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Pulsed Neutron and Formation Evaluation in the Cased Hole Environment
ABSTRACT
As the oil and gas industry continues to mature, wells are drilled in increasingly challenging environments, often putting at risk the formation evaluation program in an open hole environment. Even if the well is accessible, cost and risk of a complete multi-run open hole formation evaluation program can be prohibitive, however sufficient data is still required to identify hydrocarbons in these complex reservoirs. These and other considerations push operators more than ever to find accurate and complete formation evaluation in a rigless environment, after the completion has been run. Logging runs through the completion reduce both cost and risk, where downhole tools are better protected and efficient conveyance options such as slickline is feasible with memory-capable tools. Cased hole logging data have historically been limited as compared to the vast data that can be acquired in an open hole environment. Additionally, the range of conditions encountered in cased hole environment, and the complexity of the completions will challenge even robust environmental corrections models, putting accuracy at risk. Completions with smaller tubing force the evaluation tools further away from the formation being evaluated and limit the OD of the tools being conveyed through the completion. The statistical nature of spectral nuclear technologies are particularly heavily affected by these constraints.
Pulsed neutron formation evaluation has historically been successfully employed for water, oil, and gas saturations, whether for initial input into a petrophysical model or as part of a time-lapse well monitoring program. Advancements in electronics and scintillator materials that take advantage of high-yield neutron generators work together to improve the accuracy of the raw output. Continued improvements in MCNP nuclear modeling software coupled with a large variety of characterization lab conditions have improved validation, stretching the limits for the specified operating environment. Individual elemental yields have been fully characterized, both with the neutron generator active and from passive mode with the generator off. Processing software has been developed concurrently that incorporates the traditional parameters with several additional parameters such as cement density.
New multi-detector pulsed neutron tools can be conveyed in a wide range of borehole sizes, completion configurations, and borehole fluid types with a more accurate response across this array of challenging conditions. A comprehensive range of characterized elemental yields combined with spectral natural gamma measurements from the same tool enable valuable minerology data to be acquired thru-tubing. New tools are fully memory capable, where the full data including all energy spectra can be acquired via slickline conveyance. The inclusion of a direct measurement of total carbon yield from the inelastic spectrum lend to accurate physics-based TOC output that can aid in evaluating unconventional reservoir.
