Characterization of Light Hydrocarbon-Bearing Reservoirs by Gradient NMR Well Logging: A Gulf of Mexico Case Study
Authors: Mardon D. , Miller D. , Howard A. , Coates G. , Jackson J. , Spaeth R. , Nankervis J. 
Published: December 2007

Characterization of Light Hydrocarbon-Bearing Reservoirs by Gradient NMR Well Logging: A Gulf of Mexico Case Study D. Mardon, D. Miller, A. Howard, and G. Coates, NUMAR Corp., J. Jackson, R. Spaeth, and J. Nankervis, Vastar Resources, Inc.

Abstract
This paper presents a case study from an offshore Gulf-of-Mexico field that illustrates some of the advantages and limitations of new gradient NMR (MRI) well log interpretation methods for evaluating light hydrocarbon-bearing reservoirs. In one well, analysis of dual-wait time MRI log data using the differential spectrum method reveals the presence of free gas in shaly sand reservoirs that was invisible by the standard neutron-density "cross-over" method due to shaliness effects on the nuclear logs. Well test results have confirmed the MRI log interpretation in the one zone in this well that has been perforated to date. Effective porosity and free fluid volume readings were artificially suppressed due to the well-known effects of low hydrogen density and long T1 relaxation times characteristic of light hydrocarbons. However, the MRI irreducible water volume measurement was unaffected by light hydrocarbons and thus found to be a powerful tool for evaluating low-resistivity zones when integrated with conventional log analysis.

Introduction
Pulsed NMR logging is rapidly gaining acceptance as a powerful tool for evaluating the hydrocarbon production potential of earth formations. In the past, the most common petrophysical applications of NMR have been determination of capillary-bound fluid volume, effective porosity, and permeability estimation. Recently, however, new applications have been introduced for one tool - the Magnetic Resonance Imaging Log (MRIL) - which can be used to detect and potentially quantity the volumes of water, light oil, and gas in the near-wellbore region investigated by the tool. These new applications exploit the contrasting diffusion and NMR relaxation characteristics of these fluids which, together with the investigation characteristics of the MRIL, give each fluid a unique signature to the tool.
This article presents new examples that illustrate the advantages and limitations of MRI log data for identifying and characterizing light hydrocarbons in shaly sand reservoirs. We focus on one well in which the differential spectrum method successfully detected free gas zones and located gas-oil contacts in shaly sand reservoirs that were not visible on conventional open-hole logs, i.e., by neutron-density "crossover". This and other examples illustrate the value of NMR log data for irreducible water volume for evaluating potential low-resistivity/low-contrast pay.

Differential Spectrum Method
The differential spectrum method (DSM) employs the combined effects of T1 relaxation and diffusion contrast to separate the NMR signals from light oil, natural gas, and brine. In essence, T1 contrast is used to separate the hydrocarbon (gas or oil) and water signals and then diffusion contrast identifies the hydrocarbon type. These principles are illustrated below for the fluids and conditions applying in the well selected for detailed analysis, Well A (T=200 F, P=4500 psi). The characteristic MRIL response to each fluid type depends on the fluid properties and on the data acquisition parameters. Table 1 lists the MRIL data acquisition parameters for Well A while Table 2 shows the properties of the relevant (i.e., produced) fluids from one reservoir penetrated by Well A.
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