Produced water characterization by prompt gamma-ray neutron activation analysis

Abstract

Multiphase gas/oil/water measurement has been and still is a challenging task within the petroleum industry. The requirements of measurement efficiency and accuracy have been continuously increasing at the same time as the trend is to use subsea installations. As a response, the design of the SOFA (Subsea Online Fluid Analyser) concept was initiated by Christian Michelsen Research in cooperation with the University of Bergen. The final design of the analyser is yet to be completed. The ultimate goal of this design is the development of a permanently installed subsea metering station which is capable of characterizing all of the components that can be found in multiphase flow. One of the key challenges in this design is the detailed characterization of produced water samples which includes the identification of salt ions in the sample and their weight percentages. In this work, use of prompt gamma-ray neutron activation analysis (PGNAA) in conjunction with the so-called Monte Carlo library least-squares (MCLLS) approach is considered for characterization of the produced water component. A relatively simple produced water sample was prepared and the prompt gamma-ray spectrum of the sample was recorded using a large 6" × 6" NaI(Tl) scintillation detector. A specific purpose Monte Carlo code named CEARCPG was used to generate the pertinent elemental libraries that are required in the MCLLS approach. The results of this feasibility study have shown that the PGNAA in conjunction with the MCLLS approach would be feasible for determining the amounts of salt ions that can be found in produced water samples.

On the treatment of ill-conditioned cases in the Monte Carlo library least-squares approach for inverse radiation analyzers

“…The idea of combining a traditional gammaray densitometer with a potential PGNAA analyzer will in addition require the use of two different radioisotopes such as a Cf-252 and a Cs-137 radioisotope, thus, increasing the unwanted radiation dose to the environment. Furthermore, in some applications of PGNAA in conjunction with the MCLLS approach such as characterization of produced water samples [16], the ill-conditioning of the covariance matrix will cause the least-squares solution to produce meaningless results (results with large standard deviations). Therefore, in cases where the MCLLS approach is to be implemented for the quantitative analysis of prompt gamma-ray data, it is believed that a method which can treat these ill-conditioned cases would be of significant importance.…”

“…The treatment being considered in this work is an iterative approach that is based on minimizing the condition number of the covariance matrix in the LLS analysis. A similar approach has been applied in a recent work [16]. However, much of the actual importance and relevance of treating ill-conditioned cases were not sufficiently pronounced.…”

“…The previously described indirect approach for treating the illconditioning in the LLS analysis [16] does not involve removal of libraries or residual analysis during LLS calculations. The indirect approach is solely based on minimizing the condition number of the system of linear equations by searching for appropriate combinations of different libraries prior to performing the LLS calculations.…”

“…The salt phase was assumed to contain only NaCl The theoretical problem geometry used to generate the elemental prompt gamma-ray libraries for the MCLLS approach. All dimensions are given in cm [16].…”

Review Article : Applicable methods of evaluating peak area of gamma ray spectra

“…Consideration has recently been given to the use of the LLS technique for scatter correction in gammaray tomography systems [40] as a result of its comparatively successful application for multiphase hydrocarbon flow measurement [41] and for characterization of sub-sea fluid samples [42] using prompt gamma-ray neutron activation analysis.…”