Optimization of Mercury Intrusion Capillary Pressure Measurement for Characterizing the Pore Structure of Tight Rocks

Hydrocarbons accommodated in tight reservoirs, e.g., shale and tight sandstone has been showing increasingly economic importance. It has been indicated that the pore structure at nanometermicron scale of tight rocks primarily determines the storage capacity and seepage flow of hydrocarbon. Therefore, quantitatively characterizing the pore size distribution (PSD) of shale and tight sandstones is the cornerstone for tight hydrocarbon evaluation and exploration. Mercury intrusion capillary pressure (MICP) is the commonly used measurement technique because it can reveal the PSD in a broad range from nanometer to micron. However, the MICP operation condition has not been well optimized for measuring the porosity, PSD, and apparent density and other petro-physical properties of tight rocks. We performed systemic MICP tests on mudstone and siltstone with different equilibration time and a variety of sample preparation, and compared with the helium pycnometry method and nitrogen adsorption isotherm measurements. MICP is found applicable for characterizing the PSDs of tight rocks with size of 15 nm to dozens of microns by using optimized operation parameters. The optimal testing equilibration time is 45 similar to 60 s, and sample grain size ranges in 20-30 mesh, which can avoid incomplete intrusion into the pores or altering the sample pore structure. However, the pores smaller than 15 nm can be characterized by using N-2 adsorption isotherm based PSD curve, which can be well integrated with MICP PSD curves. The proposed optimal operation conditions for MICP measurements can improve the reliability of the characterization of pore structures in tight rocks.