Abstract: The low permeability characteristics and the contamination of drilling fluid are the main factors affecting the effective production of low permeability gas reservoirs. Based on the ultra-high temperature and high pressure resistance experimental system and CT scanning technology, the permeability stress sensitivity and drilling fluid pollution characteristics of a high temperature ultra high pressure tight gas reservoir (158.6 ℃ 146 MPa) in the southern margin of Xinjiang Oilfield were studied. The research results show that: 1) more than 90$\%$ pores in the reservoir cores are intergranular pores, and a small amount of intergranular pores are fully or half-filled by calcite cement and asphalt; 2) under the net stress of 110 MPa, the stress-sensitive permeability reduction ratio (relative to the net stress of the initial reservoir) of the two plug cores (0.001 9 mD, 0.032 5 mD) were equal to 39.28$\%$ and 16.04$\%$, respectively, which are moderately weak and weak sensitive, respectively; 3) qualitatively and quantitatively characterizing the drilling fluid invasion characteristics of the other two plunger cores (0.004 2 mD, 0.029 mD) under actual operating conditions, it was found that the drilling fluid did not break through the length of the two cores. It was tested that the drilling fluid invasion length in the 0.029 mD core was 3.86 cm, but the skin coefficient caused by it reached 6.54, which belonged to serious pollution degree. The research results provide important technical support for mastering the characteristics of high temperature and ultra-high pressure gas reservoirs, understanding of drilling fluid pollution, and for establishing subsequent reservoir protection measures.

Key words: high temperature and ultra-high pressure, tight gas reservoirs, stress sensitivity, drilling fluid contamination, CT scanning technology