A Statistical Evaluation of Formation Disturbance Produced by Well‐Casing Installation Methods

Water-resources investigations concerned with contaminant transport through aquifers comprised of very loose, unconsolidated sediments have shown that small-scale variations in aquifer characteristics can significantly affect solute transport and dispersion. Various sampling and borehole-logging techniques have been integrated into these studies to detect and measure detailed formation changes through casing or well screen along the length of the borehole. Commonly, measurement accuracy and resolution have been limited by a borehole environment consisting of an annulus of disturbed sediments produced by the casing-installation method. In an attempt to quantify this disturbance and recognize its impact on the characterization of unconsolidated deposits, three installation methods were examined and compared in a sand-and-gravel outwash at a test site on Cape Cod, Massachusetts. These installation methods were: (1) casing installed in a mud-rotary hole; (2) casing installed in an augered hole; and (3) flush-joint steel casing hammer-driven from land surface. Fifteen wells were logged with epithermal neutron and natural gamma tools. Statistical analyses of these digitized nuclear logs proceeded under the fundamental hypothesis that a large variance indicated preservation of layering in the surrounding sediments, significant variability at a small scale, and a small level of disturbance. Conversely, a small variance corresponded to mixing, poor detail, and larger disturbance. The statistical information generated from a systematic “analysis of variance” investigation confirmed an intuitive assessment of the problem by demonstrating that, under the conditions encountered at this site, augering is the most disruptive of the three casing-installation methods and that driving casing directly, though typically a more time-consuming operation, transmits the least amount of disturbance into the surrounding formation.

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