Abandoned Well Characterization: A Methodology to Evaluate Regional Hydraulic Controls on Flow From Hydrocarbon Reservoirs into Underground Sources of Drinking Water

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1994

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Between 1859 and 1990, approximately 3.3 million wells for oil and gas exploration and production were drilled in the U.S., of which nearly 2.4 million have been shut-in, temporarily abandoned, or plugged and abandoned (World Oil, 1992). Several major petroleum basins in the country contain large populations of these wells. Because drilling, completion, and abandonment practices for wells have evolved over the years, older wells that were found to be unproductive (or dry), or which had to be permanently shut-in for mechanical problems or economic reasons, may not have been adequately plugged according to modern standards or regulations. In some instances, upward movement of saltwater in such abandoned wells may pose a risk of contamination to underground sources of drinking water (USDWs).

Three main conditions must exist for contamination of a USDW to occur by fluid migration from an oil and gas production zone or a waste disposal zone: (1) presence of a USDW overlying the zone; (2) presence of unplugged or inadequately plugged abandoned wells (or natural geologic pathways) that penetrate both a production or disposal zone and a USDW; and (3) an upward-directed hydraulic gradient between the zone of interest and the USDW. The first condition exists in many of the petroleum-producing areas in the U.S. However, the second and third conditions may or may not occur. In particular, the third condition depends in part upon the changes in pressure due to fluid withdrawal and injection associated with enhanced recovery or salt-water disposal (Class II wells).

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