. When pressure increases, certain inorganic salts may redissolve, partially recovering lost permeability and porosity, although the recovery is often limited and depends on mineral composition. In contrast, heavy component precipitation behaves distinctly. Unlike inorganic salts, heavy components interact strongly with crude oil and are governed by different mechanisms such as pressure depletion and composition changes. Under identical pressure conditions, CO₂ flooding generally achieves a higher oil recovery when heavy component precipitation occurs, primarily because heavy components exhibit greater solubility in the crude oil, facilitating their removal with produced fluids and reducing net damage. With the pressure continues to rise, the solubility of heavy components in CO₂ saturated crude oil may change unfavorably. Heavy components precipitation can cause more severe plugging in both large and small pores, further diminishing permeability and porosity, even more pronounced than inorganic precipitation at elevated pressures. This study serves as a valuable reference for predicting and mitigating reservoir damage during CO₂ flooding. In field applications, engineers can adjust ionic composition of injected water and injection pressure to minimize adverse impacts, thus effectively suppressing formation damage and ultimately enhancing oil recovery.

Key words: carbon dioxide flooding, reservoir damage, nuclear magnetic resonance, inorganic ion precipitation, heavy component precipitation

摘要：

二氧化碳驱固相沉积会对储层物性产生伤害。通过物理模拟二氧化碳驱实验，原位生成无机离子沉淀和重质组分沉淀，系统表征了固相沉积对储层孔隙度和渗透率的影响，同时利用核磁共振定量计算了不同大小孔隙的堵塞情况。无机离子沉淀和重质组分沉淀会堵塞孔隙，造成渗透率和孔隙度减小，压差使流体进入更小的孔隙。饱和水矿化度升高，无机离子沉淀的沉积速度加快，造成渗透率和孔隙度减小。压力升高，大孔隙的无机离子沉淀可能重新溶解，造成渗透率和孔隙度一定程度恢复。但是重质组分沉淀的影响明显不同于无机离子沉淀。相同压力下二氧化碳驱，由于在原油中的溶解度更强，重质组分沉淀造成的采收率普遍较高。压力升高，重质组分沉淀堵塞大小孔隙的程度加剧，造成渗透率和孔隙度进一步减小。本研究为二氧化碳驱过程中可能产生的储层伤害提供参考依据，在实际应用中可以通过对水离子组成和注入压力调控，进一步抑制储层伤害，提高二氧化碳驱采收率。

关键词: 二氧化碳驱, 储层伤害, 核磁共振, 无机离子沉淀, 重质组分沉淀