A METHOD AND APPARATUS FOR MEASURING WELL CHARACTERISTICS AND FORMATION PROPERTIES

摘要

The present invention provides for method and an apparatus for determining the porosity of a geological formation surrounding a cased well. The method further comprises of generating neutron pulses that release neutrons into an area adjace nt the well, from a neutron source. Neutrons are sensed and a plurality of neutron dete ctor count rates is acquired using at least two neutron source to neutron detector sp acings. A timing measurement is acquired at one of the spacings to measure a first depth o f investigation. A ratio of the neutron detector count rates is acquired to measur e a second depth of investigation. An apparent porosity is calculated using the timing meas urements and the ratios of neutron count rates. The effect of a well casing on the calcul ated apparent porosity is determined in response to at least one of the ratio of neut ron detector count rates and the timing measurement. A cement annulus is computed based on th e ratios of neutron count rates and the timing measurement. A formation porosity i s calculated by performing a correction to the apparent porosity for the casing an d the cement annulus.