Research Institute of Petroleum Engineering and Technology
In the process of oilrecovery with carbon dioxide injection, carbonic acid is formedwhen carbon dioxide dissolves in water, which reacts with reservoir minerals and the mineral composition of rock and physical properties of reservoirare changed. In this paper, the changes of minerals, producing fluids, microstructure and permeability of low permeability reservoir under three different injection methods of CO2 flooding, alternate CO2/waterflooding and CO2 huff-n-puff were studied by laboratory experiments.The results show that the permeability of rocks can be increased by these three CO2injection methods. Among them, alternate CO2/waterfloodinghas the most obvious effect on the permeability;The concentration of calcium and sodium ions of the producing fluids increased obviously and the dissolution of carbonic acid results in the reduction of plagioclase and calcite in the core so that the content of calcite and feldspar in the core mineral decreased;The originalmicrostructureof rocks is dense, with poor connectivity and poor development of intergranular pores. After the effect of carbonic acid, the microstructure of rocks becomes loose, with good connectivity,well-developed intergranular pores and obvious dissolution.