Sourceless formation evaluation decreases HSE risks

Roger Griffiths
Petrophysics Domain Head, Drilling & Measurements, Schlumberger

The use of chemical sources in logging-while-drilling (LWD) operations poses health, safety, and environmental (HSE) risks that include direct contamination or extended close contact with the human body. Abandoning a chemical source downhole also presents an environmental risk that can have long-term effects, lasting thousands of years.

Eliminating the need for chemical sources improves operational efficiency in challenging onshore and offshore environments, where operational guidelines have become increasingly stringent and compliance more time-consuming to ensure that the risks faced by personnel and the environment, particularly in extreme or remote conditions, are kept to a minimum.

Sourceless formation evaluation has been on the industry's wish list since radioisotopic chemical nuclear sources were first used to acquire neutron and density measurements in 1942 and 1959, respectively. The industry accepted the HSE risks associated with the transportation, deployment, and storage of these sources as there was no other means to determine formation porosity and fluid content as reliably as comparing the density and neutron porosity measurements that the nuclear sources facilitated. These became so fundamental to formation evaluation that the density and neutron measurements, together with formation resistivity, became known as the industry-standard "triple-combo" – the minimum suite of measurements required to evaluate porosity, water saturation, and hence hydrocarbons in place.

The introduction of nuclear magnetic resonance (NMR) in the 1960s showed much promise as a sourceless means to evaluate porosity. The NMR measurement responds exclusively to hydrogen in the pore fluids. However, experience has shown that attempting full formation evaluation with only NMR hydrogen index is fraught with difficulties, including under-polarization and unknown fluid characteristics. In addition, very short or long decay rates that fall outside the measurement envelope can easily result in inaccurate evaluation. The elegance of density-neutron interpretation is that the measurements cross-validate each other such that measurement inconsistencies can be readily recognized, keeping the risk of inaccurate evaluation and wasted operations to a minimum.

Today, density and neutron measurements are used in the vast majority of formation evaluation workflows. Consequently, significant effort has been invested in developing techniques to acquire these fundamental measurements without the use of radioisotopic sources. The most promising approaches use electronically controlled particle accelerators to generate the required nuclear species on demand. When the accelerator is off, no nuclear particles are emitted, so tools containing this technology can be treated in the same way as other non-nuclear tools. Not only does this simplify transport and handling of tools and auxiliary equipment, it improves operational efficiency because sources do not need to be loaded and unloaded. Should the tool become stuck downhole, in most jurisdictions fishing requirements, abandonment procedures, and side tracking operations are greatly simplified.

Electronically controlled pulsed neutron generator (PNG) technology was introduced to the industry in the 1980s. In addition to providing neutrons for the traditional neutron porosity measurement, the ability to create pulses of neutrons by turning the PNG on and off enables additional measurements to be derived. Neutron capture spectroscopy and thermal neutron capture cross-section – more commonly known as sigma – require a pulse of neutrons followed by a "quiet" measurement period. Spectroscopy provides valuable information about the elemental composition and mineralogy of the formation. Sigma provides information about porosity, lithology, and fluids. Both provide complementary information to the density and neutron measurements for sophisticated formation evaluation.

While both wireline and LWD technologies have been providing neutron measurements without the use of a chemical neutron source for many years, the means to acquire a density measurement without the use of a chemical source had remained elusive.

The first quantitative sourceless neutron-gamma density (SNGD) measurement was made commercially available in 2012. The SNGD measurement uses high-energy neutrons emitted from a PNG to induce gamma rays in the formation. An array of detectors configured near the PNG characterizes the neutron distribution, induced gamma source, and the subsequent gamma ray scattering which provides formation density information.

The introduction of the SNGD measurement has finally completed the quest for the sourceless triple-combo suite. The NeoScope sourceless formation evaluation-while-drilling service is currently only available on LWD and for 8 ½-in. (216-mm) diameter holes, but the tool will be made available in other hole sizes. Wireline-conveyed sourceless density will also become available, completing the wireline sourceless triple-combo offering. Together these technologies will provide a full range of options to perform established interpretation workflows using data from state-of-the-art, environmentally responsible technologies.

The industry is just beginning to benefit from the enhanced formation characterization, reduced HSE risk, and improved operational efficiency that sourceless formation evaluation technology provides.

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