Compensated Neutron-Gamma Density (CNGD) for Formation Evaluation(New Features and Challenges)

New technologies are applied in petroleum engineering recently. Nowadays some environmental problems and operation limitations come into consideration. New challenges like drill string loosing andprohibition of using chemical resourcesin the ground are weaknesses that petroleum industry faces them. Its procedure is started by Pulsed Neutron Generator (PNG). Fast neutrons penetrate the formation and collide with nuclei. The nuclei rapidly return to the original energy state and emit gammarays, some of which are scattered back to the detectors. The counted rate of detected gamma rays depends on formation electron density.As it mentioned, one of the most important problem is drill string loosing. The first experience of usingNeutron Gamma Density(NGD) measurements is inLogging While Drilling(LWD)techniqueto minimize the impact of stuck pipe and to provide density measurements with greater depth of investigation. The first problem of applying LWD is its single detector and standoff problem.Therefore, it is so essential to reduce standoff. The first try is using caliper but it also adds more uncertainty. The next try is using standoff-compensated density with two detectors at different spaces from PNG. Borehole effects is another problem therefore it is so critical to apply spine-and-ribs algorithm. For a two-detector density tool, the plot of long-spacing versus short-spacing counted rates for different formation densities, mudcake densities and mudcake thicknesses. The plot takes its name from the spine, which is the locus of points with no mudcake, and the ribs, which show the effect of mudcake at certain fixed formation densities. The plot illustrates graphically that for a given formation density there is only one rib for all normal mudcake densities and thicknesses. Thus, although there are three unknowns, it is possible to make a correction using two measurements. The compensated NGD measurements provides standoff-compensated formation bulk density for formulation evaluation, without using a chemical source. This paper provides an overview of the compensated algorithm’s advantages, comparison with traditional Gamma-Gamma Density (GGD) and highlights limitations that are currently being addressed through ongoing development. Numerical simulations will guide us to real results and give us the ability of prediction of reservoirbehavior

Keywords:

pulsed neutron generator (PNG) , Logging While Drilling(LWD) , gamma-gamma density (GGD) , interbedded sandstone