Controls on reservoir properties in organic-matter-rich shales: Insights from MICP analysis

Mercury injection capillary pressure (MICP) analysis was used to study organic-matter-rich Devonian shales varying in maturity from marginally mature to postmature to discuss controls on drainage and imbibition in shale reservoirs. Six samples come from the New Albany Shale; their total organic carbon (TOC) ranged from 3.3 to 8.4% and vitrinite reflectance (R_o) from 0.54 to 1.42%. One Marcellus Shale sample with R_o 2.50% was used to extend the maturity range to the advanced dry gas window. Three coal samples of varying rank were also included for comparison. Porosity of the shales ranges from 1.5 to 5.0%, and MICP-derived permeability ranges from 0.17 to 9.9 mD. Injection capillary pressure curves have a range of entry pressures from 0.055 to 0.421 MPa, and no relationship of entry pressure with maturity was revealed. Maturity, however, has an influence on pore size distribution, with the higher-maturity samples having significantly lower volumes of pores with throats less than 10 nm in diameter. Withdrawal capillary pressure curves document a wide range of withdrawal efficiencies from 55% to almost 100%, and pores having throats between 3 and 50 nm in diameter play a major role in the ability of mercury to be released from the pore system. The obtained data provide valuable information about shale properties and their behavior not only in the air-mercury system but also under reservoir conditions.