Abstract

Carbonate reservoirs of the Devonian Dundee Formation represent the most productive interval in the Michigan basin, yet have suffered widespread abandonment due to improper early reservoir management and completion practices. Dundee carbonate reservoirs, deposited in marginal-marine and shallow-marine environments, have produced over 351 MMbbl oil from 137 fields, most of which were discovered in the 1930s and 1940s. Initial potentials for many Dundee wells ranged from 2000 to 9000 bbl/day, presumably due to high-permeability fractures and solution-enhanced porosity in some areas and excellent primary interparticle porosity in other areas. Crystal field, discovered in 1935 at a depth of less than 3500 ft (1060 m), was aggressively developed and produced at high rates, causing widespread water coning, such that 95% of the field was drilled and abandoned in less than 5 yr, leaving considerable interwell reserves in the ground. No cores or wireline logs existed for the field prior to 1995, when the field became the subject of a multiyear study aimed at determining the feasibility of employing horizontal drilling to recover bypassed reserves. A vertical test portion of the TOW 1-3 horizontal well, located in a structurally elevated portion of the field, cored and logged the entire reservoir interval, yielding crucial new information. On the basis of relevant data, a horizontal leg was drilled, resulting in an excellent producer with estimated recoverable reserves of 200,000 bbl. Two subsequent horizontal tests with poor results were drilled in downdip locations and oriented perpendicular to the TOW well, suggesting the importance of both structural position and azimuth. Selective use of horizontal drilling is considered an appropriate, cost-effective means for recovering bypassed reserves in Dundee fields where sufficient geologic data exist to adequately characterize the reservoir and oil occurrence. For some fields, this may require new coring and logging operations.

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