This log demonstrates the magnesium detection capability of the GEM tool. The five tracks on the left show conventional log responses. Track VI on the right is the GEM tool log showing the weight fractions of the elements. Readings of mostly calcium (light blue) at the bottom and the gradual increase in magnesium (dark blue) towards the top indicates increasing dolomitization as we move up the well. These findings are confirmed with the overlay of the n-d log as well as the PE curve.

Formation evaluation is critical to exploration and development. It allows the consideration of the basic questions on a new level: Does the reservoir contain hydrocarbons or water? Is there oil or gas? Can fluids move through the rock? What kind of rock is it? How much potential is there?

Answers to these questions allow potential reserves to be identified, bolster the balance sheet, or determine and optimize production, ultimately leading to improved profitability. Analysis of elements from drilling cuttings is a key method of evaluating the formation to help clarify these questions.

In simple terms, elemental analysis is used to determine the concentration of specific minerals that can help identify the composition of reservoir rock. On a more detailed plane, a volumetric breakdown of mineralogy can be obtained by apportioning measured elemental concentrations according to mineral chemistry. This thorough quantitative assessment can lead to improved porosity determinations and more accurate reserve estimates and can benefit the development of efficient drilling procedures for subsequent wells or the design of better stimulation techniques.

With the new GEM geochemical logging tool developed by Halliburton, operators are better equipped to obtain more accurate estimates of their reserves, design optimal completion and stimulation programs, and maximize production. The tool offers a rapid and precise evaluation of formations with complex mineralogies, complementing the company’s LaserStrat cuttings evaluation service performed while drilling and giving operators a complete elemental analysis solution for complex reservoirs.

The tool detects gamma-ray emissions produced by neutron reactions in earth formations and identifies those emissions through the chemical elements involved. The tool works in boreholes from 6 to 20 in. in diameter and can be run in combination with other wireline tools. It also functions in various borehole fluid environments, including salty or freshwater-based muds, oil-based muds, and air-filled boreholes.

The design and selection of materials used in the tool minimize the undesired influences that the surrounding borehole environment and the tool itself have on the readings, which are the crucial challenges when using gamma ray spectrography. The design incorporates borehole gamma ray and thermal neutron shielding, which reduces detected responses from the borehole and tool, thus maximizing the signal-to-noise ratio of formation element responses that enhances the accuracy of the readings. Monte Carlo modeling showed that the gamma-ray shield reduces the borehole contribution of the detected spectra by 70% in an 8-in. hole filled with saturated salt water.

The tool can measure magnesium in carbonates and aluminum in clays and shale. These are among the most important elements needed to describe the reservoir, but perhaps the most difficult to measure until now. The tool also measures manganese, a fairly common constituent of carbonates and sheet silicates. Using these three additional elements to determine mineralogy leads to improved estimates of porosity, saturation, permeability, detection of swelling clays, and rock mechanical properties.

Tool configuration
As shown in Figure 1, the tool consists of an electronics section for the tool telemetry system and a detector section that contains the large bismuth germanate (BGO) detector, which is located above the neutron source. The detector crystal, photomultiplier tube, high-voltage power supply, and multichannel pulse-height analyzer are deployed inside the Dewar flask along with an eutectic heat sink and are engineered to permit extended operations in holes feature, in addition to the attachable cooling system, allows operators to log a longer section of wells in one run and reduces recovery time before redeployment.

The design of the tool makes it possible for other tools to be run above and below it without the need for special connections. The tool’s configuration also reduces the total logging string length, minimizing the risk of getting stuck.

A typical logging configuration would be a compensated dual-spaced neutron, spectral density, and spectral natural gamma-ray tool to provide measurements of thorium, uranium, and potassium. This combination of tools enables real-time output of a sufficient number of elemental weight fractions to evaluate formation mineralogy with exactness.

Enhanced measurement, detection
The GEM tool’s measurement system uses a standard chemical americium-beryllium neutron source instead of a neutron accelerator to introduce moderate-energy neutrons to the surrounding environment. This source simplifies the measurement technique, offers better reliability, and results in an overall shorter tool length.

Neutrons from the source lose energy through scattering reactions and are eventually absorbed by materials in the borehole, the formation, and the tool itself, although to a much lesser degree. The scattering and absorption reactions cause various elements among the atoms in the surrounding materials to emit gamma rays that have characteristic energies. Emitted gamma-ray spectra are detected and recorded using the large BGO scintillation detector coupled to a high-efficiency photomultiplier and a pulse-height analyzer.

A robust spectral fitting code was developed for the tool to constrain the elemental yields within meaningful limits. Relative elemental yields from the spectral fitting code are converted to absolute weight fractions in real time by applying a normalization for environmental effects that is obtained from the oxides’ closure model. This process derives the highly precise quantification of elements measured in weight percent, providing a better understanding of formation complexity, reserves in place, and completion design.

Encouraging results
Initial tests at Sonde Acceptance Wells (SAWs) at Halliburton’s Manufacturing, Research, and Development facility in Houston yielded encouraging results. These wells were constructed with stacked slabs of quarried limestone, sandstone, and dolomite rocks. One of the wells was filled with freshwater and the other with 166-Kppm sodium chloride brine.

The high-salinity environment of the saltwater SAW in particular provides a challenging logging situation for any neutron induced gamma-ray spectroscopy measurement because of the prolific production of capture gamma rays by chlorine. Chlorine gamma rays accounted for roughly 45% of all gamma rays detected in the saltwater SAW. Even so, the tool was able to identify the Kasota dolomite layers by magnesium detection in these zones. This result is particularly exciting because magnesium has a relatively small thermal neutron absorption cross-section, which makes its detection more difficult than other major elements such as silicon and calcium.

A second test (Figure 2) was conducted in the brine-filled borehole of a West Texas well with a dolomitic interval near the top of the carbonate section. The brine in the borehole had a salinity equivalent to a 180-Kppm sodium chloride solution. The GEM log was recorded at a logging speed of 15 ft/min (4.5 m/min). Under these taxing conditions, the magnesium elemental weight percentage correlated well with the PE log in the upper portion of the interval and reflected a trend of increasing dolomitic content toward the top of the zone. Occurrences of sulfur in the interval are believed to be associated with small amounts of anhydrite or gypsum often found in West Texas carbonates.

A third test (Figure 3) took place at a well located in North Texas. The presentation format is identical to the layout of the previous example. The interval spans a sand-shale sequence that includes two thick and nearly clay-free sandstone formations. Elemental results for this example exhibit anti-correlation of aluminum and silicon together with correlations between aluminum and potassium as well as aluminum and iron that are often observed among core data.

These characteristics are frequently related to variations in clay content, which appears to be the case in this example when taken in context with the gamma-ray log and the difference between neutron porosity and density logs. Thus, the elemental weight fractions in this example reflect variations of clay mineral content in a manner consistent with other conventional nuclear logs while providing a means of understanding the clay types.

Optimization
With its enhanced accuracy and versatility, the GEM elemental analysis tool represents a step towards optimal effectiveness of geomechanical logging.

It provides the most precise quantification of magnesium and aluminum in the industry and is the first logging tool that can measure manganese, making reserves estimations in formations with complex mineralogy easier and more accurate. Its cooling system and flask allow it to operate for longer periods in hot wells, logging longer sections and needing less time to prep for redeployment.

Additionally, combining the sensitivity of the tool in the vertical sections of wells with cuttings analysis in the horizontal sections provides operators with an understanding of reservoir mineralogy for the entire well, helping to achieve more accurate reserves estimates, improved geosteering on horizontal sections, better completion and stimulation designs, and improved production.