Abstract:

In order to illustrate formation damage mechanism of tight gas-bearing sandstone and work out a protection scheme during drilling and completion, clay minerals from Jurassic to Upper Triassic sandstone in representative gas fields were studied.Thin section, XRD, and SEM experimental methods were used to analyze the content, type, occurrence and microstructure of the minerals. The study indicates those clay content decreases with increased depth. There are kaolinite, chlorite, illite,smectite, interlayered smectite/chlorite and smectite/illite. Diagenitic history of the minerals is as follows: smectite (interlayered smectite/chlorite (chlorite, smectite(interlayered smectite/illite(I, and kaolinite(iillite. Seven types of clay microstructure
are recognized and the stability of clay microstructure increases with depth. The study also shows that clay sensitivities and reservoir tightness, which is resulted from the existence of clays, are the essential factors to cause formation damage. In prevention of formation damage, the most important is to solve the clay problems. Compared with water or oil based working
fluids, gas based fluids are superior for damage prevention from clays.

Key words: Tight Sandstone, Clay Mineral, Formation Damage, Clay Microstructure, Sichuan Basin