Columbia Gas Transmission operates a 290-well underground gas storage field in north-central Ohio. The Weaver storage field, converted to gas storage starting in 1937, utilizes the Silurian-age Clinton sandstone as the storage reservoir. Clinton sand thickness, reservoir quality, areal distribution and lateral continuity are often highly variable over short distances. Many of the storage wells have only cased-hole electric logs, primarily a gamma ray with an API neutron. Stratigraphic correlation and identification of Clinton sand members can be problematic, particularly when relying on cased-hole logs and original completion records.

Compounding the task of Clinton sand correlation in the northwestern portion of the Weaver storage field is the depositional presence of "Red Rock", an iron-bearing carbonate identified through x-ray analysis as an Ankerite [Ca(Fe, Mg, Mn)(CO3)2]. Over ten square miles of Red Rock with a thickness of 10 feet or greater have been isopach-mapped. Historically, the Red Rock presents a clean, blocky gamma ray signature on electric logs with an associated neutron response similar to a gas-filled sandstone reservoir. In portions of the Weaver storage field, the Red Rock was often miscorrelated as Clinton sand and mistakenly completed in numerous wells. This resulted in an undesired division of the hydraulic stimulation between Clinton reservoir sand and non-reservoir rock. Also, due to a significant iron presence, completion acids may have reacted negatively when exposed to the Red Rock to produce iron precipitates that can degrade well performance. Accurate correlation of both Clinton sand and Red Rock carbonate, therefore, is crucial to the operational success of Weaver storage field.

To address this problem in cased-hole workover candidates, a pseudo-density logging technique was used in conjunction with an API neutron log with considerable success. This logging methodology could have additional cased-hole applications where contrasting behind-pipe porous sand with associated or enveloping carbonate is desired.

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