Introduction to Special Log Analyst Issue on NMR Logging
Harold J. Vinegar Shell Exploration and Production Technology Co.

This November-December issue of The Log Analyst covers recent advances in NMR logging. This is a timely subject, as NMR logging was reactivated in the last few years with the introduction of new pulsed NMR logging tools containing permanent magnets. The new technology advanced NMR into mainstream logging applications, with all the major logging companies now offering NMR logging, and over 2,500 NMR logs run to date. This special issue focuses on some new applications of NMR logging. To date, NMR logs have been used mostly for determining irreducible water saturation and estimating permeability. Some of the new applications highlighted in this issue are gas detection and characterization of light hydrocarbons. With NMR logging likely to be utilized more in the future for fluid characterization, this Log Analyst issue begins with a review paper, "NMR Properties of Reservoir Fluids." The next paper, "NMR Logging of Natural Gas Reservoirs," describes the gas effect and how NMR logging can identify unambiguously the gas phase in the reservoir. It introduces the differential and shifted spectrum methods for quantifying free gas zones and locating gas-oil contacts in shaly sands. The next paper, "Selection of Optimal Acquisition Parameters for MRIL Logs," develops this methodology in the time domain, which is more robust in low signal-to-noise conditions. This paper also considers practical aspects of running NUMAR's MRIL tool to obtain hydrocarbon typing information. The methods in these two papers rely on NMR pulse sequences with long wait times because natural gas has a T1 between 3 and 6 s. The pulse sequences exploit the contrasting diffusion and relaxation characteristics of the reservoir fluids at the long end of the NMR spectrum. Readers will find the fourth paper, "Operating Guide for the Combinable Magnetic Resonance Tool," to be a very useful paper on the practical aspects of running Schlumberger's CMR tool. This paper describes how to change operating conditions to obtain different types of information with the CMR tool. The last paper in this issue is an exciting development in total porosity logging; that is, extending NMR logging to ultrashort interecho times (below 1 ms) to determine total porosity. Opening up the short end of the NMR spectrum is potentially very important for at least three reasons:
1) obtaining a lithology-independent total porosity,
2) detecting gas from the difference between density and NMR porosities, and
3) obtaining new information on clay minerals, with a possible direct logging link to clay counterion conductivity. In summary, recent developments have opened up the long end and the short end of the NMR spectrum. That's the long and the short of it!

Selection of Optimal Acquisition Parameters for MRIL Logs
R. Akkurt NUMAR, New Orleans, Louisiana, M.G. Prammer NUMAR, M.A. Moore Shell Offshore Inc.

Abstract: Recently introduced NMR-based logging techniques, such as the differential and shifted spectrum methods and the time domain matched filter analysis, have added hydrocarbon typing and calculation of near-borehole water saturation to the suite of available NMR applications. The keys to the new applications are the exploitation of the relaxation and diffusion properties of reservoir fluids by utilizing the single-valued magnetic field gradient and depth of investigation of the Magnetic Resonance Imaging Logging tool (MRIL). Reservoir, fluid, and borehole properties determine the optimum mode of operation for the MRIL. Temperature, pressure, hydrogen index, oil viscosity, mud type, and invasion characteristics can impact the information available from the log. This complexity, considering the variety of the applications available from the MRIL log, requires the careful selection of optimal acquisition parameters based on expected logging conditions. The objective of this paper is to establish the background to develop basic guidelines that can be used to identify and screen particular applications for reliable and robust MRIL-only interpretation. The mechanics of selecting the optimal acquisition parameters are demonstrated for a differential spectrum method application in the Gulf of Mexico, where the primary objective is the detection and quantification of free gas by relying on MRIL as the primary log.

NMR Logging of Natural Gas Reservoirs
R. Akkurt NUMAR, formerly Shell Offshore Inc., New Orleans, Louisiana, H.J. Vinegar Shell Development Co., P.N. Tutunjian Shell Development Co., A.J. Guillory Shell Offshore Inc.,

Abstract: The phenomenon causing reduced NMR porosities in gas reservoirs, the so-called "gas effect," has lately become a subject of great interest in the petrophysical community. Contrary to the industry-wide belief, NMR logging tools can detect gas provided that the pulse sequences are chosen properly and that the logging tool has adequate depth of investigation. Furthermore, gradient-based logging tools such as the MRIL-C can be used to unambiguously identify the gas phase in the reservoir. Failure to recognize gas may result in gas being misinterpreted as bound fluid, which, in turn, may result in excessively high irreducible water saturations and incorrect permeability estimates. The NMR properties of gas are quite different from those of water and oil under typical reservoir conditions and this can be used to quantify the gas phase in a reservoir. A new NMR-only interpretation approach based on this principle, called the differential spectrum method (DSM), has been developed and successfully tested in the Gulf of Mexico. This method utilizes properly selected NMR pulse sequences and does not require resistivity or other porosity logs. The DSM can be used in reservoirs containing gas and/or oil. Another technique exploiting the diffusion properties of gas, called the shifted spectrum method (SSM), is also introduced. Hydrocarbon saturations computed using the differential and shifted spectrum methods show very good agreement with those obtained conventionally. The methods are mineralogy independent and insensitive to clay-bound water and ideal for shaly sand applications.

Measurements of Clay-Bound Water and Total Porosity by Magnetic Resonance Logging
M.G. Prammer, E.D. Drack, J.C. Bouton, and J.S. Gardner NUMAR Corp.

Abstract: Pulsed nuclear magnetic resonance (NMR) logging has until now been limited to measurements of capillary- bound water and of free fluids, the sum of which is considered the "effective porosity" of rock. Clay-bound water and fluids trapped in micropores generally exhibit NMR relaxation times too fast to be detected, given the echo sampling rates and sensitivity limitations of current state-of-the-art NMR logging tools. Core studies performed on representative clay samples confirm a linear relationship between the transverse relaxation time T2 and the water content. At 1 MHz, clays with the largest specific surface areas (smectites) exhibit T2 values in the submillisecond range; illites have characteristic T2 values of 1 ms; kaolinites, having the smallest specific surface areas, relax with T2 values in the range of 10 ms. A new MRIL(r) application was implemented based on the industry-standard MRIL logging tool. By incorporating twice the standard sampling rate and an acquisition scheme designed to boost the signal-to-noise ratio of very fast decay modes, the tool is sensitive to transverse decay components as short as 0.5 ms. During a field-test campaign, the tool demonstrated the feasibility of simultaneous acquisition of effective porosity and total porosity. Neither porosity measurement requires prior knowledge of rock matrix properties. In shaly sands, the difference between MRIL total porosity and effective porosity can be interpreted as the clay-bound water volume, relevant as the clay conduction term for resistivity analysis.