CO_(2) dry fracturing is a promising alternative method to water fracturing in tight gas reservoirs,especially in water-scarce areas such as the Loess Plateau.The CO_(2) flowback efficiency is a critical factor that a...CO_(2) dry fracturing is a promising alternative method to water fracturing in tight gas reservoirs,especially in water-scarce areas such as the Loess Plateau.The CO_(2) flowback efficiency is a critical factor that affects the final gas production effect.However,there have been few studies focusing on the flowback characteristics after CO_(2) dry fracturing.In this study,an extensive core-to-field scale study was conducted to investigate CO_(2) flowback characteristics and CH_(4) production behavior.Firstly,to investigate the impact of core properties and production conditions on CO_(2) flowback,a series of laboratory experiments at the core scale were conducted.Then,the key factors affecting the flowback were analyzed using the grey correlation method based on field data.Finally,taking the construction parameters of Well S60 as an example,a dual-permeability model was used to characterize the different seepage fields in the matrix and fracture for tight gas reservoirs.The production parameters after CO_(2) dry fracturing were then optimized.Experimental results demonstrate that CO_(2) dry fracturing is more effective than slickwater fracturing,with a 9.2%increase in CH_(4) recovery.The increase in core permeability plays a positive role in improving CH_(4) production and CO_(2) flowback.The soaking process is mainly affected by CO_(2) diffusion,and the soaking time should be controlled within 12 h.Increasing the flowback pressure gradient results in a significant increase in both CH_(4) recovery and CO_(2) flowback efficiency.While,an increase in CO_(2) injection is not conducive to CH_(4) production and CO_(2) flowback.Based on the experimental and field data,the important factors affecting flowback and production were comprehensively and effectively discussed.The results show that permeability is the most important factor,followed by porosity and effective thickness.Considering flowback efficiency and the influence of proppant reflux,the injection volume should be the minimum volume that meets the requirements for generating fractures.The soaking time should be short which is 1 day in this study,and the optimal bottom hole flowback pressure should be set at 10 MPa.This study aims to improve the understanding of CO_(2) dry fracturing in tight gas reservoirs and provide valuable insights for optimizing the process parameters.展开更多
This paper comprehensively reviews the application and research progress of CO_(2) fracturing fluids in China,highlights the existing issues and puts forward suggestions for future development.Three types of fracturin...This paper comprehensively reviews the application and research progress of CO_(2) fracturing fluids in China,highlights the existing issues and puts forward suggestions for future development.Three types of fracturing fluid systems containing CO_(2),namely,CO_(2) dry fracturing fluid,CO_(2) energized fracturing fluid,and CO_(2) foam fracturing fluid,are categorized based on the mass ratio and process difference between CO_(2),water,and treatment agents.Field applications in China reveal several problem to be resolved:(1)The application scope of CO_(2) fracturing fluids is restricted to depleted reservoirs,re-fracturing of old wells,and medium-deep reservoirs with low formation pressure coefficients;(2)different types of CO_(2) fracturing fluids require different processes and ground supporting equipment;(3)optimization of CO_(2) compatibility,functionality,temperature and salt tolerance,as well as the cost of treatment agents is necessitated;(4)existing CO_(2) fracturing fluid system fail to perform well with low friction,low filtration,and high sand-carrying capacity.(5)there lacks a targeted industry standard for evaluation of performance of CO_(2) fracturing fluid system and treatment agents.Therefore,in order to meet the goals of CCUS-EOR,CCUS-EGR,or integration of fracturing,displacement and burial by CO_(2),efforts should be made in the aspects that followed,including in-depth investigation of the mechanism of CO_(2) fracturing fluids,the adaptability and compatibility between existing equipment,different CO_(2) fracturing fluid systems and processes,and construction of treatment agents,low-density proppants and highperformance systems of recyclability and industrial-grade.In addition,optimization of CO_(2) fracturing fluid system based fracturing design is also crucial taking such related factors such as overall reservoir geological conditions,petrophysical properties,CO_(2) transportation,and well site layout into consideration.展开更多
基金support from the National Natural Science Foundation of China(No.51904324,No.51974348)the Prospective Basic Major Science and Technology Projects for the 14th Five Year Plan(No.2021DJ2202).
