| Coalbed methane(CBM)is a kind of hydrocarbon gas which is mainly stored on the surface of coal matrix in adsorption state and partially dissociated in coal pores.Its main component is methane.It is a new,clean and high-quality unconventional natural gas energy rising in the world.China's CBM reserves are about 36 trillion cubic meters,ranking third in the world,with a total recoverable amount of about 10 trillion cubic meters,of which the total recoverable amount of Qinshui Basin is more than 1 trillion cubic meters,which is the coal basin with the highest CBM production in China.CBM reservoir protection drilling fluid technology is one of the key technologies of CBM exploration and development.In recent years,clean water drilling fluid has been widely used in CBM well drilling in Qinshui Basin.However,a complete and mature CBM drilling fluid system has not yet been formed,and the technical contradiction between wellbore stability and reservoir protection is still prominent.In view of this situation,this paper carries out research and design of Qinshui Basin coalbed methane reservoir protection and wellbore stability dual energy cooperative drilling fluid system.The paper is divided into seven chapters:The first chapter introduces the research background and significance of this paper,analyzes the research status and development status of this field at home and abroad,and introduces the research content and technical route.In the second chapter,the coal reservoir in Qinshui Basin is taken as the research area,and the geological situation,physical properties,natural diversion fracture system and other aspects of the coal reservoir are discussed.On this basis,the drilling fluid system suitable for CBM Drilling in Qinshui Basin is further studied.The influence of natural fracture system of coal reservoir on drilling engineering is mainly analyzed.In addition,through literature research and field investigation,based on coal mine underground observation,the development types of natural fractures in five typical coal reservoirs in Qinshui Basin are analyzed,and the development characteristics of micro fractures in coal matrix are clarified,which lays a geological foundation for subsequent drilling fluid design and application.In the third chapter,the optimization of biological enzyme,controlled degradation surfactant and waterproof and locking surfactant was carried out,hemicellulase and cellulase were selected as single agent of biological enzyme,PEG4000,Tween80 and ODEP-98 were used as controlled degradation surfactants,Span80 is the surfactant of waterproof lock;the synergistic effect of enzyme and surfactant was analyzed,and the mechanism of surfactant regulating the degradation of polymer by enzyme was revealed.The rate of enzymatic hydrolysis reaction of drilling fluid system can be directly judged by comparing the percolation rate of cores in different time periods measured by core flow meter,and the synergistic effect of enzyme and surfactant is demonstrated;then several influencing factors of the synergistic effect of enzyme and surfactant were explored,including the different molecular structure characteristics of polymer,the source and composition of enzyme,and the concentration of surfactant.In the fourth chapter,the design and evaluation of dual energy drilling fluid system for coalbed methane are carried out.Through the optimization of viscosity improver,fluid loss reducer and inhibitor,the basic formula is determined as clear water+ 1% lime soil+0.4% guar gum+ 1% lv-pac+ 1.5% KCl+ 0.2% soda.0.2‰ cellulase+ 0.2‰ hemicellulase is selected as the composite enzyme system.The composite enzyme can obviously improve the degradation rate of polymer in drilling fluid.The optimized composite surfactant system is 0.1% Span80+ 0.05% ODEP-98+ 0.05% Tween80,which greatly improves the reservoir protection performance of drilling fluid.Finally,the optimized dual energy drilling fluid formula is clear water+ 1% calcic soil+ 0.4% guar gum+ 1% lvpac+ 1.5% KCl+ 0.2% soda+ compound enzyme+ compound surfactant,and the basic parameters such as rheology,filtration and p H were evaluated.At the same time,the expansion test,heat roll recovery test,adding foreign material test were carried out,and the inhibition,temperature resistance,salt invasion resistance and cuttings invasion resistance of the developed drilling fluid system were evaluated,and the test results met the requirements.In addition,the comprehensive evaluation on the degradation performance and reservoir damage prevention performance of dual energy drilling fluid shows that its performance is excellent.In the fifth chapter,the application effect of dual energy drilling fluid system is evaluated.In terms of logging and coring,the value of dual well diameter curve is close to or equal to the bit diameter,and the average diameter expansion rate of the whole well is only 8.4%.It can be seen that dual energy drilling fluid can effectively reduce the diameter expansion rate and greatly guarantee the stability of the wellbore compared with traditional drilling fluid.At the same time,there were no accidents on the ground,underground and other complex conditions during the construction,and the core recovery rate was 96.8%,which proved that the drilling fluid formula not only ensured the safe construction,but also ensured the core recovery.In terms of drainage and production effect,the gas production of this well is more than twice that of other CBM wells in Zhaozhuang block,which indirectly reflects that the application of dual energy drilling fluid can effectively reduce the damage to the reservoir and ensure the gas production of the later reservoir.The main innovations of this paper are as follows:(1)The synergistic mechanism of surfactant and enzyme in drilling fluid is revealed.Surfactant can reduce the surface tension of drilling fluid,reduce the ineffective adsorption of enzymes on polymer,improve the effectiveness of polymer surface,and promote the degradation of drilling fluid by enzymes.The optimized non-ionic surfactant system FP6(0.1%Span80+ 0.05%ODEP-98+ 0.05%Tween80)can improve the drilling fluid degradation rate by6.2% to 96.2%,and the permeability recovery value can reach93.7% through the regulation of composite enzyme.(2)A dual energy drilling fluid system suitable for fractured coalbed methane reservoir was developed based on the synergistic effect of enzyme and surfactant.The formula of the system is clear water+ 1% calcareous soil+ 0.4% guar gum+ 1% lv-pac+1.5% KCl+ 0.2% soda+ compound enzyme+ compound surfactant,which can reliably realize the dual energy efficiency of wall protection and plugging in the early stage of drilling and reservoir protection in the later stage of degradation,it has achieved good application effect in Zhaozhuang coalbed methane demonstration block of Qinshui Basin. |
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