Pore-Size Distributions in Vuggy Carbonates From Core Images, NMR, and Capillary Pressure

Abstract

Abstract Vugs, which are defined as pores larger than adjacent grains, are common in many carbonate reservoirs. Such pores are not always recognized by conventional wireline logs. The purpose of this study is to compare pore-size distributions in Oligocene carbonates from China using core, borehole images, nuclear magnetic resonance, and capillary pressure. The Kerr-McGee CFD2-1-2 well was drilled in the Bohai Basin in China. The reservoir consists of dolomitic limestones and fractured dolomite. The core (25 meters) was polished and inked with fluorescent paint, then photographed under black light to generate high-resolution, 2-D black and white photos. Image analysis produced detailed information about vug size and depth. Core images were then used to calibrate a Formation Micro Imager (FMI) log, so that pixel counts of vugs from the FMI would match core observations. Nuclear magnetic resonance (NMR) logs measure the T2 relaxation time, a parameter that is a function of pore size and reservoir fluid composition. Laboratory NMR measurements for six core plugs have been compared to pore-size distributions from core images. These distributions are comparable for high T2s (above 92 milliseconds). Mercury intrusion capillary pressure (MICP) measurements are used to determine pore-throat size distributions. These distributions, which can be directly compared to NMR and image analysis pore-size distributions, have very similar shapes. The net result of this study is that a technique has been developed to relate core-calibrated borehole images to NMR and MICP data. This is an important step in the difficult process of understanding NMR log behavior in vuggy carbonate rocks.