Well logging apparatus

Details Well logging apparatus Well logging apparatus

1999-06-21

2001-10-30

Williams, Hezron (Department: 2856)

Measuring and testing

Borehole or drilling

Formation logging

C073S152140, C250S269700, C250S268000

Reexamination Certificate

active

06308561

ABSTRACT:

The present invention relates to well logging apparatus and more particularly to well logging apparatus which is able to be deployed in relatively narrow boreholes in order to measure formation density.
It is advantageous to be able to deploy a logging apparatus in a small diameter borehole for several reasons. In a number of boreholes the direction of the borehole changes to follow specific strata and if a large diameter apparatus is used this can often not follow the borehole. Also, if borehole conditions are difficult then often the drill pipe may be left in place, at leave over the difficult section. It is advantageous to be able to deploy a logging device by running it through the drill pipe into the “open” hole beyond.
To achieve high quality borehole density measurements it is know that the logging device must preferably have a number of advantageous features. To achieve all of these in a relatively small diameter device has hereto been very difficulty. The present invention seeks to provide these features in a small diameter device preferably less than 2¼″ diameter as opposed to the normal 3¼″ or greater diameter of a standard device.
The features preferably required for a “density logging device” are:
1. Two or more detectors for compensation/correction for mudcake;
2. Scintillation counter detector systems for good counting statistics and therefore good measurement precision;
3. Effective shielding using Tungsten or similar to reduce radiation into the borehole, and therefore the perturbing effect on measurement of varying borehole size and fluid content;
4. To get good quality measurements, the detector system needs to be carried in a short “pad” or “skid” pressed against the borehole wall and free to follow borehole irregularities. A relatively poor measurement results if the detector system is carried in the body of the instrument (“sonde”) without any provision or mechanism for moving, tilting or pressing its detector array into contact with mudcake or borehole wall, i.e. if the device is the basic version of “mandrel” type that does not allow any lateral adjustment of its detector position.
5. A technique for measuring the “photoelectric factor” and hence indicating lithological rock types can be employed which involves measuring the energy of the detected gamma rays. The energy spectrum is split into low and high (“soft” and “hard”) received energy sections and a ratio of these sections formed which is then related to the photoelectric factor.
6. To measure low energies the detector system casing needs to be transparent to these energies.
7. To access difficult borehole conditions it is advantageous to maintain as small a diameter sonde as possible.
8. The detector skid needs to be disposed in “carrier” which is “through wired” to enable other devices to be connected below it, forming a sonde “stack”.
9. The employment of a “free to move” skid requires a flexible cable and sealed connector system prone to unreliability.
In prior art devices the transparent casing of 6. is realized by puncturing the casing and employing a window of Beryllium, Titanium, or other light material, welded, bonded, or otherwise sealed into the window aperture.
Features 3, 4, 6, 8 and 9 require sonde diameters of more than 3.5″ and more typically 4″ +. This limits the deployment in “slim” boreholes, and in particular the deployment of the device by running it through the drill pipe into the “pen”hole beyond.
Prior art systems are described in U.S. Pat. Nos. 3,654,470, 4,034,218, 4,048,495, 4,504,736, 4,578,580, 4,628,202, 4,661,700, 4,814,611, 5,134,285, 5,204,529, 5,390,115, 5,451,779, 5,528,029, 5,530,243, and 5,608,215 which may be referred to for explanation of well logging techniques in general.
It is an object of the present invention to provide a logging device capable of providing high quality measurements in a small diameter and the present invention provides a well logging device including a detector system having a long spaced detector and a short spaced detector mounted in a skid, said skid being provided with means for pressing said skid against the side of a borehole, said skid comprising an elongate tube of low energy gamma ray transparency, said tube being internally supported by an inner support means to provide support for said elongate tube over a major internal area of said elongate tube.
Preferably said elongate tube comprises a stainless steel tube.
Preferably said internal support means comprises an internal tungsten tube.
Preferably said internal tungsten tube is provided with a first relatively elongate window having length and width dimensions that are compatible with an adjacent said long spaced detector crystal.
Preferably said relatively elongate window is provided with a filler.
Preferably said internal support means is also provided with a shorter window, having length and width dimensions generally compatible with an adjacent said short spaced detector crystal.
Preferably said relatively elongate window is provided with a filler.
Preferably said internal support means is also provided with a shorter window, having length and width dimensions generally compatible with an adjacent said sort spaced detector crystal.
Preferably the shorter window is not provided with a filler but is provided with a liner to absorb X-rays from the tungsten tube. Preferably said liner is constructed from tin.
Preferably the pad is also provided with an electronics section adjacent to said reinforcing means but within the measurement skid tube.
Preferably the reinforcing means is sealed against the stainless steel tube by a plurality of ‘O’ ring seals adjacent to each end of the reinforcing means.


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