Impacts Of Hydrochemical And Environmental Factors On Elements Mobilization From Longmaxi Formation Shale

Mineral elements in shale gas reservoirs may migrate due to water-rock interaction during hydraulic fracturing.The hydrochemical characteristics of fracturing fluid,the temperature and pressure of shale gas reservoir,and the proportion of its minerals may affect the migration degree and rate of shale elements.Accordingly,the hydrochemical experiments under different initial pH,solid-liquid ratio and oxidation conditions were carried out in the laboratory.The hydrogeochemical simulations under different environmental conditions?i.e.,different temperature,different pressure and different mineral composition?were carried out,and the reverse simulations were carried out in combination with the actual fracturing fluid-backflow fluid properties.Based on the experimental and simulation results,the element migration of Longmaxi Formation shale was discussed.The main conclusions are as follows:?1?In fracturing fluid-shale experiments under different initial pH conditions,the main cationic concentration is affected by initial pH,which shows that at the end of the reaction,the ionic concentration of the initial solution under weak acidity condition is higher than that under neutral and alkaline conditions,reflecting the migration of elements in the corresponding shale.The existence of carbonate minerals in shale makes it have strong buffer capacity to acid.The difference of acidity and alkalinity of the solutions after reaction is much smaller than that of the initial solutions.?2?The increase of solid-liquid ratio can enhance the buffer capacity of systems;Ca migration is mainly controlled by the dissolution equilibrium of calcite;Mg migration increases nonlinearly with the increase of solid-liquid ratio,possibly because the element comes from the dissolution of different minerals,and the increase of Ca²⁺concentration in solution inhibits the release of Mg from dolomite;Fe element forms Fe?OH?₃ precipitate in the dissolution process of pyrite,and Fe content in solution is low.?3?Adding oxidant can promote the oxidation reaction of pyrite in shale,thus increasing the release of S.Under anaerobic conditions,the release of S in shale can reflect the existence of original sulfate,while the increase of S release after adding H₂O₂can reflect the oxidizable S content in shale samples.?4?In the simulation system of different temperature and pressure,the final pH showed that the solution was weak alkaline.In the reaction system,the migration of Ca from mineral to fracturing fluid decreases with the increase of temperature,while the migration of Mg from shale decreases with the increase of temperature.The increase of pressure increases the amount of Ca,Si and Al in shale migrating to fracturing fluid.The higher the content of brittle minerals in shale,the greater the brittle index,which leads to the increase of the migration of major elements.?5?The migration of major elements in shale along two paths in the study area is that Na migrates from the dissolution of albite,Ca migrates from the dissolution of calcite,and K migrates from different paths.The migration amount of Na,Ca and K in the two paths is quite different.