Method and instrument for controllable source neutron logging during drilling

Abstract

The invention discloses a method and instrument for controllable source neutron logging during drilling. The method comprises the steps that a pulse neutron source on a drill collar is started and quantitative high-energy neutrons are emitted to a stratum in a pulse mode; the number of thermal neutrons with the speed reduced by the stratum is measured through a long spacing detector and a short spacing detector which are arranged on the drill collar respectively, wherein the thermal neutrons are scattered back to a well hole; according to the number, measured through the two spacing detectors, of the thermal neutrons, the macroscopic capture cross section sigma of the thermal neutrons and the porosity phi of the stratum are obtained; the stratum water saturation Sw is obtained according to the macroscopic capture cross section of the thermal neutrons and the porosity phi. The instrument comprises the pulse neutron source, a detection unit, a signal processing and data analyzing unit and a power module used for supplying power for the detection unit and the signal processing and data analyzing unit. The thermal neutrons are detected through the long spacing <3>He thermal neutron detector and the short spacing <3>He thermal neutron detector, two sets of parameters including the porosity and the macroscopic capture cross section of the thermal neutrons and are obtained, and therefore the measurement of the water saturation and the porosity of the stratum with a relatively low degree of mineralization is achieved. Due to the use of the pulse neutron source, damage, caused by a radioactive source, to human and the environment is eliminated.

Description

technical field [0001] The invention relates to the field of logging-while-drilling methods for geo-steering while drilling in petroleum geological exploration, in particular to a method and instrument for logging with controllable source neutrons while drilling. Background technique [0002] Logging while drilling is a technique in which logging tools installed in drill collars are used to measure formation properties during drilling and transmit the measurement results to the surface or record them in downhole memory. This technology requires that the logging instrument can be installed in a small space in the drill collar, and can withstand high temperature, high pressure and strong vibration during drilling. LWD data is measured before the formation is invaded by mud filtrate or when the invasion is very shallow, and it reflects the characteristics of the formation more objectively and truly than wireline logging. Logging while drilling can determine the wellbore trajec...

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Problems solved by technology

[0004] Chinese patent document CN 2900786 discloses a "deuterium-deuterium controllable compensation neutron logging tool while drilling", a neutron porosity logging tool with a controllable source while drilling without a chemical source. This patent uses a controllable source for the first time In obtaining the formation porosity parameters, it uses D-D neutron tube as the controllable source part of the neutron porosity logging tool with the controllable source while drilling, the neutron ray energy is 2Mev, and the detection depth is 23.75cm. It is a D-D neutron tube, and the neutron yield of the neutron tube is low, which is easy to cause radioactive statistical fluctuation errors and low measurement accuracy

The measurement method of the thermal neutron capture cross section is the same as that of the traditional neutron lifetime logging tool. The disadvantage of this method is that it is affected by the background counts of various gamma rays in nature, and it is not suitable for relatively low salinity. low strata

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