Sensitivity Response And Mechanism Of Upper Cretaceous Lignite Reservoirs In Erlian Basin,China

The sensitivity damage to coal reservoir runs through the whole process of CBM development.It is the key to optimizing the technology of CBM production enhancement by grasping the sensitivity response and its geological control.The coalbed methane development test in China’s lignite area is generally not ideal,and an important reason is the lack of sensitivity awareness.For this reason,this paper analyzes the potential sensitivity factors of the Upper Cretaceous lignite in the typical sags of Erlian Basin,and carries out the sensitive physical simulation experiments,which reveals the sensitivity effect and geological mechanism under different fluid characteristics.The following innovative insights have been achieved.The sensitivity response law of lignite is discovered.Under a given effective stress condition,the change in lignite permeability with the variation of fluid pressure undergoes three evolutionary stages,that is stable-rise stage,decline stage,and fluctuation stage,respectively.There are two effects（sensitization and velocity sensitivity effects）,which could be quantitatively characterized and predicted by the seven indexes ofΔM₁（pressure difference of rise stage）,ΔM₂（pressure difference of decline stage）,K₁（rise stage slope）,K₂（decline stage slope）,S²₃（fluctuation stage variance）,K₃（fluctuation stage slope）and D_m（maximum permeability damage rate）.Regardless of the mineralization degree and pH of the solution medium,the relationship between the dimensionless permeability and effective stress is consistent with the negative exponential model,and the reservoirs always have medium-to-strong stress sensitivity.The stress sensitivity coefficient shows fluctuation and stability stage with the increase of effective stress.A dynamic permeability prediction model which considers stress sensitivity under the influence of two factors can be constructed.With the decrease of mineralization degree,the dimensionless dynamic permeability of lignite is represented by three stages,and the main controlling geological factors are different in different stages.A comprehensive identification method for lignite reservoir sensitivity is constructed.It is found that slope of velocity sensitivity stage（K₂）can cover and reflect the water sensitivity effect law at different pH,which is defined as a comprehensive water sensitivity index to indicate the maximum permeability damage degree caused by water sensitivity effect.It is found that the compressibility coefficient（C_f）can not only characterize the velocity sensitivity sustained range at different pH,but also reflect the stress sensitivity characteristics under the coupling action of pH and salinity.It defines a comprehensive stress sensitivity index to characterize the maximum damage degree of permeability caused by the stress-sensitive effect.It is found that the variance of fluctuation stage（S₃²）can reflect the influence of mechanical strength of coal on the velocity sensitivity effect,which is defined as a comprehensive velocity sensitivity index to indicate the the maximum permeability damage degree caused by the velocity sensitivity effect.Integrating three correlation sensitivity parameters（K₂,C_f,S₃²）to establish a comprehensive sensitivity index system of lignite reservoir,and a comprehensive sensitivity three-dimensional identification template is also established,which is used to predict the sensitivity of the No.V coal in the Jiergalangtu sag.The prediction results show that the coal has the characteristics of strong velocity sensitivity,weak water sensitivity and strong stress sensitivity,and it is necessary to take targeted technical measures to reduce reservoir damage.The geological mechanism of the sensitive response of lignite reservoirs is revealed.The pH of the solution may mainly affect affects the migration and aggregation of pulverized coal particles through wettability,with the chemical reaction between the solution and the coal material both affect the change of permeability.The influence of salinity on sensitivity is mainly reflected in the coal agglomeration under high salinity,the separation of non-expanded clay and the hydration expansion of clay minerals under low salinity.Effective stress mainly affects reservoir sensitivity by affecting pore structure and destroying coal structure.Huminite content,mineral occurrence,types of oxygen-bearing functional groups,mechanical properties of coal,and pore connectivity also affect the sensitivity.Based on the sensitivity superimposed coalbed methane well productivity model,it is found that the effective stress of reservoir under the condition of superimposition of velocity sensitivity and stress sensitivity effect is greater than that of reservoir only experiencing stress sensitivity damage.