Cuttings Sampling And Lithology Description

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Accurate cuttings descriptions are essential for the following 3 reasons:

  • To pick out individual formation sequences as they are drilled, to establish stratigraphic position at all times, and hence to determine casing and coring points.
  • To ensure that the well approaches the correct TD.
  • To enable construction of a Well Completion Log, with all lithologies accurately described. This log allows detailed correlation to be made with neighbouring wells for development studies.

1       Sample Collection And Preparation

All mudlogging instruments should be checked and calibrated prior to spudding the well. It is particular important to check Total Gas, Chromatograph and H2S detectors.

Sample descriptions are required every 5 m in the reservoir.

2       Collection of Cuttings Samples

The overall objective is to obtain representative, clean samples, with an accurate assurance of the depths from which they are drilled. In order to achieve this it is essential to have an organised method planned for the sampling programme. The following are guidelines in this respect:

1) Samples should contain material collected from all shale shaker levels, and from the desilter/mud        cleaner. Use of sieves with suitably sized mesh for collection will facilitate rinsing of the samples. After rinsing, samples for immediate examination should be placed in sample trays,

2) Samples must be identified by well number and sample interval. It is essential that the marker pens used for labelling are insoluble to all types of drilling fluid. The sample depths must be corrected for bottoms up lag time, which should be theoretically checked by running carbide            checks.

3) One set of samples should be kept on the well site for reference until the completion of the next well. The remaining samples should be sent onshore at each section TD.

3       Cuttings Preparation

Samples shall be cleaned prior to description. Wash only enough samples to give a thin layer on the examination dish. Do not overwash, or grind the cuttings while washing - both may result in disintegration of the sample. Care must be taken not to rinse away soluble soft clays.

The cuttings shall be described in accordance with the Guide for Lithological Description of Sedimentary Rocks (Tapeworm 1966 version). The apparent subjectivity of cuttings description can be reduced by working through the different properties in a methodical manner, with consistent reference to the "tapeworm".

Cuttings descriptions are recorded by the mud loggers on a work sheet with provision for lithological descriptions, percentages of lithology for various intervals, penetration rates, gas levels, etc..

 

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