Study On Low-damage Cement System For Coalbed Gas Well Cementing In Eastern Yunnan And Western Guizhou

Coalbed gas is a kind of clean,high quality and unconventional natural gas resources.The coalbed gas resources of eastern yunnan and western guizhou region are abundant in our country,and the resources available for exploration account for about 10% of the national coal bed gas resources.The low pore pressure gradient and developed cleats and cracks of the coalbed gas reservoir in this area are easy to cause the intrusion damage to the reservoir by cementing slurry.Aiming at this problem,this paper discovered coalbed gas reservoir cement damage mechanism in eastern yunnan and western guizhou through the study of the damage degree and damage law of cement slurry to coal and rock,and developed multistage strong cohesive low density plastic cement slurry system with low damage and excellent performance.The microstructure and mineral content of coalbed gas reservoir in eastern yunnan and western guizhou were tested by scanning electron microscopy and XRD method,the physical and chemical properties and potential damage methods of coal and rock are analyzed;a coal reservoir cement slurry pollution evaluation method based on high resolution three dimensional X ray microscope coal core microstructure scanning and ultrasonic nondestructive measurement of compressive strength before and after coal core pollution is established,the quantitative evaluation of the damage of the cement slurry to the coal reservoir is realized;the damage regularity and damage degree of cement slurry to coal and rock are analyzed,and the damage mechanism of cement slurry in coal and rock is revealed;according to coal rock cement slurry damage mechanism,the four-bit integrated plugging idea of "Bridge + Construction + Network + Cementation + Multistage filling" is proposed to reduce the intrusion of cement slurry,the multistage strong cohesive low density plastic cement slurry system was developed through the optimization experiments of the plugging materials and admixtures,the slurry properties of the system were tested,and the damage degree of the system to coal and rock was evaluated.The results of the study show that,the minerals of coal rock in eastern yunnan and western guizhou mainly include calcite,quartz,and the kaolinite and illite which expand rapidly when contact with water,pores and cracks are developed,cracks are interlaced,and the connectivity of cracks and pores is good;The impact of intrusion of cement slurry on the microstructure of coal rock is obvious under pressure difference,cement particles block cracks and pores,and overlay on the surface of coal samples;After coal rock pollution,the permeability is reduced by up to 65%,and the greater the original permeability,the greater the degree of damage;Cement particle filled coal rock pores densely,and fully cemented the matrix of coal rock,the compressive strength of coal rock is increased by nearly 30%,which affects the later fracturing effect;The damage degree of cement slurry to coal rock is much greater than the effect of cement slurry filtrate on the alkali sensitivity and speed sensitivity of coal rock;The developed multistage strong cohesive low density plastic cement slurry has the characteristics of good flow,less water loss,high strength and strong plasticity,which can block the coal rock crack effectively and reduce the invasion depth,with the minimum leakage up to 8mL;The damage degree of the system to the compressive strength and permeability of coal and rock is 6.03% and 5.34%,effectively reduced the abnormal rise of fracture pressure during fracturing and protected the gas circulation channel.The study provided a low damage cement slurry for CBM well cementing in eastern yunnan and western guizhou,and provided technical support for the effective development of coalbed methane resources in this area.