Investigation of Prompt Gamma-Ray Neutron Activation Analysis for Determining Phase Amounts in Multiphase Flow

A. M. Mutiso, G. A. Johansen, and R. P. Gardner

Center for Engineering Applications of Radioisotopes (CEAR), Nuclear Engineering Department, North Carolina State University, Raleigh, NC, 27695, Email: gardner@ncsu.edu

ABSTRACT

Prompt Gamma-Ray Neutron Activation Analysis (PGNAA) is considered for the measurement of the in situ multiphase flow amounts of oil, gas, and sea water in a deep sea oil well. PGNAA has the advantages for this application that: (1) useful characteristic prompt gamma rays are produced by neutron interactions with almost all elements, (2) it is a rapid non-destructive measurement method, (3) a large sample volume is measured, and (4) it can be used under the relatively extreme conditions present for under sea oil recovery. Feasibility calculations have been made with the previously developed Monte Carlo – Library Least-Squares (MCLLS) measurement approach used with the specific purpose Monte Carlo code named CEARCPG that was previously developed at CEAR for PGNAA bulk material analysis.

A slight modification of the MCLLS measurement approach previously developed for the non-linear PGNAA and Energy Dispersive X-Ray Analysis (EDXRF) measurement applications is used for the present application. This approach allows the use of the very accurate forward Monte Carlo calculation of the PGNAA response. The modification is the use of the three components oil, gas, and sea water as library spectra rather than the normal use of individual elemental libraries. This approach allows one to determine the three parameters of primary interest directly. The experimental arrangement considered is a Cf-252 neutron source and a large NaI detector placed on opposite sides of a right circular cylindrical sample holder for an assumed homogeneous mixture of oil, gas, and water. An alternative approach consists of adding a Cs-137 source to also use gamma-ray densitometry for better sensitivity to the gas phase.

In this work, nine elements that yield reasonable PGNAA responses that are included in oil, water and gas are simulated and quantitative spectrum analysis is performed by using the MCLLS approach to measure the three phase amounts in hypothesized realistic samples. A background that was previously obtained experimentally in bulk analysis applications was added to the response here to make the calculations more reasonable. Results indicate that this approach would be accurate and is feasible.

Keywords Prompt Gamma-Ray Neutron Activation Analysis, Multiphase Flow