Abstract:

The Yanchang oilfield is an ultra-low permeability reservoir with low matrix permeability, high proportion of nanoscaled pore throats, complex pore-throat structures, high salinity of formation water, and high mass concentration of Ca²⁺. The interactions among CO₂-formation water-rock and CO₂-crude oil exert more complex impacts on reservoir properties and fluid characteristics in such reservoirs. Targeted studies were conducted at three CO₂ flooding areas （Huaziping, Qiaojiawa, and Baibao） to investigate these interaction mechanisms. The results showed that long-term CO₂ injection significantly affected the reservoir properties and fluid characteristics in ultra-low permeability reservoirs. Due to the water-rock interaction, the core porosity, permeability, and contact angle were all decreased, with the magnitude of decrease initially increasing then decreasing as injection pressure was raised. Meanwhile, with the increase of pressure, the mass concentrations of HCO₃^- and Ca²⁺ increased and then reached a plateau. Under actual injection pressures, the overall core blockage rates in the three tested areas were 7.71%, 8.65%, and 13.88%, respectively, with more severe plugging observed for smaller pores （corresponding blockage rates were 21.36%, 26.66% and 32.81%）. The porosity and permeability under the influence of CO₂-crude oil interaction showed similar trends to those under water-rock interaction. Increasing injection pressure enhanced the C₃-C₁₄ and C₁₅-C₂₉ components but reduced the C₃₀₊ fractions and asphaltene content in produced oil, so the viscosity of crude oil correspondingly decreased rapidly. After the minimum miscible pressure was reached, the composition and the viscosity of crude oil became unchanged. Under actual injection pressures, the core blockage rates were 15.81%, 18.85% and 20.69%, respectively, with blockage predominantly in medium and small pores. Under miscible conditions, the core blockage rate in Baibao area in these pores was 20.84% and 11.92% higher than those in Huaziping and Qiaojiawa, respectively, reflecting stronger CO₂-oil interactions during miscible flooding.

Key words: ultra-low permeability reservoir, CO₂ flooding, water-rock interaction, asphaltene deposition, Yanchang oilfield