Development and Field Testing of a Novel Technology for Evaluating Gravel Packs and Fracture Packs

摘要

Many completions require some sort of pack to prevent fines migration associated with the high production rates necessary for economic recovery. Traditional evaluation of these pack completions has generally been accomplished using a combination of pressure analysis, material balance calculations, and basic logging information, often including the use of radioactive tracers. Radioactive tracers introduce significant hazards relating to health, safety, and the environment, and therefore are under strict regulations. This paper presents a new alternative pack-evaluation technology which eliminates these radioactivity-related issues. The new technique utilizes a recently introduced non-radioactive tracer containing a taggant material with a high thermal neutron capture cross-section. This tagged proppant can also be used as, or mixed with, conventional gravel or frac packing materials prior to downhole placement. The non-radioactive taggant is detected using standard pulsed neutron capture (PNC) logging tools, with detection based on the high thermal neutron absorptive properties and/or capture gamma ray spectral properties of the tagged pack material relative to other downhole constituents. The tagged pack material is indicated from: (1) changes in after-pack PNC detector count rates relative to corresponding before-pack count rates, (2) increases in PNC formation and borehole component capture cross-sections (Σfm and Σbh), and/or (3) increases in the computed elemental yield of the neutron-absorbing tag material, derived from the observed PNC capture gamma ray energy spectra. This technology has been successfully employed in induced fracturing operations in over 200 wells to determine fracture height, and in many situations also indicates relative fracture width. By further optimizing the concentration of the tag material in the proppant and the time windows utilized for PNC data processing for pack applications using Monte Carlo software (MCNP5), the resulting non-radioactive pack tracer (NRPT) technique can now not only evaluate gravel packs, but also fracture height behind the casing/gravel pack at the same time. Moreover, enhancements to this technique have also been developed to eliminate the before-pack log in some situations. As a result, these recent developments significantly simplify and shorten the logging procedure, and therefore reduce operational costs. This paper begins with a brief review of prior published MCNP5 modeling data on gravel pack and frac- pack evaluation, and then discusses recent additional modeling data utilized in NRPT taggant concentration optimization, utilizing the borehole geometry of the well in the field log example in the paper. The effectiveness of the new NRPT technology is then demonstrated with the field example, which has only after-pack logs. In the field log evaluation, the natural gamma ray log, silicon activation log, borehole sigma log, formation sigma log, and gadolinium (the NRPT taggant) yield log are all analyzed. The most suitable logs and log combinations for evaluating gravel pack and fracture height were identified based on the comprehensive analysis, and quantitative evaluations for gravel pack and fracture height were obtained. This new technique is especially useful in evaluating onshore and offshore pack completions where the issues and hazards associated with the use of radioactive tracers can be significant and in situations where periodic monitoring of the condition of the GP is important. Time monitoring is impossible with radioactive tracers due to the short half-lives of the tracers being used.