Logging depth uncertainty, and the consequences we have to cope with  

Authors / CoAuthors

Miyazaki, S.

Abstract

Depth is the most fundamental logging parameter, but its accuracy is usually overlooked and remains enigmatic. It is not unusual that logger?s casing shoe and total depths are substantially different to driller?s. Wireline log interpretation booklets always contain correction charts for borehole size, mud weight and mudcake thickness. However, no chart is available for depth correction, probably because the sources and magnitude of depth errors are unique and temporally variable at each depth measurement point. The sources of depth errors include borehole doglegs, drill pipe stretch, incorrect lengths for drill pipes, wireline cable stretch and tides. Tidal effects on a floating drilling rig are significantly larger in the northwest continental shelf of Australia than in the North Sea or Gulf of Mexico. Depth mismatches occur between different suites of the same wireline logging run. Depth tie-in adjustments with the first suite are a common industry practice, but no guarantee is given to assure us that the depth of the first suite would be more reliable than that of subsequent ones. Sidewall cores taken from the middle of a thick monotonous shale interval, which is not uncommon in Northwest Australia, may not have an adequate depth tie-in reference point. This could result in obtaining off-depth sidewall core samples for biostratigraphic analysis. Depth mismatches also occur between MWD and wireline logging, between core cutting and wireline logging, and between equivalent wireline suite runs in multiple wells. The degree of wireline logging depth accuracy is a crucial factor for investigating pressure communication between wells and for calculating the hydrocarbon volumes of a multi-well gross geological structure. The economic impact of depth uncertainty on reserve estimates is growing because many recent discoveries are fairly small in size and complex in structure. For a small and geologically complex structure, a depth discrepancy of 0.1% (for example, 3m in a 3000m well) between wells could be too great to declare the project economically viable. Depth uncertainty often results in an open-ended discussion about whether two nearby wells have intersected the same continuous hydrocarbon pool or two separate pools. It also causes tension within a company over the decision making process of a field appraisal or development, between companies over a field unitisation process, and between a company and government agencies over an application for a location, retention lease, production licence or ?new oil? excise exemption.

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nonGeographicDataset

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61116

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• External PublicationAbstract

Australian and New Zealand Standard Research Classification (ANZSRC)

• Earth Sciences

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Publication Date

2004-01-01T00:00:00

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Australian Government Security ClassificationSystem    

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Creative Commons Attribution 4.0 International Licence    

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geoscientificInformation

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