ABSTRACT

Cased oil saturation is an essential parameter for middle and late oilfield development, which can provide the basis for the development program design. Neutron-gamma logging has been widely used in the determination of cased oil saturation. Recently, Carbon/Oxygen (C/O) logging and pulsed neutron capture (PNC) logging extract carbon-oxygen ratio (C/O) and capture cross-section (Sigma) from the measured energy and time spectra to evaluate the saturation. However, the spectral quality seriously affects the performances of the two classical methods and requires complicated spectral processing and environmental correction. Therefore, a novel cased oil saturation determination method is proposed based on the neutron-induced gamma-ray and thermal neutron counts. The proposed method does not require additional spectral processing and has higher statistical accuracy. First, the relationship between the detected capture gamma rays and the thermal neutron counts was described in theory to illustrate the feasibility of using the gamma-thermal neutron response to evaluate the cased oil saturation. Then, the test pit experiments were performed to collect the capture gamma spectra and calibrate the Monte Carlo simulation model to ensure the simulation results could reflect the real response of the instrument. After the benchmark, a new practical cased oil saturation evaluation parameter was introduced by the counts of the dual thermal neutron detectors and a single scintillator detector. Furthermore, Monte Carlo simulated case studies were conducted to analyze the response characteristics of the proposed evaluation parameter under different environmental conditions. The cased oil saturation can be calculated from the envelope established by the evaluation parameter in response to the oil and water layer. Finally, the field examples were presented to validate the effectiveness and applicability of the new method.

INTRODUCTION

Cased oil saturation is significant to oilfield development, especially later, which can guide potential layer exploration and development program optimization (Song, et al., 2022; Chen, et al., 2019; Liu, et al., 2018). Due to the limitation of completion type, wellbore fluids, casing and cement sheath materials, neutron-gamma logging as a powerful method to evaluate the cased oil saturation has been used in all kinds of conventional or unconventional reservoirs, because the fast neutrons emitted by the instrument can easily penetrate the casing into the formation and react with matter (Duci, et al., 2019; Guo, et al., 2010; Padilha, et al., 2002; Simpson, et al., 2001). The neutron-gamma logging can reflect the cased oil saturation by monitoring the secondary gamma-ray energy distribution (energy spectrum) or decay rate (time spectrum) induced by the inelastic scattering or radiative capture reaction, known as Carbon/Oxygen (C/O) logging and pulsed neutron capture (PNC) logging (Gilchrist, et al., 1999; Jacobson, et al., 1998).

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