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PublisherSociety of Petrophysicists and Well-Log AnalystsLanguageEnglish
Document ID 2009-73568
Content TypeConference Paper
TitleA NEW APPROACH FOR IDENTIFYING GAS RESERVOIRS USING MULTIPLE LWD DENSITY MEASUREMENTS
AuthorsRubi Rodriguez, Geoff Weller, Mike Evans, Hendrayadi Prabawa, Schlumberger; Thomas Zalan, Chevron Nigeria
SourceSPWLA 50th Annual Logging Symposium, June 21 - 24, 2009 , The Woodlands, Texas
Copyright2009, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors.
PreviewA recently-released Logging-While-Drilling (LWD) tool introduces a new formation bulk density measurement that uses a pulsed neutron generator (PNG) source instead of a chemical logging source. This new measurement is known as Neutron Gamma Density (NGD*) and makes it possible to acquire a formation bulk density measurement without having to use a chemical logging source. Since this LWD tool also optionally includes the traditional cesium-based density measurement, it now also becomes possible to acquire two independent formation density measurements at the same time.  

The development and theory behind the NGD measurement is discussed. We also compare NGD with the more traditional cesium-based density measurement to highlight the similarities and differences between the two methods for determining formation bulk density. One of the advantages of NGD is that the measurement has a greater depth of investigation (DOI) compared to the cesium-based density, and it is therefore less sensitive to filtrate invasion. This feature can be particularly interesting in gas reservoirs, especially those that have undergone some filtrate invasion.  

We present a methodology to compute hydrocarbon density and to identify the type of hydrocarbon that uses two different LWD bulk density measurements acquired at the same time. Illustrated by an example from West Africa, we show how it is possible to make a more positive identification of gas-filled reservoirs. In addition, we quantify the improvements obtained in the estimation of formation properties such as porosity and saturation in order to make a more realistic calculation of reserves.

INTRODUCTION  

The formation bulk density measurement is widely used to compute the petrophysical properties of a reservoir, and is generally considered to be a key input to log interpretation. Together with neutron porosity, density plays an important role in formation evaluation. The combination of both logs in applications such as the density-neutron crossplot and the density-neutron overlay, are widely used in determining lithology, porosity and identifying gas-bearing zones.  

In a clean reservoir, an increase in apparent density porosity together with a decrease in apparent neutron porosity – the so-called gas crossover – is often used as a potential indicator of gas. The difference between the apparent porosities and the actual formation porosity depends on several factors including the gas saturation, lithology, and gas density. Under reservoir conditions, the situation will become complicated and the gas crossover effect reduced or even hidden because the amount of gas “seen” by the different measurements may be quite small due to filtrate invasion, low formation porosity, formation shaliness, or low gas saturation. To better understand the influence of these variables requires multiple measurements, preferably with different depths of investigation.  

The EcoScope* LWD tool offers a comprehensive suite of measurements in a single collar, as shown in figure 1. Enabling tight integration of these measurements, a pulsed-neutron generator (PNG) positioned under the resistivity array removes the need for a chemical neutron source. The electronically-controlled pulsed neutron source delivers more neutrons of higher energy than the chemical Americium-Beryllium (AmBe) source that it replaces. Improved neutron source control enables sigma, spectroscopy and NGD measurements, all of which are new to the LWD domain.
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