Oilfield service company Schlumberger is allowing for better evaluation of geological formations during drilling without traditional chemical sources.
NeoScope, the Schlumberger formation evaluation-while-drilling service, is the industry's first and only sourceless formation evaluation technology, said Akshay Sagar, global business development manager for measurements, Schlumberger Drilling & Measurements.
While chemical sources have traditionally been used for measuring while drilling (MWD) and logging-while-drilling (LWD) technology, introducing chemical sources into a drilling bottomhole assembly (BHA) involves a level of risk greater than similar tools deployed in wireline.
"In addition, stringent regulatory requirements are imposed on the transport, storage, handling, abandonment and eventual disposal," Sagar told Rigzone in a recent interview.
Traditional acquisition technology employs americium-beryllium radio-isotopic neutron and cesium 137Cs gamma ray sources to provide respectively neutron-porosity and bulk density measurements.
NeoScope's pulsed-neutron-generator (PNG) tool allows neutrons to be generated on demand, eliminating the need for traditional chemical sources that are associated with all other nuclear services. Most of the NeoScope measurements have been used for years in wireline and logging-while-drilling, but the sourceless neutron-gamma-density (SNGD) is a new measurement to the industry and completes the portfolio of sourceless triple-combo formation evaluation measurements.
Designed around the PNG, the NeoScope formation evaluation-while-drilling service uses technology that resulted from collaboration between Schlumberger and Japan Oil, Gas and Metals National Corporation (JOGMEC). Through that collaboration, Schlumberger and JOGMEC sought to develop PNG technology for deployment in logging while drilling, with the intention of eliminating the need for traditional chemical sources.
"The PNG and the comprehensive suite of measurements in a single collar are key components of the NeoScope service that deliver game changing LWD technology," Sagar said.
The service, through a single 25-foot collar, offers azimuthal gamma ray, array resistivity, dual ultrasonic calipers, a full drilling mechanics package and a full set of nuclear measurements, without the use of chemical sources. Eliminating of the "always on" chemical sources reduces risk and the logistical complexity in moving, storing and using chemical sources. Because the source can be switched off, it can operate at higher count rates during operation.
"This provides measurements with better statistics and hence improved precision compared to measurements made with a chemical nuclear source as these are count rate limited by the need to ensure personnel safety during deployment," Sagar commented.
NeoScope can be used in a wide range of environments, including onshore and offshore, as well as for exploration and production wells, mature and emerging plays and conventional and unconventional reservoirs, said Sagar.
For exploration drilling, development drilling or everyday logging, NeoScope:
NeoScope also provides an extensive suite of sourceless measurements for unconventional plays to evaluate reservoir quality and provide critical data to optimize completion quality. The service also provides sourceless measurements in real-time for better well placement, and removes transportation and handling risks and the costs of chemical sources.
Tullow Oil has used NeoScope in its French Guiana operations to acquire formation evaluation measurements in a remote exploration well in the Zaedyus field. In North Africa, the NeoScope service successfully logged a well in which wireline conveyed measurements were unable to access the entire open-hole interval due to deteriorating hole conditions. NeoScope has also been used in the Wolfcamp shale play in North America.
In Africa, NeoScope was used to acquire a full suite of petrophysical measurements in a directional exploration well, where poor borehole conditions prevented wireline tools from reaching the entire interval of interest. The customer was able to save seven days by eliminating the time and cost associated with chemical source deployment while providing the data required to successfully evaluate the well.
The PNG-based measurements were compared to wireline-conveyed conventional chemical nuclear source-based neutron porosity and density measurements in the shallower section. Sagar said that the SNGD data and other LWD measurements compared favorably in the interval where both set s of data were available, "providing confidence for use of the PNG-based measurements for formation evaluation over the entire interval logged with the NeoScope tool."
"The cost and environmental concerns surrounding exploration in frontier areas increase demand for safe, environmentally friendly technologies delivered with excellent service," said Sagar. "While exploring new frontiers, the NeoScope service eliminates the need for chemical sources, avoids complex abandonment procedures, provides comprehensive petrophysical measurements in real-time, and saves rig time through early evaluation to optimize drilling and logging programs."
Schlumberger spent the past four years field-testing the NeoScope service extensively for a wide range of formations and environments from tight carbonates and unconventional reservoirs to conventional clastics to validate the measurement response. Schlumberger's technical experts have also verified the challenging measurement physics through extensive modeling and experimentation both in the lab and in the field.
While the NeoScope service offers a number of advantages, Sagar noted that the slightly lower accuracy than the traditional bulk density, which uses a cesium source. The sourceless measurement also uses a more complex two-step process in which neutrons induce gamma rays that are then detected in the tool.
"The use of high energy neutrons means that the resulting density measurement cannot be focused azimuthally to form an image," Sagar commented.
The fact that the both NeoScope and EcoScope utilize radioactive materials also presents health, safety and environmental (HSE) risks, whether through direct contamination or extended close contact with the human body. Abandoning a chemical source downhole also presents an environmental risk that can last hundreds or thousands of years, Sagar said.
"Government regulations rightly consider that the industrial use of these materials requires stringent control," Sagar commented. "Service companies apply tight controls and comprehensive training programs to ensure strict adherence to procedures that minimize operational risk. These are particularly important in logging while drilling operations due to the severity of the drilling environment and because rig personnel often assemble the tools in the BHA onsite."
Several technologies have been implemented to minimize human exposure to radioactive materials and maximize the ability to safely retrieve sources when tools become stuck downhole.
"Nevertheless, the use of chemical sources inherently poses a risk, and the opportunity to entirely eliminate the need for them provides benefits not only for HSE risk reduction, but also for improving operational efficiency and avoiding restrictions and delays relating to compliance with local legislation," Sagar commented.
The pulsed neutron generator used in both the EcoScope and NeoScope services is an electronically-controlled source of high-energy neutrons. The PNG can be switched on and off, providing significantly safer operations than deploying a chemical nuclear source. It also allows additional formation evaluation measurements such as sigma and the SNGD measurement to be acquired, Sagar noted.
A series of safety interlocks have been implemented, which include password protection, use of a physical key to enable the PNG, intra-BHA tool recognition and power control to ensure power is only supplied to the PNG when mud is circulating through the BHA, and a pressure switch to prevent accidental neutron emission near surface.
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