Applications of 2D-NMR maps and geometric pore scale modeling for petrophysical evaluation of a gas well

Abstract

This paper presents the petrophysical evaluation of a gas reservoir from the Gulf of San Jorge basin, Argentina, by means of two-dimensional nuclear magnetic resonance (2D-NMR) maps and a geometric pore scale modeling of the NMR response of the wetting phase. The reservoir contains gas and irreducible water saturation at the top with a sharp transition into the water zone. The well has been logged with conventional tools such as gamma ray, neutron, density, and also the MRExplorerSM (MREXSM) to acquire the NMR data for determining the irreducible water saturation. Data from special core analysis, scanning electron microscopy, thin sections, and capillary pressure tests have also been acquired. The core-log evaluation shows a very good agreement between the laboratory and log field data, especially in terms of irreducible water saturation, porosity, and permeability, which was also modeled using the Timur–Coates equation for purposes of field delivery. The 2D-NMR maps as T1 vs. T2 apparent (T2app) and diffusivity vs. T2 intrinsic (T2int) from both hydrocarbon and water zone lead to characterize the clay-bound and capillary-bound water and the under-called porosity due to the low hydrogen index of the gas. We use a pore scale model to quantify the effect of pendular water on T2 distribution. The methodology is based on constructing physically representative model rocks numerically, which allows precise geometric description of pore space. Unlike many other approaches to pore-level modeling, this approach introduces no adjustable parameters and can be used to produce quantitative, a priori predictions of the rock macroscopic behavior.