An Experimental Study Of CO₂-Formation Water-Sandstone Interactions Under Burial Conditions And Its Geological Applications

Interactions between the corrosive fluid and the reservoir rock under burial condition is an important mechanism for the development of secondary pores and dissolved reservoirs.CO₂ is an important acid gas.The dissolution of CO₂ will lead a significant drop of the pH of the formation water and break the water-rock equilibrium,leading to minerals dissolution or precipitation.In addition,injecting CO₂ into depleted or nearly depleted oil and gas reservoirs is a significant method to achieve CO₂ geological storage and CO₂-enhanced oil recovery（CO₂-EOR）.Hence,it has the vital implications for understanding the development of secondary pores and dissolved reservoirs and CO₂ geological storage and CO₂ enhanced oil recovery that study on the interaction of the CO₂-formation water-rock.For the above reasons,this paper mainly has carried out three aspects of the work:（1）The accurate control for the experimental temperature has an important effect on the reliability and accuracy of experimental results,based on the specific liquid-liquid separation phenomenon of the sulfate aqueous solution in fused silica capillary tubes,we proposed a new method for calibrating the temperature of a heating stage.（2）Combining in situ analysis of Raman spectroscopy and bubble point method,we make a constructive attempt about quantitative measurement of CO₂ solubility in formation water by Laser Raman.（3）Taking the arkose in the lower part of the first member of the Paleogene Shahejie Formation in the Pucheng oilfield as example,we conduct an experiment of water-rock interaction with supercritical CO₂,and discussed the mechanism of the development of dissolution reservoirs and the implications for CO₂ storage and CO₂-EOR.The details are as follows:Some sulfate solutions（MgSO₄,ZnSO₄,CdSO₄,Li₂SO₄）exhibit a special liquid-liquid immiscibility phenomenon under high temperature conditions,that is to say,aqueous solution was separated into a sulfate-rich and a sulfate-depleted liquid phases at elevated temperatures.Current studies showed that sulfate solutions exhibited a wide range of liquid-liquid phase separation temperature（150～360 °C）under saturated vapor pressure affected by the solution components.For example,the addition of low dielectric constant components（such as alcohol）can significantly reduce the liquid-liquid phase separation temperature,while the addition of sulfuric acid will increase the liquid-liquid separation temperature.For the specific component of the sulfate solution,the liquid-liquid phase separation temperature was relatively stable and could be highly reproduced and the effect of the percentage of the sample filling is negligible.Hence,fused silica capillary tubes with a specific component of the sulfate solution can be used as a standard for calibrating the temperature of a heating stage,we established a new process and method for calibrating the temperature of a heating stage.Based on the results of the Raman spectra of 3 mol/kg NaCl solution with saturated CO₂ at 25～120 ℃ and 10～40 MPa,we found that at a certain temperature,the peak area ratio（PAR）between the CO₂ peak and the vs（H2O）band was positively correlated with the solubility of CO₂（R2 =0.9788,80 ℃）,indicating that it is completely feasible to quantitatively analyze the solubility of CO₂ by laser Raman.On the basis of the salinity of the formation water in the lower part of the first member of the Paleogene Shahejie Formation in the Pucheng oilfield,we used 1 mol/kg CaCl2-2 mol/kg NaCl as the simulated formation water.Combining with the experimental technique of high pressure optical cell and the observation of bubble point pressure,we measured the CO₂ solubility in simulated formation water at 50-125 ℃ and 4-61 MPa)。The PAR of the simulated formation water is positively correlated with the CO₂ solubility（R2 =0.977,85℃）under a certain temperature conditions.Therefore,laser Raman can be used to quantitatively analyze the solubility of CO₂ in formation water（mixed solution）.According to the temperature and pressure conditions of the reservoirs of the lower part of the first member of the Paleogene Shahejie Formation in the Pucheng oilfield,we conducted the water-rock reaction experiments of CO₂-formation water-sandstone at 85 ℃,10 MPa and 20 MPa.By using weighing,ion chromatographic analysis,Scan electronic microscope（SEM）and PHREEQC simulation,we clarified the dissolution/precipitation of the minerals after CO₂ injected.The dissolution of calcite is principal parts,followed by potassium feldspar,while the dissolution of albite is not obvious.The dissolution of the minerals increases with the increasing of the CO₂ partial pressure,and the type of the formation water has an important effect on the dissolution of the mineral.PHREEQC simulation and geological examples shows that the CO₂ injection will distinctly reduce the pH of the formation of water,resulting in dissolution of carbonate and other alkaline minerals,which is in favor of the development of secondary pores and dissolved reservoirs.In addition,our study also has some implications for CO₂ geological storage and CO₂-EOR.