Time-spectrum processing technology of neutron lifetime logging based on regularized non-negative least squares method

Abstract

Neutron lifetime logging can measure the thermal-neutron capture cross-section of the formation using the neutron/gamma time spectrum information, and has a wide application in dynamic monitoring and recovery of remaining oil. However, the current time-spectrum processing technology is easily interfered by the borehole information and counting statistics in the neutron/gamma time spectrum, especially in a complex logging environment. To ensure the accuracy of neutron lifetime logging, a new time-spectrum processing technology based on regularized non-negative least squares (RNNLS) method was proposed. By presetting an array composed of different capture cross-section values, a multi-exponential decay function was established to describe the decay law of the time spectrum. Taking the multi-exponential decay function as a reference, an overdetermined linear equation system was constructed using time spectrum data. Finally, using the regularized non-negative least squares (RNNLS) method to solve the equation system, the contribution distribution of different cross-section values in the preset array to the time spectrum was obtained to indicate the capture cross-section of the formation. Studies show that, the new technology can automatically separate the cross-section contribution of the wellbore and formation to the time spectrum, and the cross-section contribution distribution has an obvious contribution peak which can be used for evaluating the real formation cross-section. In addition, by adopting the RNNLS method, this new method successfully gets rid of the influence of noise information and counting statistics on the capture cross-section results. Compared with the original technology, the new time-spectrum processing technology is no longer limited by the selection of time gates, and shows more accurate results in various logging environments.