Traditional proppant placement evaluation in hydraulically induced fractures uses detection of radioactive tracers such as iridium-192, scandium-46 and antimony-124, which are manufactured in nuclear reactors and then shipped to the wellsite and pumped downhole with the frac slurry. Although this technique has proven useful, it involves environmental, safety, and regulatory concerns/ issues. Recently, a new technology has become available that offers a viable alternative to radioactive tracers and also eliminates these concerns. The new technology uses a nonradioactive ceramic proppant that contains a high thermal-neutron-capture compound (HTNCC). This HTNCC is inseparably incorporated into each ceramic proppant grain during manufacturing in sufficiently low concentration so as not to affect proppant properties. The nonradioactive tracer proppant (NRT) taggant is detected using standard pulsed-neutron-capture tools (PNC) or compensated-neutron tools (CNT), with detection based on the high thermal-neutron absorptive properties and/ or capture-gamma-ray spectral properties of the tagged proppant relative to other downhole constituents. The presence of proppant is indicated from:
decreases in after-frac PNC and/or CNT detector count rates relative to corresponding before-frac count rates,
increases in PNC formation and borehole component capture cross-sections (Σfm and Σ bh), and/or
increases in the PNC derived elemental yield of the neutron-absorbing tag material, computed from the observed capture-gammaray energy spectra. In some applications, enhancements to these methods have also been developed to eliminate the requirement for the before-frac log. Monte Carlo modeling data and field examples presented in this paper demonstrate the viability of both the PNC and CNT proppant detection technologies.
