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Method for Calibrating Natural-Gamma-Ray Tools to Measure API Values without Using the API Calibration Formation

IP.com Disclosure Number: IPCOM000247886D
Publication Date: 2016-Oct-10
Document File: 4 page(s) / 269K

Publishing Venue

The IP.com Prior Art Database

Abstract

Natural-gamma-ray tools have historically been calibrated in the API formation at the University of Houston. This formation has two major drawbacks: it is too small to accommodate LWD tools and it is soon to be decommissioned. This disclosure describes how to calibrate (determine the sensitivity) of both wireline and LWD NGR tools without using the API formation.

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Method for Calibrating Natural-Gamma-Ray Tools to Measure API Values without Using the API Calibration Formation

Abstract

Natural-gamma-ray tools have historically been calibrated in the API formation at the University of Houston. This formation has two major drawbacks: it is too small to accommodate LWD tools and it is soon to be decommissioned. This disclosure describes how to calibrate (determine the sensitivity) of both wireline and LWD NGR tools without using the API formation.

Previous Method of Calibration

The American Petroleum Institute built a calibration formation in the late 1950s for calibrating natural-gamma-ray (NGR) tools. The formation has been located at and maintained by the University of Houston. The formation is described in a paper by Belknap et al.1 The formation is composed of cement. It contains a borehole through the middle that is lined with casing. The formation is divided into three vertical zones. The middle zone was doped with potassium, uranium, thorium, and radium to emulate a shale formation.

A tool is calibrated by recording count rates measured by the tool when it is pushed against the casing wall. A count rate is measured in the bottom and the middle zones. The count rate from the bottom zone is subtracted from that of the middle zone. The difference is defined to be 200 API. API is an arbitrary unit with no physical significance other than being a relative measure of radiation.

The casing has an inner diameter of 4.89 inches, so most logging-while-drilling (LWD) tools cannot fit in it. To calibrate LWD tools, a secondary standard constructed from naturally radioactive granite is sometimes used. Such blocks are owned and stored by the user. A wireline tool must be calibrated in the API formation and then used to assign a value to the granite block. An array of blocks with a wide range of borehole diameters is required to calibrate tools of different diameters. Also, these blocks are typically not uniform, which adds an error to the calibration.

New Calibration Method

The new calibration method is composed of five steps.

1. Define an openhole digital formation (conceptual formation used in computer modeling) that emulates the API formation. When the formation contains a borehole filled with fresh water and a wireline tool is positioned against the borehole wall, the resulting measurement should be essentially the same as the difference between measurements in the middle and lower zones of the API formation. That means that the energy distribution on the surface of the tool and the total count rates should be the same. This ensures that the formation corresponds to 200 API for a wide range of tool designs. However, the source intensity of the formation could be reduced so that the formation represents a different API level, for example 1 API.


2. For each tool to be calibrated, generate a computer model that emulates the tool response.

1 Belknap, W.B., Dewan, J.T., Kirkpatrick, C.V., Mott, W.E.,...