Numerical modeling of influence of horizontal borehole surrounding heterogeneity on the response of the gamma-gamma density tool

The response of the spectrometric gamma-gamma density tool in borehole conditions has been numerically simulated. A transport of gamma rays from a point source situated in a borehole tool through the rock medium to the detectors has been simulated using the MCNP code. An influence of heterogeneity of a rock medium surrounding the borehole on the response of detectors in the probe has been examined. The heterogeneity results from the presence of the interface between two different geological layers parallel to the borehole axis. Such conditions happen in horizontal logging practice when the borehole is drilled along the boundary of geological layer. It is possible to assess the distance from the boundary on the basis of the response of the gamma-gamma density tool, using the classic interpretation procedure '' spine-and-ribs''. We take advantage of the effect of different densities of the bordered layers. When the boundary is far-away the response of the probe fits to the '' spine '' curve and when the boundary is close - the response fits to the '' rib ''. The presented examples show the wide possibilities of numerical modeling of the complex borehole geometry and of applications to solve difficult interpretation problems of the nuclear borehole logging. (authors)