Positive Comparison of LWD Data Acquired With and Without Chemical Radioactive Sources in the Vankor field, Russia, Paves the Way for Radioisotope-Free Formation Evaluation

Abstract

Abstract Safety issues, tightening of environmental regulations, and efficiency are among many reasons the petroleum industry has long searched for a way to acquire standard formation evaluation measurements without the use of chemical radioactive sources. Pulsed neutron generator (PNG) technology provides electronically controlled pulses of neutrons. This radioisotope-free source of high-energy neutrons has been in use for decades in wireline-conveyed logging tools for various commonly used measurements such as neutron porosity, spectroscopy, and thermal neutron capture cross section (sigma). For several years, a ruggedized PNG technology that performs under the harsh shock and vibration conditions of an LWD tool operation has allowed these measurements to be made while drilling. Recent developments have extended the suite of measurements to include a radioisotope-free bulk density measurement. In conjunction with the other PNG-related formation measurements, this makes it possible to perform comprehensive formation evaluation without chemical radioactive sources. The sourceless neutron-gamma density (SNGD) measurement is initiated by neutrons, in contrast to the standard gamma-gamma-density (GGD) measurement. The high-energy neutrons from the PNG experience inelastic collisions with the atomic nuclei of atoms surrounding the tool. The gamma rays emitted as the nuclei return to the ground state create a distributed gamma ray source in the formation. These gamma rays then experience Compton scattering in the same way as gamma rays emitted from the chemical radioactive source used for the standard GGD measurement. The count rate of detected gamma rays at a gamma-ray detector at a distance from the PNG is proportional to the electron density of formation, which is then transformed to bulk density in the standard manner. This study compares the new sourceless neutron-gamma density measurement with the traditional source-based bulk density measurement, both of which were acquired in the same well in the Vankor field, Siberia, Russia. Conditions encountered include oil, gas, shale, bad hole, and coal-bearing intervals. A comparison of the final petrophysical analysis derived from the traditional source-based density and the new sourceless SNGD measurement is also made. Our evaluation indicates a positive comparison between density recorded using a chemical radioactive source and neutron-gamma density recorded using a PNG without a chemical radioactive source.