Characterization of 3D Pore Structure in a Carbonate Reservoir Through Rock Typing: a Case Study of Khasib Limestone, Ah Field, Iraq

Abstract The Khasib limestone reservoir in the Ah oil field of Iraq is characterized by a variety of pore types and pore interconnectivity resulting in strong heterogeneity in fluid flow capability. To elucidate the underlying factors contributing to the elevated water cut and swift penetration of injected water in horizontal wellbores, this study proposes a novel methodology for characterizing pore structure variation within the three-dimensional reservoir space of the Khasib limestone, utilizing a rock typing approach. Core slabs, thin sections, scanning electron microscopy (SEM) and mercury injection capillary pressure (MICP) curves obtained from cored wells are subjected to quantitative analysis. A multi-hyperbolic-tangent function is created to quantify the MICP curves and pore-throat size distribution is obtained. By integrating lithofacies and petrophysical properties, 24 rock types are identified in cored wells and spatial distribution of these rock types is elucidated through well-log clustering; Pore structure parameters, including pore types and pore-throat radii, are upscaled for each rock type by introducing a volumetric weight factor. Consequently, a 3D visualization of pore structure variation within the Khasib reservoir is achieved. The Khasib reservoir is stratified into seven distinct horizons, with its effective pore-throat radius varying from 0.09 μm to 9.2 μm, showing an increasing trend upwards and a decreasing trend westward along the major axis of the structural anticline. Among these horizons, the Kh2-1-2L horizon, characterized by an average thickness of 0.8 m, predominantly comprises two rock types typified by interparticle porosity. These two rock types exhibit an average effective pore-throat radius of 8.53 μm and a permeability of 278 mD, surpassing neighboring rock types by several orders of magnitude. It is designated as the high-permeability "thief zone," serving as a preferential conduit for fluid flow and representing the principal factor influencing the rapid breakthrough of injected water. The findings are corroborated through production logging and discernible variations in resistivity observed in vertical wells. These results hold significant implications for optimizing protocols to stabilize oil production and effectively manage water influxes. The method presented in this paper also provides a reference for production optimization in other types of reservoirs.