BACKGROUND Porosity is a key parameter for exploration and evaluation of shale gas reservoirs. Rapid and accurate determination of shale porosity is crucial for reserve calculation. At present, there are helium gas method, liquid saturation method and image analysis method applied to shale porosity determination, but the comparison between methods and laboratories is lacking.

OBJECTIVES To evaluate the applicability of different methods for determining shale porosity.

METHODS The characteristics of 69 shale samples from the Longmaxi Formation were studied by analyzing and testing techniques such as X-ray diffraction, argon ion polishing-scanning electron microscopy, and mercury injection-nitrogen adsorption method. Comparison experiments for porosity measurement were conducted among helium method, liquid saturation method and image analysis method.

RESULTS The Longmaxi Formation shale had high clay content, many organic pores, small pore throats, and poor seepage capacity, which made it difficult for foreign liquids and gases to quickly enter the pores, which required high resolution of core image data. The liquid saturation method (alcohol method) used 20MPa pressure to saturate the sample for 24 hours, resulting in core damage, and the measured porosity deviated from the true value. The image analysis method ignored the micropores (pore size < 2nm) and mesopores (pore size < 50nm) in the mineral matrix and other factors, making the porosity measurement result unreliable. The porosity measurement result of the helium method was closely related to the drying temperature and stabilization time. The higher the temperature, the larger the measured porosity, and if the stabilization time is too short, the porosity result will be smaller than the real values.

CONCLUSIONS After drying the core at 60℃ for more than 24 hours to a constant weight, by setting a reasonable stabilization time, accurate porosity results can be obtained by the helium method. Properly increasing the injection pressure helps to speed up the experimentation rate.