Micro mechanism of reservoirs damage caused by CO2 displacement in tight reservoir

Abstract

CO2-fluid-rock interactions during CO2 injection in tight oil reservoirs, i.e. asphaltene precipitation and geochemical reactions, alter the pore-throat structure and wettability, which will in turn affect the CO2 displacement efficiency. In this work, pore structures of tight sandstone from the Ordos Basin, China were characterized comprehensively using the X-ray diffraction, cast thin sections, scanning electron microscopy, nuclear magnetic resonance, and high-pressure mercury intrusion. In addition, pore structure and asphaltene precipitation before and after CO2 displacement were analyzed to clarify the influences of CO2-fluid-rock interactions on the pore throat structure and wettability of tight sandstone reservoirs, including mineral dissolution, asphaltene precipitation location and aggregation mode. The results indicate that the Chang8 tight sandstone can be divided into three classes based on pore throat combination: narrow channel intergranular pore type (I), wide channel and dissolution pore type, intergranular pore type (II), and mixed type (III). The porosity of the three types gradually decreases and the percentage of small pore throats increases. With the injection of CO2, secondary pores are formed due to the dissolution of feldspar in Class I reservoirs, and asphaltene precipitates in the macropores, mainly distributed on the surface of mineral particles and clustered in a point-like manner. The impact on the macropore throat is relatively small, and the wettability is reversed towards oil wetting. In the Class II reservoir, feldspar dissolution produces a large number of secondary pores, and the proportion of pore throat of fine and small pores developed by asphaltene precipitation in Clay mineral increases. Asphaltene content in Class III reservoir is precipitated in a large amount in small pores and throats, and accumulates in clusters. Asphaltene precipitation is usually concentrated more closely in the pore throat structure where Chlorite group and Illite are more developed, and core wettability is constantly changing to the direction of strong oil wetness. The asphalt content and wetting reversal index of Class I are the highest, while Class III is the lowest.