Análise da aplicação do método de perfilagem gama-gama na estimativa da espessura e densidade de camadas de carvão: um estudo de caso

Abstract

In coal deposits where the rock is friable, it is not always possible to obtain core recovery samples, the main direct method for collecting data about the thickness, density and characteristics of coal and waste layers (hard data). When the core recovery is not feasible, indirect methods (soft data) are adopted in order to know the attributes of the coal seams. This study aims to compare the density and thickness data of coal layers acquired through the gamma-gamma logging method, using the Long-Spaced Density (LSD) and High-Resolution Density (HRD) sensors, with those obtained through core recovery data analysis, to assess the accuracy of the gammagamma geophysical method and determine if one sensor exhibits greater accuracy when compared to the other. The focus on analyzing the density and thickness of coal seams is because these are two limits that influence the decision of whether a layer will be mined or not: (1) the lower the coal seam density (equal to or less than 2.0 g/cm³) consequently, the higher the calorific power and, the higher the economic value associated; (2) the thickness of the layers, must be equal to or greater than 0.4 meters, for potential economic viability, accordingly to industry standards. Data acquisition was conducted in four drillholes in the coal area known as B3, located in the Butiá city, 86 km away from the capital of Rio Grande do Sul, Porto Alegre, Brazil. In terms of the geology of the area, the analyzed coal packages belong to the Rio Bonito Formation, a unit of the Guatá Group in the Paraná Basin. The limits established for the layers to be analyzed via geophysical methods, were that cited above and commonly used in evaluating coal deposits. After analysis, the data obtained through geophysical methods was compared with geological data using the WellCad™ software, uni and bivariate statistics. Additionally, a comparison was made between geological data and data obtained via gamma-gamma logging using two different sensors, LSD and HRD. After the analyses conducted, it was concluded that the gamma-gamma method, using both sensors, is effective in identifying coal seams, their thicknesses, and densities. Furthermore, it is concluded that the gamma-gamma method is a valuable tool in data acquisition, providing comprehensive information about a coal deposit, and can be used in locations where core recovery drilling is not possible.