Traditional proppant placement evaluation in hydraulically induced fractures utilizes detection of radioactive tracers pumped downhole with the "frac" slurry. Although this technique has proven useful, it involves environmental, safety, and regulatory concerns/issues. The fracture height determination method described in this paper eliminates downhole placement of radioactive materials. A high thermal neutron capture compound (HTNCC) is inseparably incorporated into each ceramic proppant (CEP) grain during manufacturing in sufficiently low concentration that it does not affect mechanical strength, conductivity, durability, or density of the particles. The proppant is detected using standard compensated neutron logging tools (CNT) and/or pulsed neutron capture (PNC) logging tools, with detection based on the high thermal neutron absorptive properties of the compound relative to downhole constituents. Since the HTNCC is placed permanently in the proppant, logging for proppant detection can occur at any time(s) after the frac job, with no requirement for special handling or mixing considerations. To clarify, the new detectable proppant is not radioactive, and the benign detectable material is uniformly mixed into the ore during proppant manufacturing, so there is no chance for segregation of the detectable agent from the proppant.
Specifically, the proppant is detected using after-frac CNT logs, sometimes combined with corresponding before-frac logs. The increased thermal neutron absorption by the HTNCC reduces count rates in the near and far detectors, with approximately the same percentage reduction observed in each detector, leaving the near to far detector count rate ratio (N/F ratio) unchanged. One detection method utilizes a comparison of before-frac log count rates and after-frac count rates, with reduced after-frac count rates observed in zones containing proppant. Another detection method, especially useful if formation gas saturations change, requires only the after-frac log. Since the N/F ratio is essentially unaffected by proppant, after-frac count rates predicted from the ratio will also be unaffected. These synthetic count rates will be greater than the observed after-frac count rates in intervals containing proppant. When PNC tools are utilized in a third method, tagged proppant can be identified from count rate suppression in the detectors and also from increases in the measured formation and/or borehole component capture cross sections. In tools utilizing spectral gamma ray detectors, the HNCC may also be detected using capture gamma ray spectral deconvolution.
Monte Carlo modeling data are presented demonstrating the utility of these techniques employing CNT tools. Field examples illustrating proppant detection using compensated neutron tools are also presented.