文摘CO_(2) dry fracturing is a promising alternative method to water fracturing in tight gas reservoirs,especially in water-scarce areas such as the Loess Plateau.The CO_(2) flowback efficiency is a critical factor that affects the final gas production effect.However,there have been few studies focusing on the flowback characteristics after CO_(2) dry fracturing.In this study,an extensive core-to-field scale study was conducted to investigate CO_(2) flowback characteristics and CH_(4) production behavior.Firstly,to investigate the impact of core properties and production conditions on CO_(2) flowback,a series of laboratory experiments at the core scale were conducted.Then,the key factors affecting the flowback were analyzed using the grey correlation method based on field data.Finally,taking the construction parameters of Well S60 as an example,a dual-permeability model was used to characterize the different seepage fields in the matrix and fracture for tight gas reservoirs.The production parameters after CO_(2) dry fracturing were then optimized.Experimental results demonstrate that CO_(2) dry fracturing is more effective than slickwater fracturing,with a 9.2%increase in CH_(4) recovery.The increase in core permeability plays a positive role in improving CH_(4) production and CO_(2) flowback.The soaking process is mainly affected by CO_(2) diffusion,and the soaking time should be controlled within 12 h.Increasing the flowback pressure gradient results in a significant increase in both CH_(4) recovery and CO_(2) flowback efficiency.While,an increase in CO_(2) injection is not conducive to CH_(4) production and CO_(2) flowback.Based on the experimental and field data,the important factors affecting flowback and production were comprehensively and effectively discussed.The results show that permeability is the most important factor,followed by porosity and effective thickness.Considering flowback efficiency and the influence of proppant reflux,the injection volume should be the minimum volume that meets the requirements for generating fractures.The soaking time should be short which is 1 day in this study,and the optimal bottom hole flowback pressure should be set at 10 MPa.This study aims to improve the understanding of CO_(2) dry fracturing in tight gas reservoirs and provide valuable insights for optimizing the process parameters.
基金support for this work from National Natural Science Foundation of China (Grants No,U21B2071).
文摘This paper comprehensively reviews the application and research progress of CO_(2) fracturing fluids in China,highlights the existing issues and puts forward suggestions for future development.Three types of fracturing fluid systems containing CO_(2),namely,CO_(2) dry fracturing fluid,CO_(2) energized fracturing fluid,and CO_(2) foam fracturing fluid,are categorized based on the mass ratio and process difference between CO_(2),water,and treatment agents.Field applications in China reveal several problem to be resolved:(1)The application scope of CO_(2) fracturing fluids is restricted to depleted reservoirs,re-fracturing of old wells,and medium-deep reservoirs with low formation pressure coefficients;(2)different types of CO_(2) fracturing fluids require different processes and ground supporting equipment;(3)optimization of CO_(2) compatibility,functionality,temperature and salt tolerance,as well as the cost of treatment agents is necessitated;(4)existing CO_(2) fracturing fluid system fail to perform well with low friction,low filtration,and high sand-carrying capacity.(5)there lacks a targeted industry standard for evaluation of performance of CO_(2) fracturing fluid system and treatment agents.Therefore,in order to meet the goals of CCUS-EOR,CCUS-EGR,or integration of fracturing,displacement and burial by CO_(2),efforts should be made in the aspects that followed,including in-depth investigation of the mechanism of CO_(2) fracturing fluids,the adaptability and compatibility between existing equipment,different CO_(2) fracturing fluid systems and processes,and construction of treatment agents,low-density proppants and highperformance systems of recyclability and industrial-grade.In addition,optimization of CO_(2) fracturing fluid system based fracturing design is also crucial taking such related factors such as overall reservoir geological conditions,petrophysical properties,CO_(2) transportation,and well site layout into consideration.
