Neutron Derived Porosity-Influence of Bore Hole Diameter

Abstract

Introduction The neutron-gamma log has been used for stratigraphic correlation by the oilindustry for a number of years. In the past few years, the quantitativeapplication of the log to provide information with respect to porosity hasreceived considerable attention. The basic concept of quantitativeinterpretation has been pointed out in the literature. It shall be the purposeof this paper to develop by empirical means a method whereby the relationshipof porosity to neutron-gamma deflection may be predicted for various bore holediameters. This relationship is applicable to the present commerciallyavailable neutron-gamma log run in open hole. Data available at present areinsufficient to establish similar correlations for cased holes. Through thedevelopment of such a correlation, the importance of the various factors whichinfluence the log become evident. Neutron-Gamma Curve The logging tool consists of a source of neutrons and a means of detectingand measuring gamma rays, the detector being shielded from the direct emissionfrom the source. The gamma rays reaching the detector have three sources oforigin:the natural gamma radiation emitted by the formations,induced gammaradiation resulting from neutron bombardment,gamma rays emitted by thesource and scattered by the formation. It has been found that a neutron derived porosity correlation is best forshale-free formations. Under this condition the natural gamma ray radiation isquite low, and therefore, will be neglected in this work. The induced gamma ray radiation resulting from neutron bombardment ispredominantly a function of the hydrogen atom concentration of the substancepenetrated. Due to the geometry of the presently used system, the neutron-gammacurve indicates low radioactivity opposite high hydrogen concentration and highradioactivity opposite low hydrogen concentration (Figs. 1 and 2). Having acurve which measures hydrogen atom concentration, it follows that if the matrixof the formation penetrated consists only of atoms other than hydrogen, and ifthe fluid filling the pores is either oil or water, the curve should be anindication of porosity (both oil and water have practically the same number ofhydrogen atoms per unit volume). The above criteria for a porosity correlationare fulfilled in clean sandstones and limestones in which the pore volume iscompletely filled with liquids (oil or water), but not in formations containingeither free gas or shale or both. T.P. 3377