A successful field test of a logging-while-drilling (LWD) tool in a Nexen Petroleum UK Ltd. Central North Sea well, resulted in 6 hours of rig-time savings, translating into more than US $65,000 in rig-time costs. The new multifunction LWD tool provides drilling and formation evaluation measurements in one short collar close to the bit. It provides more accurate formation evaluation while drilling than previous LWD tools, precise well placement and a safer, more efficient drilling operation.

Tool overview
The single-collar LWD tool consists of three distinct sections (Figure 1). The bottom section contains an azimuthal gamma ray (GR) detector and various drilling measurements, including annular pressure while drilling (APWD). Locating the natural GR measurement at the bottom permits it to be made closer to the bit and also prevents interference with the pulsed neutron generator (PNG) on top. Along with the APWD sensor, a three-axis shock and vibration sensor and an inclinometer are located at the tool's lower end.
The middle section contains an ultrasonic caliper and a cesium-based azimuthal density reading that is measured using the same principles as previous-generation tools, but is designed around a side-loaded cesium-137 logging source. Side-loading allows an optimal source position for increased count rates and density response for improved measurement precision and density image resolution at higher rates of penetration (ROPs). In addition to the ultrasonic caliper, the tool provides through overall density measurement improvements a more reliable density caliper over wider mud property ranges.
The tool's top section contains a PNG with associated detectors, which are co-located under a resistivity antenna array that provides phase-shift and attenuation propagation resistivity measurements. The PNG section provides several new LWD measurements, its near and far neutron detectors providing the count rate ratio-based neutron porosity measurements that include the thermal neutron and Best-Phi porosity measurements. A GR detector in between these neutron detectors is used for GR spectroscopy for detailed lithology; formation capture cross section (sigma) for a resistivity alternative to calculate fluid saturations; and the new neutron gamma density (NGD), which is a GR-generated density measurement from neutron interactions with the formation. A second GR detector provides the primary input for the NGD. The tool must be powered for the PNG section to generate neutrons, eliminating the need for an americium beryllium (AmBe) chemical neutron source.
To reduce tool length and move all measurements as close as possible to the bit, the PNG and its detectors were placed underneath the resistivity antenna array. Many technical challenges were overcome to place these resistivity antennas so close to the neutron generator and detectors, their successful co-location allowing five different measurements to be made at the same place and time. Previously, making such measurements introduced uncertainties regarding effects like invasion that could have occurred over time. The novel integrated design enables all measurements to be made within 16 ft (4.9 m) of the bottom of the tool.

Tool design
Key considerations in tool development were increased safety, reliability, operational efficiency and ROP capability. Integrating all measurements into one collar reduces the handling associated with LWD operations, wherein previously, at least two LWD collars were included with the bottomhole assembly (BHA). The integrated design improves safety by simplifying logistics and reducing lifting operations and loads. Another safety design aspect is the use of the PNG, which permits nuclear logs to be obtained without a chemical logging source, reducing exposure, handling and operational risks. In addition, employing a non-retrievable, side-loading cesium source improves safety as well as measurement performance. Moreover, the PNG provides a density measurement, giving the option of not running the cesium source at all.
Reducing associated LWD rig time and cost was integral to tool design. Shortening the flat lines on the drilling curve is achieved by cutting the time it takes to pick up and lay down the BHA, largely achieved by integrating the measurement sensors into one collar. Reducing to one chemical source also improves tool efficiency by eliminating the AmBe source to generate neutrons. Significant rig time savings also results from the faster cesium source loading and unloading process. Throughout the new tool's field-test period, an average 50% time savings in BHA handling was obtained, compared with that of traditional LWD tools. Additionally, a more reliable tool design approach helps minimize potential nonproductive time, including the use of dedicated chips on electronic boards and detectors to record inherent environmental parameters, improving maintenance efficiency and LWD platform reliability.
Rotary steerable system use has resulted in consistently faster drilling, which was taken into account in the new LWD design to obtain high-quality logs at fast ROPs. This accomplishment can be attributed to the tool's detectors for nuclear measurements, which provide better precision and repeatability at faster ROPs. Improved measurement specifications coupled with increased recording rates and memory capacity allow the new LWD tool to record two data points per foot at 450 ft/hr (137.2 m/hr).

Field test
Nexen Petroleum ran the EcoScope multifunction LWD service on a sidetrack of an existing well in the Central North Sea, United Kingdom. The sidetrack was drilled in 121¼4-in. hole down to the top reservoir, and then in an 81¼2-in. hole, 35° angled section through the reservoir. The BHA included the new LWD tool run above a positive displacement mud motor and below a telemetry tool and a previous-generation density neutron LWD tool. To evaluate the sidetrack's hydrocarbon bearing sands, the LWD objectives for the reservoir section were to acquire GR, resistivity, density-neutron porosity and caliper. In addition, the BHA run was designed to evaluate the new LWD tool's measurements and compare them with those of the traditional density-neutron tool for data quality. The run was also designed to identify this BHA's potential operational efficiencies gained and time savings realized compared to a traditional triple-combo LWD BHA.
During the sidetrack operation, about a 6-hr time saving was identified for future runs, translating to a saving of more than $65,000 in rig-time costs. The major time savings identified for future runs included: 1-hr saving by not having to load and unload the old-style radioactive source; 11¼2-hr saving by not having to pick up and lay down the traditional density neutron tool; and 1-hr saving by having a shorter and faster logged repeat section.
The operator noted safety benefits for the new LWD tool on future sidetrack jobs, namely less handling of the source, tubulars and lithium batteries (there are no lithium batteries involved in the operation). Also, when pulling out of hole, the BHA can be racked straight back in the derrick without having to break it down to unload the old-style density neutron tool source.
A petrophysics plot displays the new tool's measurements made in the well and how, by combining them, physical properties of the reservoir can be calculated (Figure 2). Clearly shown is a clean gas productive sand having 15% porosity, high permeability and a low clay volume from the capture spectroscopy mineral analysis. The data interpretation is based on an innovative method that uses the spectroscopy data as a primary input. The answer plot shows calculated productivity through the 10,360- to10,485-ft sand together with the lithology column from the elemental capture spectroscopy data. Also shown is the computed volumetric display.
In summary, the benefits of an integrated LWD platform in terms of drilling efficiency are evident through the 6 hours of rig-time savings achieved in this Central North Sea field test. Key to this achievement was the use of just one chemical logging source and the PNG, which also added new measurements and safety improvements to the LWD environment.