Real-time fluid type identification and quantification in a complex carbonate sequence was used to support completion decisions.

The oil and gas industry represents one of the world’s toughest markets. As new technology gains market traction, users can be confident that some of the world’s most demanding geoscientists and engineers have carefully evaluated all its features and potential applications.

The Schlumberger EcoScope multifunction logging-while-drilling (LWD) tool, designed and built in a single 26-ft (7.9-m) collar, offers a long list of services, including the following:

• 2 MHz and 400 KHz propagation resistivity;

• Elemental capture spectroscopy;

• Neutron-gamma density;

• Thermal-neutron porosity;

• Capture cross section, Sigma;

• Azimuthal bulk density;

• Azimuthal photoelectric factor;

• Azimuthal natural gamma ray;

• Density caliper;

• Ultrasonic caliper;

• Annular pressure and temperature while-drilling;

• Triaxial shocks and vibration; and

• Near-bit borehole inclination.

The tool co-locates the top five measurements on the list, meaning that they are essentially measuring the same formation volume under the same static and dynamic conditions simultaneously; this simplifies interpretation because the same environmental conditions apply to all five measurements, allowing a more precise, integrated petrophysical evaluation. All measurements are considerably closer to the drill bit than if they were spread out over several tool systems run in tandem. This gives a considerable advantage when using real-time tool measurements to support geosteering decisions.

This tool has replaced the traditional chemical neutron source by an electronic pulsed neutron generator. In fact, only the azimuthal bulk density measurement and photoelectric factor require a chemical gamma ray radioactive source. If the user prefers to forgo the traditional gamma-gamma density log to avoid having to use a chemical radioactive source, it can now be substituted by the neutron-gamma density. This is a new measurement that uses high-energy inelastic neutrons available from the electronic neutron generator to generate gamma rays, which then interact with the formation to provide a bulk density measurement.

Track record

In the relatively short time since its introduction, this tool has performed operations for 65 different clients worldwide while drilling more than 1 million ft (305,000 m) in 50 different locations. These include the longest run time of 317 pumping hours and the longest run distance of 6,234 ft (1,900 m). Maximum circulating temperature logged to date is 325ºF (163ºC).

Using the new Orion II high-speed telemetry system, the multifunction formation evaluation tool is capable of transmitting high-quality real-time data while drilling at a speed up to 500 ft/hr (152.4 m/hr).

Having all the measurements in one collar also reduced the number of connections in the bottomhole assembly and thus increased the reliability of the system.

Complementary measurements

The tool provides a rich dataset including lithology and mineralogy, porosity, and fluid saturation measurements that are delivered in real time to support drilling and evaluation decisions. While most of the tool’s measurements are well-known, having been introduced earlier in wireline-conveyed systems, the ability to acquire these data simultaneously from a single point during drilling is new to the industry and has enabled new applications of these measurements to be developed.

Lithology and mineralogy knowledge helps resolve complex geology and reservoir heterogeneity issues. The principle source of these answers comes from the elemental capture spectroscopy measurements. By bombarding the formation with neutrons, captured gamma rays are given off as the neutrons interact with various elements of the formation. The gamma energy produced by each element comprises a measurable, characteristic spectrum, which is a function of the relative proportion of the elements present in the reservoir rocks. Elements positively identified include silicon, calcium, magnesium, iron, titanium, sulfur, and gadolinium.

From the elemental analysis, formation mineralogy and matrix properties can be provided as an essential aid to interpretation and reservoir characterization. Using a comprehensive database of more than 1,500 hundred core samples analyzed for both chemistry and mineralogy, empirical relationships supported by nuclear modeling have been developed to derive these petrophysical parameters.

Formation mineralogy is deduced using an empirical model that determines the weight-fractions of the following minerals and groups: clay, quartz/ feldspar/mica, calcite, dolomite, pyrite, anhydrite-gypsum, coal, and halite. Matrix density, matrix neutron, and matrix sigma, which are necessary to convert the bulk density, neutron, and sigma measurements to porosity and water saturation, are determined directly from the elemental concentrations.

In complex lithology, continuous logs of mineralogy and matrix properties can be useful in correlation of stratigraphic sequences and seismic attribute analysis and are invaluable for the accurate calculation of porosity and water saturation.

As for porosity, the tool offers compensated thermal neutron porosity, hydrogen index porosity, azimuthal density porosity, and neutron-gamma density porosity. Lithology, independent density porosity, and neutron porosity can also be calculated based on the use of the matrix properties available from the spectroscopy measurement.

The use of an electronic pulsed-neutron generator also allows the measurement of a thermal neutron decay time, or the formation thermal neutron capture cross section, sigma. The sigma measurement has proved to be an exciting addition to the LWD formation evaluation portfolio as it provides an alternative way to determine water saturation when traditional resistivity-based methods are complicated by low-resistivity pay or uncertain interpretation parameters, such as Archie’s m and n.

Saturation is traditionally derived from porosity and resistivity. The fact that these measurements are co-located greatly simplifies formation evaluation. It ensures that they are acquired under the same formation conditions, eliminating uncertainty about time and environmental differences when comparing the measurements. This removes a great deal of the uncertainty that often complicates measurement interpretation.

A versatile evaluation system

One of the advantages of a multifunction LWD tool is the adaptability of the measurement suite. Data can be presented in a way that complements the objectives of the acquisition, so users can make the most efficient use of the information.

A gas-bearing sand-shale sequence in a low-angle deviated well drilled with synthetic oil-based mud is shown in Figure 1. Natural gamma ray and two caliper curves appear in Track 1, followed by a lithology by- weight presentation in Track 2. These are followed respectively by resistivity, porosity (sigma, Hydrogen Index, Density), and saturation curves in Tracks 3, 4, and 5. Tracks 6 and 7 compare fluid volumes derived from resistivity and the combined nuclear measurements, respectively. The saturation curves shown in Track 5 compare calculations made from each method. The good agreement between the two cross-validates the interpretation parameters and confirms the water saturation as it has been derived by two independent techniques.

The second example is a suite of real-time measurements acquired for detailed lithology and porosity evaluation in a carbonate reservoir (Figure 2). The client wanted to determine gas, oil, and water saturations in addition to hydrocarbon density in a complex carbonate with varying lithology and fluids. Mud filtrate invasion was a major issue that played a role in the decision to acquire the necessary data while drilling, and as close to the bit as possible.

In this example, phase-shift resistivity at three separate depths of investigation is presented on a logarithmic scale in Track 1. Thermal neutron porosity, bulk density, and photoelectric factor can be seen in Track 2. Formation sigma along with dry weight percent of sulfur and magnesium are presented in Track 3. Hydrocarbon density and fluid saturations are shown in Track 4, and the volumetric mineralogical analysis is depicted in Track 5.

A clear picture of formation lithology, fluid volumes, and saturations can be seen in Tracks 4 and 5. The spectroscopy measurement provided the elemental reservoir rock constituents, and sigma provided quantitative hydrocarbon type and saturations. Integrated interpretation of the tool’s measurement suite permitted evaluation of downhole hydrocarbon density. The real-time information guided well-completion decisions and provided a cross-check for subsequent enhanced oil recovery simulations.

The Schlumberger Carbonate Center of Excellence focuses on continued development of improved measurement and interpretation techniques in complex lithologies typical of carbonate reservoirs. EcoScope measurements are an important addition to its toolbox.