Developments in Precision Casing Joint and Radioactive Measurements for Compaction Monitoring
Authors: Allen Dennis R. 
Published: December 2007

Summary
A method has been developed in Wilmington field, CA, for measuring oil zone compaction and expansion by the deformation in well casing. Possible formation compaction is also directly investigated by locating radioactive bullets previously placed in the formation. Early logging techniques were described fully in 1969. The addition of a downhole odometer and different recording techniques have improved measurement accuracy. Random joint lengths have been repeatedly measured and remeasured under field conditions with a standard deviation of 0.0159 ft [4.8 mm]. An alternative system, developed by Ruedrich et al., utilized multiple collar locators and specially milled casing joints. Both systems can be applied to field situations where random joint lengths are found; however, the odometer system should be more accurate under these conditions.

Introduction
The basic problems of surface subsidence and related formation compaction in Wilmington field are well known. Field investigations have shown that when the formation rocks are unconsolidated, formation compaction and expansion are approximately reflected by casing length changes. Formation compaction is not reflected exactly by casing deformation because of slippage between the formation and pipe. However, in Wilmington field correlation has been good, perhaps as great as 95%. These problems, along with logging tools for detecting and measuring casing-length deformations within a few hundredths of a foot per joint [+/ –9 mm/ +/–13 m] were described by Allen in 1969. At that time the system used two or three collar locators (and/or radioactive bullet locators for direct measurement of formation compaction) spaced about one joint length apart. Calculations were made from film recordings made at a scale of 50 to 60 in./100 ft [1.27 to 1.52 m/30.48 m] of hole logged. Subsequently, this equipment has been improved and other investigators have developed at least one alternative system.

Statement of the Problem
Measurement of subsurface compaction has been a problem in Wilmington field for a number of years and all the older in-situ casing-joint-length investigations were related to this problem. Recently, data requirements for environmental impact reports and studies in both oil and geothermal fields have revived interest in these measurements. In-situ baserun casing-length measurements on new wells have been required in certain instances. Investigations of potential formation compaction in geothermal areas are under way, including government grants for development of measurement systems. Current state-of-the-art tools will be used as a base for developing similar tools that can be used effectively at geothermal reservoir temperatures.
To detect small amounts of compaction, by either casing deformation or radioactive bullet displacement, downhole measurements accurate to a few hundredths of a foot [9 mm] or less are necessary. A major problem in this type of logging has always been tool "bounce." Bounce is defined as an erratic tool speed in the hole, caused by drag and cable stretch, while the footage recording device at the surface is recording-at a uniform scale. Obviously, erroneous measurements will be made when this happens. This problem is accentuated when logging older wells wherein scale, corrosion, and tubing wear marks have roughened the pipe interior. Deviated holes, which are common in Wilmington field, also increase bounce.
Several other instrument systems have been used in very shallow water wells and in engineering geology applications, but these systems generally are not applicable in a deepwell environment.

Tool Development
To overcome the bounce problem, a downhole tracking system with small odometer wheels was devised by the City of Long Beach and built by Dresser-Atlas. The tool body is ex-centered by springs that push the wheels into a firm contact with the casing. Two tracking wheels are used, each having five magnets spaced around its periphery. These magnets trigger a reed switch each time passed and a "blip" is recorded at the surface. Each blip records that the tool has traveled about 2 in. [5.1 cm], regardless of the amount of cable that may have emerged from the hole during this same interval.
Two wheels are used to provide a redundancy factor; if both are in contact with the hole wall, they should record about the same amount of tool travel (they are not synchronized). Because of occasional lift-off of the casing wall by the upper wheel, resulting from varying cable pull, the lower wheel has proved the most reliable.
JPT
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