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

摘要

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 crosssection 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.