Comprehensive pore size characterization of Midra shale

Abstract

Although the Middle East's carbonate reservoirs were the first and most studied rocks in the world, Middle Eastern shale reservoirs are still poorly understood compared with other unconventional formations in the US. We investigate the pore-throat size and pore-body size distributions of Midra shale in Qatar to quantify the pore-scale features that control transport properties at the core scale. We measured the capillary pressure by injecting mercury into samples in drainage. The capillary pressure variation with wetting phase saturation exhibits a plateau-like trend, which differs from the linear trend observed in most shales in the US. The capillary pressure measurements quantify the pore-throat size distribution. We also measure nitrogen adsorption and analyze the measurements to determine the pore-body size distribution. This study shows that the pore-throat size has a narrow distribution, and its average is close to 22 nm. In addition, the pore-body size has a wide distribution, and its average is 18 nm. Thus, the transport properties dependent on the pore-throat size require modifications to account for the pore proximity to represent subsurface conditions. The transport properties, such as density, relevant to the pore volume in the matrix can be estimated with reasonable accuracy from the gas composition in wider conduits. The presented results have applications for the development of unconventional gas, which is the cleanest fossil fuel.