Diamond Coring in the Rangely Field, Colorado

Abstract

Abstract This paper presents the development of diamond coring of the Weber sand sectionin the Rangely Field, Colorado. The description and operation of thediamond-coring equipment is included as well as the economics and resultsobtained by its use. Diamond coring is compared with the other methods used fordrilling the Weber sand. Two types of diamond coring are discussed. The firstand the one used most extensively is the regular 6 1/8 X 3 ?:-in. diamond bitand bottom hole 50'Ft barrel. The second type, which has also been usedsuccessfully by Stanolind, consists of a 4 13/16 X 2 11/32 in. cutting bit andreverse circulation coring equipment. The first type will be referred to asregular or conventional diamond coring while the latter type will be referredto as reverse circulation diamond coring. Introduction Diamond coring has been used for many years in mines, quarries, surface coringat dam sites, laboratory work, and so on, but, until recently, had never beenused very extensively in coring oil sands and other oil-bearing strata. It wasdemonstrated early in its development that hard formations could be cut rapidlyand that core recoveries far exceeded those obtained with conventional hardrock bits and quite often 100 pct recovery was obtained. However, the poorlyconstructed diamond-coring equipment and resulting high costs of early diamondcoring slowed down its progress. Early in the development of the Rangely Weber field, Rio Blanco County, Colorado, the Rangely Engineering Committee selected key wells that were tocore the Weber section in its entirety. These cores were to be analyzed andvarious sections tested to gather sufficient information so that accuratereservoir studies could be made. The number of wells selected were kept at aminimum inasmuch as the productive Weber section, which is found at anapproximate depth of 6000'Ft, is thick, being as much as 600 to 700'Ft on topof the structure. The Weber section is also comparatively hard to drill. Forinstance, one well took 60 days to core 490'Ft of Weber sand with conventionalcoring equipment. It was noticed, however, that Core Laboratories diamond plugcutter could quickly cut a plug from one of these hard cores. This fact led tothe serious consideration of diamond coring the Weber sand. The first diamond coring at Rangely was performed by Stanolind in August 1946and, although we experienced some difficulties, the results definitelyindicated that diamond coring would be successful at Rangely for coring theWeber sand. As more wells were cored and the equipment improved somewhat, diamond coring became less expensive than drilling the Weber sand with theconventional rock bit. In less than one year diamond coring has definitelyproved its usefulness in oil-field drilling operations in the Rocky Mountainarea where formations quite often drill very hard. The practice of spot coringin hard oil-bearing formations with conventional coring equipment is giving wayto diamond coring of the entire sections, thus giving an operator the completepicture. T.P. 2301