New Developments in Remote Elemental Analysis of Rock Formations
Authors: Scott H.D. 
Published: December 2007

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose:To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology.

Introduction

In oilwell logging, measurements are made of the physical properties of therock formations surrounding the borehole to properties of the rock formationssurrounding the borehole to determine the mineral fractions and fluids present. Basic measurements make use of electromagnetic, sonic, and nuclear technologiesin about equal proportions. Traditional methods use resistivity, density, neutron, and sonic logs. The use of nuclear techniques to provide elementalanalysis of borehole formations will be described here. This procedure makesuse of gamma ray spectrometry for use in open and cased boreholes.

Formation Elements

The first reason for measuring formation elements is the need todifferentiate between hydrocarbons and water. The second is to quantify theminerals present in the rock matrix. The ability to produce fluids effectivelydepends on such properties as pore-size produce fluids effectively depends onsuch properties as pore-size distribution and permeability, which are bothstrongly affected by mineral - e.g., producibility is affected by the quantityand distribution of clays. Early knowledge of this critical information beforea large initial investment in production facilities can determine the successof a project. Table 1 provides a list of formation elements currentlydetectable in situ by gamma ray spectrometry methods. Although the sensitivityto each element is different, most can be quantified to good accuracy. In thistabulation, examples of some of the most common minerals in sedimentaryformations are given, although not all possible clay types are listed indetail. Essentially, all major elements in sedimentary rocks are detectable. Several papers have been published recently on the use of elementidentification for clay typing.

Gamma Ray Spectrometry

Spectrometry is the measurement of the intensities of a range of wavelengthswhich, taken together, form a spectrum. The stimulation and detection ofspectra play an important role for element identification in several fields. Ingamma ray spectroscopy, the atomic nucleus is excited to higher energy statesby such processes as neutron collision scattering (inelastic scattering), thermal neutron capture, neutron-induced radioactive decay (activation), andnatural radioactivity. As the nucleus decays from the excited states to theground state, gamma rays are emitted. For a given element, the gamma rayenergies (wavelengths) are characteristic, and the intensities are proportionalto the abundance of that element.

Technology and Applications

Instrumentation. For many years, the most suitable detector for oilwellgamma ray spectrometry has been the sodium iodide scintillation crystal, activated with thallium. This detector has the advantages of a fast response, awide temperature range, large size, and cost-effectiveness. The most seriouslimitation, however, is the gamma ray energy resolution, which is about 70 keVat the standard energy of 1332 keV. Consequently, it works best for naturalgamma ray logging where only three elements (Th, U, and K) require separation. A new scintillation crystal creating interest is bismuth germanate. Thiscrystal has about twice the density of sodium iodide (higher efficiency) butrequires a stable temperature for best operation. Energy resolution is not yetas good as sodium iodide. Dramatic progress in energy resolution has beenachieved with a solid-state detector made of high-purity germanium. Thisdetector is the most important part of the enhanced resolution tool now indevelopment and of other similar systems. Resolution capability at standardenergy is close to 3 keV, more than a factor of 20 better than currentcommercial sodium iodide-based systems. A comparison of spectra from germaniumand sodium iodide-based tools is shown in Fig.

To achieve this fine energyresolution, the germanium detector is operated at a very cold temperature, close to liquid nitrogen (- 321 degrees F [77 K]). Design and construction of arugged cryostat to allow tool operation to 302 degrees F [150 degrees C] hasbeen completed successfully and several field tests have been run. Germaniumdetectors are made from the purest manmade material and have fewer than oneimpurity atom per trillion atoms of germanium. They are expensive, and crystalsizes are limited. Sodium iodide-based spectroscopy systems typically recordmultiple spectra with 256 energy channels. Consequently, it is easy to exceed10 kilobits of data per borehole foot. To record high-resolution germaniumspectra, 4,096 channels are typically used, significantly increasing the datatransmitted to the surface for analysis. Stimulation of gamma ray spectra isachieved with a small source of neutrons, either a miniature pulsed acceleratoror a continuous radioactive isotope.

Applications. Fluid Measurements. Chlorine dominates gamma ray spectrometrylogging because it has a great ability to capture thermal neutrons and ispresent in most formations in the form of salt water. Spectroscopists tookearly advantage of this to develop chlorine logs to measure the absence of saltwater and to infer the presence of hydrocarbons.

JPT

P. 711