Schlumberger Technology Corporation

Ralph M Tapphorn

Jeffrey S Schweitzer

In the illustrative embodiments of the invention disclosed, the lithology and salinity of an earth formation traversed by a well bore are investigated by irradiating the formation with a neutron source and generating an energy spectrum of the activation gamma rays resulting therefrom. From the spectrum thus obtained, the level of the activation gamma radiation emitted by .sup.24 Na and that emitted by .sup.38 Cl are determined. The two intensity measurements are then combined, e.g., by forming a cross-plot or ratio thereof, to provide an indication of the lithology and salinity of the earth formation.

Salinity and lithology determination from the sodium and chlorine

In the specific embodiments of the invention disclosed, a semiconductor radiation detector located in a logging tool sized for passage through a borehole is maintained at cryogenic temperatures by a cryostat housed in the tool. The cryostat includes a cryogen chamber containing a melting-solid type cryogen. Thermal insulation in the form of radiation heat shields and a vacuum chamber surround the cryogen chamber. The integrity of the vacuum insulation is maintained by an active vacuum pump. The pump may be mounted either externally of the cryostat, such that its operation is independent of temperature conditions within the cryostat, or internally thereof, so as to be cooled by the cryogen. Alternatively, a second cryostat may be provided to cool the pump.

Cryostat for borehole sonde employing semiconductor detector

In the illustrative embodiments of the invention disclosed, the lithology of an earth formation traversed by a well bore is investigated by irradiating the formation with a neutron source and generating an energy spectrum of the activation gamma rays resulting therefrom. From the spectrum thus obtained, the levels of the activation gamma radiation emitted by .sup.27 Mg and that emitted by .sup.49 Ca are determined. The two intensity measurements are then combined, e.g. by forming a cross-plot or ratio thereof, to provide an indication of the lithology of the earth formation and, in particular, of the extent of dolomitization of the formation.

Lithology determination from the calcium and magnesium activation lines

A method of determining the flood front profile created during the production flooding of an oil-bearing formation utilizes cased observation boreholes located between the injection wells and the producing wells. The time and depth of arrival of the flood front at an observation borehole are detected by gamma ray spectroscopy examination of the formation. Tracer elements having characteristic gamma ray emission energies are employed to facilitate detection of the flood front and its direction of travel. The tracer elements may be naturally radioactive substances or they may be normally stable elements which are rendered radioactive by neutronbombardment. Elements having interfering spectral lines may be separated on the basis of half-life measurements, selective detection periods or the response of the elements to different energy neutrons. By repeating the detection process at different depths and times, the profile of the flood front as it approaches the producing wells may be developed. This information may be used to control the flooding operating to prevent or localize premature breakthrough to the producing wells.

Growing community of inventors

Boulder, CO, United States of America

Ralph M Tapphorn