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A slim hole gamma ray and directional surveying MWD tool has been developed specifically for real-time geosteering applications in horizontal wells. It is the first system which is totally retrievable and replaceable by slick line. This minimizes risks associated with stuck pipe and "lost-in-hole". In addition, it cuts costs by minimizing the number of round trips to change tools in case of failure.

The system can be run in drill collars with internal diameters as small as 2.19", and has been run in collars with 3 1/2" ODs. It has also been run in coiled tubing drilling operations.

This paper describes the downhole system and associated calibration procedures. Comparisons with conventional wireline logs are made.

Several field applications from horizontal wells are described. In particular we demonstrate how the gamma ray can be used to "geosteer" the horizontal drainhole to remain in the hydrocarbon section and track formation dips while drilling. This reduces the risks of horizontal drilling and increases payback potential


The first medium radius horizontal well was drilled in the Austin Chalk in 1986, by Arco in Rockwall County, Texas, and had a displacement of 1630. Typical wells now have horizontal displacements of 4,000 ft in target windows of 10 ft TVD, at inclinations of 87 degrees to 93 degrees. The Austin Chalk lends itself to horizontal drilling for a number of reasons:

  1. The Austin Chalk is a vertically fractured, fairlyhomogeneous formation.

  2. The wells can be left as open hole completions.

  3. The Chalk has several pay zones, separated by as much as 150 ft or as little as 20 ft, each of which can be drilled horizontally from the same vertical well.

  4. P. I. increases of 10:1 have been achieved in horizontalwells compared to vertical wells.

The area of current interest stretches from the Dilly / Pearsall area, south west of San Antonio, to the north east as far as the Louisiana border at the Toledo Bend Reservoir, in a band about 40 miles wide. Fig. 1. As the Chalk trends north east, the formation dips from about 6,000 it in Pearsall to about 11,000 ft in the Toledo Bend area.

The tools and techniques originally used to drill these wells were adaptations of equipment used to drill conventional long reach wells, but as more wells were drilled, the problems facing the Operating and Service Companies changed from "how far out can we go" to "how can we better define the pay zone and how can we stay in it".

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