Numerical simulation on azimuth gamma ray detection of coal and rock interface

In order to meet the requirements of geosteering drilling with LWD azimuth gamma, the response value of gamma ray intensity under different surrounding rock thicknesses and absorption coefficients was studied by numerical simulation. Meanwhile, the variation of special and physical characteristics reflected by API value measured by gamma detector under the radioactive formation conditions was simulated and analyzed. On this basis, the geological model of the three layers of coal, mudstone and limestone of 8-sectors azimuth gamma ray drilling was established to simulate the identification process of roof and floor of coal bed drilled. The simulation results show that the gamma ray intensity is reduced by half when the absorption coefficient of rock layer is changed from 0.08 to 0.10. Generally, the gamma amplitude could reflect the lithology of the drilled formation, and the upper and lower gamma change order can indicate the location of roof, floor and interface of the coal bed penetrated. In the case of penetration at small angle, the change of physical properties of the 8 m, 6 m and 2 m formation ahead of drilling could be monitored if the distance from detector to bit is 1 m, 3 m and 7 m respectively. The simulation of bedding drilling could provide technical guidance for the geosteering project, so as to improve the drilling efficiency and reduce the invalid footage.