High-energy photon transport modeling for oil-well logging

Abstract

Nuclear oil well logging tools utilizing radioisotope sources of photons are used ubiquitously in oilfields throughout the world. Because of safety and security concerns, there is renewed interest in shifting to electronically-switchable accelerator sources. Investigation of accelerator sources opens up the opportunity to study higher-energy sources. In this thesis, sources with a 10 MeV endpoint are examined, a several-fold increase over traditional techniques. The properties of high-energy photon transport are investigated for potential new or improved well logging measurements. Two obvious processes available with a high-energy photon source are pair production and photo neutron emission. A new measurement of formation density is proposed based on the annihilation radiation produced after the pair production of high-energy source photons in the rock formation. With a detector spacing of 55 cm, this measurement exhibits a sensitivity to density with a dynamic range of 10 across a typical range of formation density (2.0 - 3.0 g/cc), the same as traditional measurements. Increases in depth of investigation for these measurements can substantially improve the sampling of the formation and thus the quality and relevance of the measurement. Being distributed in angle and space throughout the formation, a measurement based on anni-hilation photons exhibits a greater depth of investigation than traditional methods. For a detector spacing of 39 cm (equivalent to a typical spacing for one detector in traditional approaches), this measurement has a depth of investigation of 8.0 cm while the traditional measurement has a depth of investigation of 3.6 cm.

(cont.) For the 55 cm spacing, this depth is increased to 9.4 cm. Concerns remain for how to implement an accelerator source in which energy spectroscopy, essential for identifying an annihilation peak, is possible. Because pair production also depends on formation lithology, the effects of chemical composition on annihilation photon flux are small (<20 %) for the studied geometry. Additionally, lithology measurements based on attenuation at high energies show too small an effect to be likely to produce a useful measurement. Photoneutron production cross sections at this energy are too small to obtain a measurement based on this process.