Controlled source neutron logging method and instrument while drilling

Abstract

The invention discloses a controllable source neutron logging method and an instrument while drilling. The method comprises: starting a pulsed neutron source on a drill collar to emit quantitative high-energy neutrons to the formation in a pulsed manner; The long source distance detector and the short source distance detector respectively measure the number of thermal neutrons scattered back to the wellbore after being decelerated by the formation; according to the number of thermal neutrons measured by the two detectors, the thermal neutron macroscopic capture cross section of the formation is obtained Σ and porosity φ; the formation water saturation Sw is obtained according to the thermal neutron macro-capture cross section and porosity φ. The instrument includes: a pulsed neutron source, a detection unit, a signal processing and data analysis unit and a power supply module, which are used to provide power for each of the above units. The present invention utilizes long and short source distance 3 The He thermal neutron detector detects thermal neutrons, obtains two sets of parameters of porosity and thermal neutron macroscopic capture cross-section, and realizes the measurement of water saturation and porosity in formations with relatively low salinity. The use of pulsed neutron sources eliminates the hazards of radioactive sources to humans and the environment.

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 and deuterium controllable compensation neutron logging tool while drilling", a controllable source neutron porosity logging tool 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 a controllable source while drilling. The neutron ray energy is 2Mev, and the detection depth is 23.75cm. The disadvantages are D-D neutron tube, in which the neutron yield of the tube is low, it is easy to cause radioactive statistical fluctuation errors, and the measurement accuracy is low

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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