| As a critical factor restricting social and economic development,the development and utilization of energy are key for improving the comprehensive national strength and realizing the rapid development of society.Shale gas is extremely rich in reserves in China.The clean and efficient characteristics make the utilization of shale gas resources better than other conventional fossil fuels such as coal and oil.The vigorous development of shale gas is significant to alleviate China's energy shortage and climate change.In shale gas development,the ultra-low permeability seriously prevents the development and utilization of shale gas.The transformation ability of conventional physical methods is limited,making the chemical oxidation method has attracted much attention in recent years.Nevertheless,the research on chemical oxidation for reforming shale reservoirs is still in its infancy,lacking systematic discussion on the interaction mechanism and feasibility analysis.Therefore,Shuijingtuo shale in the Yichang area with relatively high carbonate mineral content is selected,based on the current shale gas exploration and development firm in China,to systematically study the water-rock reaction mechanism and feasibility of shale reservoir alteration during oxidative dissolution.It is expected to provide a reference for the in-situ application of oxidative dissolution for shale permeability enhancement.The main research contents and results of this thesis are as follows:(1)Water-rock interaction in the process of shale oxidative dissolutionThe dominant chemical reactions and influencing factors in the oxidative dissolution of shale are clarified based on the batch experiments between shale powder and three oxidants(H2O2,Na Cl O and Na2S2O8).Results show that the chemical reactions in acidic and alkaline environments are dominated by the dissolution of carbonate minerals,quartz and aluminosilicate minerals respectively.At the same time,the addition of an oxidant can also effectively dissolve pyrite and organic matter.In the process of oxidant stimulation,the dissolution of inorganic minerals contributes more to the mass loss of shale than the oxidation of organic matter.Strong acidic Na2S2O8 has an obvious dissolution effect on carbonate,resulting in the greatest potential of permeability enhancement.Alkaline Na Cl O has a strong oxidation ability to organic matter,but the generation of secondary precipitation such as Fe(OH)3 has the risk of blocking pores.Nearly neutral H2O2processes weak water-rock reaction due to severe thermal decomposition.The increased concentration of oxidant strengthens the water-rock reaction.The carbonate minerals in shale buffer the p H of the reaction system and slow down the dissolution of other minerals.In addition,carbonate dissolution and subsequent gypsum precipitation also regulate the oxidation process of organic matter.(2)Effect of oxidative dissolution on shale seepage channelThrough the water-rock reaction of shale in different sizes(shale chips and debris)with different oxidants(H2O2,Na Cl O and Na2S2O8),the influence of oxidative dissolution on the development of shale fractures and pores is clarified.Research shows that the generation of a large amount of O2 in the H2O2-treated process will promote the generation of bedding-induced fractures in shale.But a slight change in the shale pore structure is observed due to the rapid thermal decomposition of H2O2.Na Cl O has poor matrix accessibility because of its alkaline environment.Nevertheless,Na Cl O can transform the nano micropores into mesopores and macropores by organic matter oxidation.The Na2S2O8 can transform the shale matrix through severe water-rock interaction to generate many dissolution pores and induced fractures.During the reaction process,the medium pores and macropores associated with precipitated gypsum increase the pore volume and the complexity of pore structure in shale.(3)Mechanism of oxidative dissolution on shale alterationAfter identifying the critical components during Na2S2O8 oxidative dissolution on shale,batch experiments and geochemical simulation are carried out based on the method of component addition to quantify the reaction kinetic parameters of key components and clarify the oxidation-acidification-catalysis interaction mechanism.The results show that pyrite will catalyze and compete with kerogen oxidation.Dissolution of carbonate minerals leads to gypsum precipitation and shale mass increase,but it is also conducive to prolonging the longevity of Na2S2O8.The corrosion of chlorite will consume the acid to generated Fe2+,which will also promote the catalytic oxidation of kerogen.The chemical reaction rate of each component in the reaction process is as follows:calcite>pyrite>dolomite>kerogen>chlorite in the dissolved component,gypsum>albite in the precipitated component.(4)Permeability enhancement of shale under flooding conditionThe feasibility and mechanism of Na2S2O8 injection for improving shale reservoir permeability are clarified through the core-flooding experiments.The results show that the permeability of shale with high carbonate content can be increased by6.8–12.4 times after Na2S2O8 injection,which is mainly ascribed to the dissolution of pyrite and calcite and the oxidation of organic matter.For shale with low carbonate mineral content,Na2S2O8 can only slightly improve the permeability of shale.This process is mainly attributed to the intense water-rock reaction to reduce the roughness of the fracture surface,which is conducive to fracture closure in shale.At the same time,the dissolution of released aluminosilicate minerals also leads to the blockage of shale pores.Both the fracture in shale and the flow rate of oxidant affect the severity of water-rock reaction in the shale-Na2S2O8 system,which also directly leads to the difference in the transformation of shale matrix and fracture conductivity.Based on the results,the shale matrix alteration mechanism of Na2S2O8 is analyzed,and the feasibility and potential of oxidative dissolution for reservoir permeability enhancement are also discussed. |
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