Formation Chlorine Measurements Using Slimmer Pulsed Neutron Devices Enhance Reservoir Monitoring
Authors: Mostefai I. , Saldungaray P. , Ma S. , Al-Nasser M , Xu L. , Rose D. , Ghadiry S. , Abdallah W. 
Published: November 2024

Chlorine (Cl) is present in the formation almost exclusively in water as dissolved salts and it strongly affects properties used in petrophysical interpretations. It affects water conductivity, thermal neutrons cross-capture section (sigma) and also dielectric permittivity, thus the computed water saturation from resistivity, pulsed neutron (PN) sigma, or dielectric dispersion logs. Therefore, uncertainty in the water Cl concentration directly impacts the uncertainty of the petrophysical analysis. A method to measure formation chlorine from nuclear spectroscopy was introduced in the past using a large diameter PN tool; in this paper we will extend the technique to a slim PN device, which can be deployed through tubing with tighter restrictions for reservoir monitoring.

Chlorine has a large capture cross-section, hence is an element easy to measure from the capture spectra of spectroscopy tools. The difficulty is that the Cl signal is coming from both the formation and the borehole in variable proportions depending on the formation properties and borehole environment. To extract a clean formation Cl signal the borehole contribution must be removed. For this purpose, different Cl spectral standards are used, and the extracted yields are combined in proportions dynamically adjusted to account for the environmental effects of borehole size, borehole fluid density and neutron transport migration length. The difference between the formation and borehole Cl spectra is very small, and therefore accurate and precise spectroscopy devices featuring high resolution detectors and high yield PN generators are required to obtain robust results. The challenge is more significant for slim PN devices, because the number of gamma ray counts and measurement precision are proportional to the scintillator crystal volume, in addition to the stronger influence of borehole fluid to the slimmer tools.

For this study several data sets were acquired in barefoot completed wells in shut-in and flowing conditions. The later ensured that any re-invasion by borehole fluids was minimized to obtain more representative results of the formation. The formation Cl concentration was combined with the water volume derived from PN carbon/oxygen ratio analysis to compute the water salinity and obtain a formation salinity profile along the reservoir. This is particularly useful in fields subjected to water injection to understand better the reservoir sweep. The computed salinity also enables the possibility of using salinity dependent workflows to estimate saturation, e.g. with sigma or resistivity logs, independently or combined applying a multi-physics approach. In one "validation well" the Cl measured with the slimmer PN tool was compared to a larger openhole proven device, both logged with drilling mud in the borehole in consecutive runs, achieving good match. As an additional quality control, the measured formation sigma was compared to a reconstructed sigma from formation Cl, porosity and matrix sigma from mineralogy.

The technique developed in this study will extend applicability of Cl logging for reservoir monitoring purposes in wells with minimum restriction able to accommodate 1-11/6" OD tools.