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The common practice of using geological well logging only on discovery wells precludes the collection of sufficient data to study the relations between production and well-log curves. The continuing development of the Vernalis gas field is a notable exception, since more than 90% of the wells in this field have been logged.
Optimum operating conditions were maintained by using the same drilling rigs throughout, excellent mud control, "controlled drilling" of the productive horizons, and by de-gassing the mud column prior to penetration of new producing intervals. The curves obtained in this manner were highly definitive of the producing sands.
Examination of the curves of many zones that are comparable from a production standpoint show large variations in amplitude that are apparently due to small changes in the mud weight.
The small effort and expense necessary to standardize as many physical conditions as possible in order to provide optimum well logging conditions appear well justified in the increased reliability of the gas curves. The apparent sensitivity to small changes in mud weight casts doubt on the reliability of many previous well logs made in extra-heavy muds and suggests that care be taken in the future to insure that excessive mud weights do not weaken one of the most reliable tools in the evaluation of gas sands.
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