Experimental Study On Interactions Of Reservoir Rock-CO₂–Formation Water（or Fracturing Fluid）

The injection of carbon dioxide CO₂ into hydrocarbon reservoir is an efficient way to enhance the recovery of crude oil and natural gas,which is a hot topic in the world in recent years.Geochemical reactions of CO₂-formation water-rocks were complicated.The investigation of such water-rock interactions would contribute to the understanding of the influence on the physicochemical properties of reservoir rocks,reaction mechanisms as well as the environmental effects.The simulation experiments of CO₂-formation water-sandstone interaction were conducted by static high temperature and high pressure experiment and dynamic core displacement test.The results showed that the calcite and its cement were dissolved rapidly and the feldspar was also eroded simultaneously,resulting in the increasing the porosity of the rocks.Whereas,other clays and aluminosilicates were released,migrated and accumulated by the dissolution of minerals.Such process was also benefit to the migration and deposition of secondary minerals.The measurements of permeability of the sandstone core demonstrated that the permeability was increased and then remained stable during the CO₂ driving process.This result indicated that the dissolution,migration and accumulation of minerals would not cause the risk of blockages.And it further confirmed that the interaction of CO₂ –water-rock favors the formation of pores and raised the permeability,which would enhance the recovery efficiency of crude oil.The simulated experiments were conducted on the CO₂-fracturing fluid-shale interactions at the temperature of 90 ℃ and the pressure of 10 MPa.The aim of this study is mainly about the changes of the rock or minerals and the change of the flowback fluid composition after injection of the hydraulic fracturing fluid and the subsequent supercritical CO₂.The results showed that the fracturing fluid could cause the shale to be eroded,and the presence of supercritical CO₂ could further enhance the dissolution of minerals,which resulted in enlarging the pores and generating more micropores,providing more channels for migration of shale gas and oil.However,the environmental threats could be existed and not to be ignored.Many elements were dissolved from the shale,such as a lot of Ca,Mg,Si and a small amount of Fe,Mn.Under acidic environment with high temperature and pressure,the volatile organic compounds（VOCs）were easily produced by the reaction system of supercritical CO₂-shale with rich organic matter-surfactants in the fracturing fluid.The VOCs remained in the stratum and very likely leaked through the pores or faults and polluted the aquifer.On the other hand,the treatment of flowback fluid became more difficult since varieties of VOCs and elements were mixed with high salinity solution.The experimental data and the results would help to understand the water-rock interactions and the potential environmental risks during shale gas exploration.