Formation water salinity is a crucial parameter for reservoir evaluation. Traditionally, sigma logging, resistivity logging and core-sampling method are used for the water salinity determination. Due to the high capture cross section of the elements (such as boron and gadolinium) existing in the borehole or formation, the salinity results from the sigma logging may be unreliable. For the resistivity logging, the salinity results always are affected by the conductive minerals, and there are some limitations when it is working under the cased-hole. Generally, the salinity results derived from coresampling method are the most reliable for geological analysis. However, the sampling method is time consuming and easily affected by the formation heterogeneity. As a result, accurate water salinity measurements always require a joint interpretation of sampling and logging data.
The neutron slowing down or capture ability is depending on the formation cross section. Usually, the lithology, porosity and borehole fluid condition will affect the salinity determination when using a neutron logging method. Here, we developed a neutron induced gamma ray spectroscopy logging method for determining formation water salinity. The measurement system Controllable Neutron Element Tool (CNET) consists of a D-T generator, one gamma detector (LaBr3), and two thermal-neutron detectors (He-3). The LaBr3 detector is covered with a thin boron-containing material for absorbing the thermal neutrons. The timing sequence of the D-T generator is designed according to the energy and time windows for the gamma-ray and thermal-neutron acquisition. Therefore, the energy spectrums of gamma rays in the inelastic and capture stage can be recorded, as well as the time spectrums in the attenuation stage.
As well known, chlorine content is a key index for formation water salinity. The characteristic gamma rays of the chlorine and other formation elements overlap on the capture gamma energy spectrum. By using the weighted least squares method, the capture spectrum with a specific energy range can be resolved to derive the gamma counts related to chlorine. Then, formation porosity can be given by the thermal-neutron count ratio. Combining the gamma counts related to chlorine with thermal neutron counts and porosity, the water salinity is derived. Meanwhile, the formation sigma information from gamma-ray time spectrum can be used as a reference parameter for the water salinity results. In addition, corrections for borehole water salinity and borehole size need to be considered.
This paper introduces a new method and its performance in different formation and borehole conditions. The effects of formation lithology and elements with high capture cross section on the salinity measurement are eliminated. Borehole and formation sigma responses for water salinity can be a good comparison for more believable results. The logging method is applied in two wells with different lithology and borehole fluids. Formation water salinity of different depths is calculated and comparing with sampling information to validate the effectiveness of this logging method.