Application of Coal Bed Methane Drilling Fluid & Completion Fluid in Xiaozhuang Coal Field

Through the detection of mineral composition of coal and rock and the study of damage mechanism of coal, we propose the protected drilling and completion fluid system combining with sensitivity experiments. The results show that the drilling fluid system has a strong inhibitory effect to prevent the expansion of clay mineral hydration and to ensure the stability of the well, and has a low filtration loss（12.6 mL/30 min）, has a moderate viscosity （0.45 Pa/（mPa·s）） which can effectively carry rock and clean the bottom hole. The graded ultrafine calcium carbonate can effectively block coal seam fractures and reduce reservoir damage. Well completion fluid system: the water lock effect is reduced by surfactant, the cellulose is degraded by enzymes, the superfine calcium carbonate is dissolved by hydrochloric acid to remove the wall-mud and the carbonate in coal are dissolved by hydrochloric acid too. CO₂ generated by dissolution is adsorbed by fissure surface of the coal and can act as a power source to push completion fluid to carry out further action and increase coal reservoir energy for drainage. The precipitation of iron hydroxide is prevented by iron ion stabilizer. Finally the completion fluid can effectively improve the permeability of coal reservoirs.