For many years there has been a need to find an alternative to the radioisotope-based gamma-gamma density (GGD) measurement. The traditional GGD measurement uses the scattering of 662-keV gamma rays from a 137 Cs radioisotopic source to determine formation density. A statistically precise measurement requires a 40-GBq or higher source strength and such a logging source, with a 30.17-year half-life, may pose health, security, and environmental risks.Pulsed-neutron generators have been used in the industry for several decades in wireline tools and more recently in logging-while-drilling tools. These generators produce 14-MeV neutrons, many of which interact with the nuclei in the formation through inelastic collisions. These inelastic interactions are typically followed by the emission of a variety of highenergy gamma rays. Similar to the case of the GGD measurement, the transport and attenuation of these gamma rays is a strong function of the formation density. However, the gamma-ray source is now distributed over a volume within the formation, where gamma rays have been induced by neutron interactions and the source can no longer be considered to be a point as in the case of a radioisotopic source. In addition, the extent of the induced source region depends on the transport of the fast neutrons from the source to the point of gamma-ray production.Even though the physics is more complex, it is possible to measure the formation density if the fast neutron transport is taken into account when deriving the density answer. This paper reviews the physics underlying the sourceless neutron-gamma density (SNGD) measurement, explains the various facets of the algorithm used for its computation and details the different environmental effects that may influence the measurement.The successful application of the method is shown in several log examples.

Abstract. When designing nuclear tools for oil exploration, one of the first steps is typically nuclear modeling for concept evaluation and initial characterization. Having an accurate model, including the availability of accurate cross sections, is essential to reduce or avoid time consuming and costly design iterations. During tool response characterization, modeling is benchmarked with experimental data and then used to complement and to expand the database to make it more detailed and inclusive of more measurement environments which are difficult or impossible to reproduce in the laboratory.We present comparisons of our modeling results obtained using the ENDF/B-VI and ENDF/B-VII cross section data bases, focusing on the response to a few elements found in the tool, borehole and subsurface formation. For neutron-induced inelastic and capture gamma ray spectroscopy, major obstacles may be caused by missing or inaccurate cross sections for essential materials. We show examples of the benchmarking of modeling results against experimental data obtained during tool characterization and discuss observed discrepancies.

Abstract. Oilfield service companies help identify and assess reserves and future production for oil and gas reservoirs, by providing petrophysical information on rock formations. Some parameters of interest are the fraction of pore space in the rock, the quantity of oil or gas contained in the pores, the lithology or composition of the rock matrix, and the ease with which fluids flow through the rock, i.e. its permeability. Downhole logging tools acquire various measurements based on electromagnetic, acoustic, magnetic resonance and nuclear physics to determine properties of the subsurface formation surrounding the wellbore.This introduction to nuclear measurements applied in the oil and gas industry reviews the most advanced nuclear measurements currently in use, including capture and inelastic gamma ray spectroscopy, neutrongamma density, thermal neutron capture cross section, natural gamma ray, gamma-gamma density, and neutron porosity. A brief description of the technical challenges associated with deploying nuclear technology in the extreme environmental conditions of an oil well is also presented.

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractRadioactive chemical logging sources have been used in the E&P industry for many years to help operators obtain valuable information about their reservoirs. Until recently, much of the information obtained using these sources could not be obtained with any other method. While the potential risks involved with the use of such sources have always been known, more awareness in the industry has led to increased efforts towards the reduction or even elimination of the use of chemical sources where possible.A new Logging-While-Drilling (LWD) tool has been developed, using innovative technology to provide a complete suite of formation evaluation measurements without having to use a chemical radioactive logging source. The use of a nonchemical radioactive source significantly reduces the environmental and operational risks normally involved with traditional LWD tools.The data delivered by this service include not only the traditional measurements such as gamma ray, resistivity, density, and neutron porosity, but also measurements not previously available in LWD such as formation capture cross section (sigma) and elemental analysis from neutron capture spectroscopy used to compute formation mineralogy. An entirely new LWD measurement has also been introduced with the tool, making it possible for the first time to determine formation density without the use of a chemical logging source.A case study is presented of a well situated in a field in southern Italy, inside an environmentally sensitive national park. Because of the location of the well, it has not been possible to use radioactive logging sources for formation evaluation. As a result, fully evaluating the reserves has been an ongoing challenge for the operator. In addition, because of the move towards high angle and horizontal wells, wireline

Radioactive chemical logging sources have been used in the E&P industry for many years to help operators obtain valuable information about their reservoirs. Until recently, much of the information obtained using these sources could not be obtained with any other method. While the potential risks involved with the use of such sources have always been known, more awareness in the industry has led to increased efforts towards the reduction or even elimination of the use of chemical sources where possible. A new Logging-While-Drilling (LWD) tool has been developed, using innovative technology to provide a complete suite of formation evaluation measurements without having to use a chemical radioactive logging source. The use of a non-chemical radioactive source significantly reduces the environmental and operational risks normally involved with traditional LWD tools. The data delivered by this service include not only the traditional measurements such as gamma ray, resistivity, density, and neutron porosity, but also measurements not previously available in LWD such as formation capture cross section (sigma) and elemental analysis from neutron capture spectroscopy used to compute formation mineralogy. An entirely new LWD measurement has also been introduced with the tool, making it possible for the first time to determine formation density without the use of a chemical logging source. A case study is presented of a well situated in a field in southern Italy, inside an environmentally sensitive national park. Because of the location of the well, it has not been possible to use radioactive logging sources for formation evaluation. As a result, fully evaluating the reserves has been an ongoing challenge for the operator. In addition, because of the move towards high angle and horizontal wells, wireline formation evaluation acquisition has been, at times, replaced by LWD. The new LWD tool allowed the operator to not only improve efficiency, but also brought with it the possibility of more completely evaluating the diagenetically complex tight carbonate reservoir through the use of the new measurements. The favourable environmental conditions (low temperature and low mud salinity) made it possible to obtain additional spectral data, which was used to correct the neutron porosity measurement for lithology effects, despite the presence of dolomite. The results of the formation evaluation studies are presented, together with an analysis of the impact on operational efficiency and environmental and operational safety as a result of not having to use a chemical logging source. Introduction and Geological Setting Understanding the porosity distribution and type within a reservoir is the first step in order to accurately quantify reserves and design proper field development scenarios. Porosity heterogeneities within complex carbonate sequences can cause the potential of a reservoir to be inaccurately evaluated when only conventional approaches, such as density, neutron and sonic logs, are used. Image logs and surface logging data have been recognized as being essential for correct petrophysical characterization of these reservoirs.