Application of Boron(Gadolinium) Neutron Lifetime Logging in Oil Field Development
Authors: Wang Wu , Wan Xinde , Chen Huijun , Zhang Yunxiang , Wang Junliang 
Published: December 2007

Boron (Gadolinium) Neutron Lifetime Logging is a new logging method to detect and determine residual oil distribution of sandstone reservoirs at the stages of high and super-high water cut. For most oilfields in China, because formation water salinities are generally lower than 20000ppm, the conventional neutron lifetime logging prevailing in high salinity of formation water will not be effective. However, the use of boron or gadolinium as indicator to carry out neutron lifetime logging has been proved to be very effective in these oilfields. By using this method, a correct water plugging operation was then done for six production wells, which were at high water cut stage in a certain development zone of Daqing Oilfield. The results showed that, for each well, the average oil production rate could be increased by 6.2 t per day, and the average water production rate could be decreased by 70.6 cubic meters per day, the ratio of net income to total investment was expected to be 5:1. Both the repeated logging data and the comparison with other logging data, such as fluid entry profile, temperature and noise data etc., demonstrated that this method presented in the paper is reliable and feasible, regardless of the variations of lithology, formation water salinity and casing size.

Introduction

Residual oil saturation is an important parameter for oilfield development. At the later development stage of high water cut (the combined water cut is more than 80%), oil and water distribution is very complex, water flooded and unflooded areas are jigsaw-like, the strength of water drive is great disparity in such multi-zonal oilfields as Daqing. The establishments of the measures of tapping potentials, the schemes of well pattern infilling and tertiary oil recovery all require to accurately, at least basically accurately realize the amount and distribution of residual oil. The distribution research of residual oil becomes the focus and difficulty in oilfield development.

At present, the methods for determining residual oil saturation are much more, such as sealing core drilling, single well tracer, well logging, material balance method and so on. By comparison, geophisical well logging has maximum potentialities, since it can continuously measure the whole interval at any time according to the actual requirement, and it is benefit as well as convenient. Neutron lifetime logging is one of pulsed neutron logging, it is also one of the main methods for determining residual oil saturation and generally used in high water salinity and high porosity formation(the total salinity of formation water is more than 150000ppm, the porosity is over 15%). Since neutron lifetime logging data are effected by various factors, the number of unknown variables are increased in interpretation equations and the accuracy of traditional neutron lifetime quantity interpretation is effected. Therefore, logging-injection-logging technology was put forward and applied to eliminate the difficulties in various parameters determination and effects resulted from the uncertainty of multiple unknown variables.

Since most oilfields in China belong to low water salinity oilfields, the total salinity of formation water is generally below 20000ppm, the application of neutron lifetime logging is restricted. Some oilfields have attempted to increase the salinity of injected water with NaCl and make logging-injection-logging experiments in low water salinity formation, otherwise, since it was difficult to make up and the work load was heavy, the deviation of measured capture cross-section was not big enough, the reliability of data and economic benefit were not good, it was unable to popularize. Neutron lifetime logging with boron or Gadolinium compound solution, the interpretation method and the results of field application are presented in this paper. Proved by practice, the data interpretation results are accurate and reliable and widely applicable. The method has a widely popularizing prospect.