Schlumberger Technology Corporation

Urmi DasGupta

Christian Stoller

Peter D Wraight

Nihal I Wijeyesekera

Olivier G Philip

Donald C McKeon

An advanced method for determining formation density in an array-detector density tool uses three or more detectors to yield an improved accuracy and precision of the formation density measurement even in the presence of a large standoff between the tool and the formation. A more accurate photoelectric factor is determined through a new single detector algorithm. Use of the information on the photoelectric effect and the density from the three detectors allows the measurement of a photoelectric effect compensated for stand off and the photoelectric factor of the mudcake. The use of the multi-detector density answers allows for a consistency check and therefore a much improved quality control of the density measurement.

Method for determining formation density and formation photo-electric

The present invention is an improved method and tool for determining formation density by using an array of gamma-ray detectors. This invention can correct for large standoffs encountered in abnormally shaped boreholes and in particular for the increased standoffs typically encountered by mandrel tools. In this invention, the collimated detectors have varying depths of investigation into the formation. At small standoffs a short spaced (SS) detector investigates mainly the mud and mudcake and a shallow layer of the formation. Unlike the SS, a mid spaced (MS) detector has a deeper depth of investigation and is sensitive to borehole and formation even at increased standoffs. A long spaced (LS) detector is mainly sensitive to the formation density and its density reading is corrected by using the standoff information from the MS and SS detectors. In addition to measuring density, this invention can measure the photo-electric factor (PEF) of the formation. Because photo-electric absorption preferentially removes low energy gamma-rays, the tool housing needs to allow passage of low energy gamma-rays. This can be accomplished through the use of a window of a material with a low atomic number (Z) or through the use of a low-Z housing material like titanium. Typical window materials are beryllium and titanium. Housing materials can be titanium or for lower pressure requirements graphite or high-strength carbon compounds.

Method and apparatus for measuring formation density and the formation

The formation porosity of a partially invaded gas reservoir in subsurface earth formation is determined from the sum of percentages of density and neutron tool porosities. During the method of this invention, density tool and neutron tool porosity measurements are taken of the formation. These porosity measurements are fitted to a predetermined porosity to determine the percentage of each porosity measurement that will be summed to estimate the formation porosity of the gas reservoir. The percentages of the density and neutron porosity measurements are dependent on a correction factor that is determined from the fitting process.

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Houston, TX, United States of America

Urmi DasGupta