Determination of Coal Ash Content by Neutron-Neutron Logging

Abstract The research is aimed at assessing the possibility of studying the ash content of coal seams using neutron-neutron logging in coal exploration wells drilled at the outcrops of coal seams under loose deposits using the materials of experimental work in the South Yakutsk coal basin. The prospects of using neutron-neutron logging to study coal well sections, on the one hand, is determined by the fact that hydrogen content in coal seams normally exceeds its content in the host rocks, and, on the other, by the small cross-sections of neutron capture by carbon. Within the same coal grade, an increase in its ash content both means a decrease in its hydrogen content and an increase in the content of elements with a higher capture cross section. Experimental studies were carried out at the Syllakh coal deposit. An IBN-8-1 type plutonium-beryllium source with an output of 5*104 neutrons/s was used as a fast neutron source, and a highly efficient SNM-17 type helium gas-discharge counter was used as a slow neutron detector. The logging depth provided by the equipment was 10 - 30 cm. During the processing of the obtained field data, a correlation was established between the count rate of neutron-neutron logging and coal ash content. It should be noted that the functional dependence of the count rate of neutron-neutron logging on ash content is not continuous – in the range of ash content of 45 - 55% a certain discontinuity point is observed, after which the functional dependence changes. To simplify the assessment of the dependence, the range of rock ash content above 45% is neglected since bituminous coals with an ash content of more than 45% are classified as carbonaceous rock and are not of industrial interest. A close correlation is established between the count rate of neutron-neutron logging and coal ash content. According to the results of statistical processing, the correlation coefficient is 0.97, which makes it possible to quantitatively determine the ash content according to neutron-neutron logging data. The absolute errors in ash content determination by neutron-neutron logging over the entire dataset are up to 3.625 %. The degree of analytical moisture influence on the data of neutron-neutron logging in determining coal ash content is estimated. No regular changes in the count rate of neutron-neutron logging due to a change in the analytical moisture index have been established. It is noteworthy that, with a sufficiently large dispersion value of the analytical moisture index, the trend line of this parameter regularly changes synchronously with the trend lines of ash content and count rate.