Abstract:

Air drilling technology, an underbalanced drilling method, affects the neutron porosity logging. The neutron porosity calculation model in open hole has been established, and the neutron transport processes were simulated with Monte Carlo method under the condition of different porosity, lithology, shale content, hole size and air pressure.After studying the ratio of thermal neutron count and porosity, it could be concluded that the thermal neutron count increase at first, then after a point, decrease with the increase of porosity. The count ratio from limestone is higher than that of sandstone; and the higher the shale content, the larger the ratio. Moreover, the ratio is influenced by hole size; it is especially significant for large porosity formation; the bigger the hole size is, the smaller the ratio is; thus makes the determination of neutron porosity more difficult. However, the gas pressure has little effect over the ratio of thermal neutron. As a result, in air drilling, neutron porosity should be re-calibrated and correction should be done according to lithology, shale content and borehole size.

Key words: air drilling, neutron porosity, Monte Carlo method