Effect Of Oil On The Cnl

It has been assumed that in porous rocks the CNL and similar tools respond primarily to the hydrogen density of the formation and secondarily to other nuclei present. According to this model, since the hydrogen density of water and most crude oils are close in value, the neutron tool should give nearly the same response with either oil or water in the pores of the same rock. Experimental measurements which have been made in laboratory formations support the view, however, that the tool response to oil in the pores can be different than when the pores are water filled. These measurements have been made with both thermal and epithermal detection. For both systems, the data show that the difference between a 35% porosity sand formation saturated 100% with water and 100% with crude oil is 4–5 p. u. Similar data were obtained with a highly refined oil. Using mathematical models of the CNL tool, calculations of the magnitude of this effect have been done to corroborate the experiments and to determine the circumstances under which oil appears different from water on the neutron log. Details of these results are given as well as two physical interpretations.