Detecting cement voids or borehole wash=out(s) - by using fast neutron source to irradiate surrounding earth formations

摘要

Borehole washouts or cement voids between a casing and earth formations are located by irradiating the earth formations in the vicinity of the borehole with fast neutrons from a relatively high intensity neutron source. The epithermal neutron population at a first shorter speed distance from the source is detected and then the epithermal neutron population at a second longer spaced distance from the source. Then the detection of the thermal neutron population at the detectors is discriminated. The population measurements are then combined by taking a ratio of the measurements to derive a first, compensated indication of porosity. A second, uncompensated indication of formation porosity is derived from the measurement at the shorter distance alone. The compensated and uncompensated porosity indications are compared by deriving a percentage compensation parameter C to locate the present of borehole washouts or cement voids. Appts. is less sensitive to disturbing environmental parameters in the detection of washouts and cement voids due to its use of the dual detector compensated porosity measurement.