Simulation Of Fluid-shale Interactions During Of Hydraulic Fracturing In Shale Gas Exploitation

The fracturing fluid-shale interaction exists in the whole process of hydraulic fracturing including the flowback,whose water is influenced by the composition of the fracturing fluid,the shale composition and the reservoir environment.Difference exits not noly between fracturing fluid formulation and reservoir environment in different areas,but also in the flowback water.This paper focused on the interaction mechanisms of fracturing fluid-shale in shale reservoir during hydraulic fracturing by laboratory experiments and hydrogeochemical software simulation.The main conclusions are as follows:?1?During the interaction.,the H⁺generated from the fracturing fluid is rapidly neutralized by the carbonate in the shale,which exhibited a strong buffering capacity of acid,to keep the reservoir in a weak alkaline environment all the time.The acidic fracturing fluid is more conducive to dissolute calcite.The concentration of Mg²⁺(Ca²⁺)is higher in the solution,the dolomitization?dedolomitization?is easier to occur during this process.?2?Higher ambient temperature and higher dissolved oxygen content are more beneficial to the oxidation of pyrite during the fracturing fluid-shale interaction.In the reservoir environment?there is no CO₂?,the H⁺produced by pyrite oxidation can promote the dissolution of carbonate minerals.There is a positive correlation between the dissolved load of calcite and the oxidation amount of pyrite when the reaction reaches equilibrium.?3?Increasing the ion strength of fracturing fluid can enhance the release of metal ions when the fracturing fluid reacts wtih shale.Solution of high ion strength can effectively reduce the activity of the metal ions and inhibit the clay mineral adsorption of metal ions,thus improves the release of metal elements during the process.Under the condition of the same solution in ionic strength,NaCl solution can promote the dissolution of pyrite and inhibit feldspar dissolution.Moreover,the CaCl₂ and MgCl₂ can accelerate the dissolution of illite and the precipitation of montmorillonite and chlorite.?4?The fluid:shale ratio?g:m L?has a barely effect on the dissolution of calcite and dolomite and pyrite oxidation during the fracturing fluid-shale interaction.However,high ratio can increase the contact area between shale and fluid,which is more conducive to release of the trace elements,such as Ba and Sr in shale.?5?The ammonium persulfate in the fracturing fluid can oxidation the pyrite and react with other components?such as organic matter?in the shale,but only at the beginning of the reaction.The citric acid in the fracturing fluid can promote the dissolution of calcite,which is indicated by a higher Ca²⁺/Mg²⁺molar ratio in the solution during the whole reaction process.In addition,this acid can be complexed with metal ions to effectively inhibit the precipitation of metal ions(such as Ba²⁺and Fe³⁺)and increase the release of metal ions in shale.