Experimental Study On Water-Rock-Interaction Of Inorganic Characteristic Pollution Factors In Unconventional Gas Produced-water Reinjection

As the potential extraction of traditional oil and gas resources declines,clean energy with large-reserves such as unconventional gas are valued.Unconventional gas,such as shale gas and tight gas,is extracted by hydraulic fracturing.However,experiences have shown that after mining,the produced water is simply treated and injected back into the formation,which may lead to high inorganic salt or heavy metal pollution and geological changes due to poor compatibility.In this paper,indoor static and dynamic experiments and numerical simulation are used to simulate the reaction conditions of the actual injection process and to predict the possible contamination for such factors as ion strength,pH value and temperature in the layer.The results of static experiments show that the sandstone minerals have a certain purification effect on the heavy metal barium ions,and the temperature,water-rock ratio rise,pH decrease and the existence of divalent cation have negative effects on the effect of purification.In the condition of TDS = 30000 mg/L,the change of pH and the purification of heavy metals are reduced.The concentration of the main elements in the solution increases when the temperature increases,the pH decreases and the water-rock ratio increases.The results of dynamic displacement experiment are consistent with the results of static reaction.The errors are within 10%.The adsorption of Ba is complete in rocks within 24 h.During the reaction,the pH value first rises and then decreases,and elements such as Si and Mn are precipitated.In the condition of TDS = 30000 mg/L,the concentration of main elements did not change significantly.After the reaction,the porosity and permeability of the rocks increase slightly,The maximum increase of porosity and permeability is 0.7% and 0.05 MD respectively.And the surface and interior of the rocks are dissolved to some extent.The results of simulation by PHREEQC are in good agreement with the experiments,and the processes of reinjection in the conditions of pH=1.5 and T=175 ?are predicted.The results show that the reactions are more severe when the temperature is increased or the pH value is decreased.When the composition of the solution is changed,the mineral content of the formation will change accordingly.On this basis,this paper puts forward the existing problems and development direction of the research in this field,which provides reference for the research on the environmental impact of the produced-water reinjection in China.