The interaction of CO₂-rich fluid with sandstone and its significance for sandstone reservoirs of Permian Longtan Formation in Huangqiao area, Subei Basin

The interaction between CO₂-rich fluid and sandstone is the main factor for forming secondary porosity in sandstone reservoirs. As an important CO₂ production area in China, the problem about the sandstone reformation by CO₂-rich fluid in Permian Longtan Formation has attracted much attention. To reveal the interaction feature of the CO₂-rich fluid and its influence on sandstone reservoirs, the authors carried out systematic core description and micro-observation on typical coring wells as well as in-situ observation and major element analysis based on EPMA. The results show that carbonate cements in sandstone near the fracture zone have been almost completely dissolved with some displacement siderite left, especially in the upper wall where the CO₂-rich fluid influx is active. Meanwhile, the dissolution of clastic grains such as K-feldspar is strong, and there are much kaolinite precipitation, quartz secondary enlargement and dawsonite, which indicates effects of high concentration of CO₂. There exists the combination of typical minerals related to the CO₂-rich fluid including dawsonite, kaolinite, secondary quartz and siderite. In the area away from faults where the CO₂-rich fluid has relatively insignificant effects, the dissolution is weak and much calcite and siderite precipitated on the contrary. As for the former, reservoir has better secondary porosity, while for the latter the reservoir is much tighter. On such a basis, the genetic mode related to the interaction between CO₂-rich fluid and sandstone as well as its effects on the objective reservoir is set up in consideration of the fact that the deep faults play the main role in the sandstone reservoir improving. This viewpoint offers a new view angle for petroleum exploration and development.