A new method for grading and assessing the potential of tight sand gas resources: A case study of the Lower Jurassic Shuixigou Group in the Turpan-Hami Basin, NW China

A grading system from a pore structure perspective for assessing the quality of tight gas reservoirs in China is proposed based on an analysis of wettability, adsorption and pore throat size distribution and experiments including X-ray diffraction, contact angle measurement and static nitrogen adsorption, upon tight sand samples, from wells in Turpan-Hami Basin, western China. The hydrophilicity of tight sand has nothing to do with the overall clay content; instead, it is linked to the content of illite. Higher illite content means stronger hydrophilicity of the rocks. Indonesia's formula is proven to be more suitable for the interpretation of gas saturation measurement in tight reservoirs with higher content of illite. The microscopic pore structure of tight reservoir is clearly related to the porosity. The tight sandstone reservoir with low porosity is characterized by high proportion of small pores and high drainage pressure, while the tight sandstone reservoir with high porosity is characterized by high proportion of large pores, low drainage pressure and large average pore size. In view of endowment and flowing of tight sand gas, a new reservoir potential assessment parameter is established by porosity, permeability and gas saturation. The parameter relates to the porosity in a clear manner of trichotomy. Given the porosity of 6% and 4% as threshold, the tight sand gas can be divided into three types: I (Good), II (Potential) and III (Challenging). The trichotomy corresponds to a consistent porosity limit with the pore size change.