Abstract: There is no consensus on the understanding and measurement of effective porosity of shale gas reservoirs under irreducible water conditions in LAB. It gives rise to a great variance of core-measurement results,even on the same sample,and brings about difficulties and confusions in evaluation of shale gas reservoirs.To cope with the problem,both total and effective porosity are measured on paired-samples from the Longmaxi Formation in Sichuan Basin according to the criteria of GRI and SY/T 5336-2006,respectively.In addition,mineralogy,clay content and TOC are also measured.The results show that the total porosity and effective porosity of the paired-samples vary by a factor of 1 to 3.The large difference is mainly caused by the different treatment approaches of clay-related irreducible water volume in shale gas reservoirs.The gas-filled porosity (effective porosity) shows strong positive correlation with TOC.The coefficient of correlation is over 0.9.It implies that the organic matter pore is the main contributor to effective porosity in shale gas reservoirs.The irreducible water-filled porosity shows significant positive correlation with clay content.It implies that the clay-related micro-pores are occupied by irreducible water and are ineffective pores.A quantitative equation of effective porosity is established based on the above-mentioned understandings.Meanwhile,a logging evaluation method of total porosity is established based on the dry clay volume model.Both total porosity and effective porosity are calculated by using actual logging data,and the results coincide well with the core-measured results.

Key words: total porosity, effective porosity, logging, shale gas, Longmaxi Formation, Sichuan Basin