| Organic shale reservoirs are rich in reducing components such as pyrite,chlorite and organic matter.The working fluid in the development of shale gas reservoirs contains oxidizing substances.Once the oxidizing working fluid enters the organic shale reservoir,it will oxidize the reducing components,cause mineral transformation,release solid phase particles,change the pore structure,and damage to the seepage capacity of organic shale reservoirs.Understanding and evaluating the oxygen sensitivity of organic shale reservoirs is of great significance for protecting and improving the productivity of shale reservoirs.Taking the organic shale of the Longmaxi Formation in the Pengshui area of the southeastern Sichuan Basin as the research object,the geological characteristics of the shale reservoirs in the study area are described.The reduced minerals such as pyrite and chlorite in the organic shale account for 5.81%.The average effective porosity of the upper to lower shale in the Longmaxi Formation is 3.03%?4.84%,the average TOC is 1.46%?4.20%,the permeability is 0.011?0.159mD.Potential damage factors of organic shale reservoirs are also analyzed.Through the experiments of water-rock interaction with organic shale and oxidizing liquid,the interaction mechanism between organic shale and oxidizing liquid is revealed.Oxidizing liquid can oxidize and dissolve reducing substances such as pyrite,chlorite and organic matter in organic shale,and produce H+to further dissolve carbonate minerals,causing selective dissolution of minerals,increasing ion concentration in solution,forming corrosion pores,and producing chemically precipitated solid phase particles and shale cuttings solid phase particles such as Fe2O3,Fe(OH)3,siderite(FeCO3),dihydrate gypsum(CaSO4·2H2O),MgSO4 and BaSO4,and The water-soluble solid phase particles have a particle size of 0.3 to 200 ?m and have a potential plugging effect on shale reservoirs.The concept of oxygen sensitivity in organic shale reservoirs is Condensed.The oxygen sensitivity of organic shale is a unique property of organic shale.It is characterized by the formation of dissolution pores and cracks in the organic shale after contact with oxidizing liquid,and the production of solid phase particles,which have potential dual effect for demaging or improving permeability.An oxygen-sensitive profit-loss ratio model for organic shale was constructed.The relationship between the chemical oxygen demand(COD)and the content of reducing components in organic shale was explored.The numerical analysis showed that the oxygensensitive profit-loss ratio was positively correlated with the ratio of pyrite content and calcite/dolomite content,and negatively correlated with chlorite content.A fine evaluation method and an industrial evaluation method for oxygen sensitivity of organic shale have been basically established.In the fine evaluation experiment of oxygen sensitivity of Longmaxi Formation organic shale reservoirs in Pengshui area,the permeability change rate of fractured rock sample is 20.2%?60.1%,and the oxygen sensitivity degree is weak to moderately strong.In the industrial evaluation experiment,the permeability change rate of fractured rock sample is 51.6%?66.7%,the oxygen sensitivity degree is medium to strong,and the critical Eh of oxygen sensitivity damage is 475 mV.The method of controlling the permeability of oxygen-sensitive organic shale reservoirs is discussed..The working fluid containing low concentration oxidizing substance can easily cause oxygen sensitive damage to organic shale reservoirs.Conventional water-based fracturing fluid can use acetone oxime to remove oxygen,which has low cost and can effectively protect shale reservoirs and make the fracturing flow-back fluid easier to treat.The appropriate concentration of oxidizing solution can make the organic shale form oxidationinduced cracks and corrosion pores in a moderate treatment time,and improve the seepage ability of the organic shale to a certain extent. |
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