The developments of gas fields and laboratory experiments present that there are great differences in the well performance and well productivity between different wells with the same reservoir petrophysics. The characters and distributions of the tight gas reservoirs are not yet well interpreted by the current formation evaluation techniques. Based on the laboratory experiments by using the rate controlled mercury injection, nuclear magnetic resonance (NMR) and physical simulation technologies, the pore microstructure features of tight gas reservoir and its influence on the formation petrophysics were analyzed. Three new formation evaluation parameters for tight gas reservoir are proposed involving mainstream throat radius, movable water saturation and gas threshold pressure gradient (GTPG). Results of the constant-rate controlled mercury injection indicated that the mainstream throat radius is one of the key controlling factors for the petrophysics of tight gas reservoir. A good correlation-ship is obtained between the permeabilities and the mainstream throat radius. Results of core flooding experiments indicated that the pore structure have eminent influence on the GTPG. The GTPG is positive correlated with initial water saturation, and negative correlated with permeability. Results indicate that the absolute open flow potential (AOF) decreases with increasing GTPG. Water-blocking would occur and reduce deliverability of gas reservoirs, when gas displacement gradient is below GTPG. The GTPG plays an important role in determining the effective drainage radius and the abandonment pressure of gas well. The movable water and irreducible water could be identified by NMR method. Results show that moving water saturation (Smw) should be an assessment indicator to the water inflow of gas well. Incorporated with petrophysics, mainstream throat radius, gas threshold pressure gradient and movable water saturation a comprehensive evaluation method has been established. Applications of this method to some of the tight gas reservoir in China get reliable consistent results.

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