‘Sourceless’ LWD improves horizontal analysis

The Auk field, located in the Central North Sea, comprises oil bearing aeolian sandstones of the Rotliegend formation. In 2009, Talisman Energy (UK) Limited planned to drill three new horizontal development wells into the Auk North field, which was originally drilled over 30 years ago. The well design consisted of long 8 ½-in. production intervals of between 5,000 ft (1,525 m) and 6,500 ft (1,980 m). The first two wells have been drilled and were evaluated using the Schlumberger EcoScope multifunction logging-while-drilling (LWD) tool.

One of the issues in the petrophysical evaluation of the Auk field is the necessity to discriminate between the dune slipface facies and dune windripple facies, since the latter is non-productive. Talisman was interested in evaluating the sigma and spectroscopy measurements available from the LWD tool to determine if they could be used to differentiate between the two facies.

The Auk field sits on the western margin of the Central Graben, in Block 30/16 of the UK North Sea under 240 to 270 ft (73 to 82 m) of water. It is about 169 miles (270 km) east southeast of Aberdeen. (Image courtesy of Schlumberger)

Integrated formation evaluation in single collar
The EcoScope service combines standard LWD measurements of formation density, propagation resistivity, gamma ray and compensated neutron response with the previously wireline-only measurements of elemental capture spectroscopy and thermal neutron decay time (Sigma), all in a single 26-ft (7.9-m), 6.75-in. diameter collar. The additional measurements are possible due to a major change in the technology used for measurement of formation neutron response. The traditional chemical neutron source has been replaced with an electronic neutron generator. In addition to the increased flexibility of measurements, the use of a Pulsed Neutron Generator (PNG) reduces risk by minimizing the requirement for chemical sources in the well. The multifunction LWD tool still contains one chemical source, which is required for the measurement of formation density; however, a new technique for measuring a sourceless formation density using the neutron generator is under development. By using this measurement, known as neutron gamma density (NGD), it will be possible in the future to run EcoScope for formation evaluation without putting a chemical source in the hole.

While drilling the lateral section in the first well, an unusual amount of severe wear was encountered on all of the bottomhole assembly (BHA) components, including wear on the EcoScope stabilizer that effected the bulk density measurement. The data processing center was able to use the tool memory data to compute the NGD density, an independent formation density that was not affected by the stabilizer wear.

Analysis of this data compared with the conventional density in the first run gave sufficient confidence in the new density porosity measurement to evaluate the reservoir, saving the time and cost of a dedicated logging run.

The EcoScope service combines standard LWD measurements (formation density, propagation resistivity, gamma ray and compensated neutron response) with previously wireline-only measurements of elemental capture spectroscopy, and thermal neutron decay time (Sigma) all in a single 26-ft (7.9m), 6.75-in. diameter collar. (Image courtesy of Schlumberger)

Sourceless real-time density measurement
Prior to drilling of the second well, several changes were made to the BHA to minimize the tool wear. To monitor the effectiveness of these adjustments and to give confidence in the real-time density measurement, it was decided to compute downhole and transmit in real time the sourceless NGD in addition to the traditional gamma-gamma density.

By monitoring these two density curves, Talisman confirmed their confidence in using the sourceless NGD data to evaluate the first well and were able to establish while drilling that data quality in the reservoir section was good. A comparison between the two measurements shows excellent agreement between the sourceless NGD and traditional density. In this first instance of employing the real time NGD measurement in the North Sea, it was proven that sourceless density can be delivered reliably for use in real time formation evaluation.

Talisman established that they could rely on the NGD density for future applications where it was not appropriate to use a chemical source in the tool.

New data for a mature field
In the case of the Auk North field development project, the field evaluation files were populated mainly with historical vertical well log data. By acquiring advanced data in two new horizontal wells, the quality of reservoir evaluation can take an important step forward. The newly acquired data was used to produce correlations that allowed the reservoir model to be upgraded. Talisman’s evaluation of the EcoScope measurements acquired in the first two wells indicated that variations in the sigma response could be used to confirm the classifications of dune slipface facies and dune windripple facies, assisting determination of reservoir quality. Furthermore, Orion II high speed telemetry provided very high quality density images to be transmitted in real time for improved identification of faults and dune bedding dips.

Azimuthal density images from the EcoScope tool from well #2 (left) help identify and quantify faults and dune dips essential to reservoir characterization. On the right, the gamma-gamma density curve (red) is seen to track the sourceless NGD curve (black) validating its use for formation evaluation. (Logs courtesy Talisman Energy (UK) Limited)

Future developments
The ability to provide consistent nuclear measurements-while-drilling (MWD), exclusive of chemical sources, with no compromise on quality, historical comparability or workflow compatibility will be valuable for our industry. Replacing the chemical neutron source with the PNG results in an 85% reduction in immediate surface exposure risk and a nearly 100% reduction in long-term contamination risk downhole. Further removal of the chemical gamma source will eliminate the HSE risks associated with handling of radioactive material on surface and provide for zero downhole radiation contamination risks. EcoScope empowers users to obtain the critical density and porosity measurements, in the form and quality they are used to, while taking advantage of technical innovations that makes the rig site safer and poses less environmental risk.

The Orion II telemetry platform enables more of the valuable downhole information to be available in real time than previously. Improved data density at high rates of penetration with quality drilling and formation evaluation parameters will have significant impact on the user’s operations and time sensitive decisions. Decisions based on improved geomechanical and petrophysical evaluation will help the operator to drill successfully in marginal mechanical environments and better place wells in the reservoir for improved production. A more frequent update for downhole parameters will allow the driller to better control the surface parameters to ensure optimum penetration rates and safe treatment of the bit and drilling assembly for longer drilling intervals.