Assessment of Environmental Implications of Abandoned Oil and Gas Wells
Authors: Warner D.L. , McConnell C.L. 
Published: December 2007

Summary
A detailed study was made of the potential for abandoned oil and gas wells in the Lower Tuscaloosa sand of Mississippi and Louisiana to act as conduits for movement of saline water from the Lower Tuscaloosa into underground sources of drinking water (USDW's). Finite-difference numerical modeling determined the extent that water might be forced from the Lower Tuscaloosa sand into a USDW as a result of injection into the Lower Tuscaloosa. Within the range of conditions modeled, water from the Lower Tuscaloosa never traveled into a USDW. On the basis of the modeling, we concluded that it is unlikely that abandoned oil and gas wells in the Lower Tuscaloosa would serve as conduits for water movement from the trend into a USDW. The procedures developed in this study should be readily applicable to analysis of the potential for abandoned wells to act as pathways for contaminant flow into USDW's in other oil and gas producing areas of the country.

Introduction

Purpose and Scope of Study.
Because of the increasing focus by Purpose and Scope of Study. Because of the increasing focus by U.S. regulatory agencies on abandoned oil and gas wells, the U. of Missouri-Rolla conducted research to assess the potential for abandoned oil and gas wells in the Lower Tuscaloosas and oil-producing trend of Mississippi and Louisiana to act as conduits for flow of saline water from the Lower Tuscaloosa into USDW's. Fig. 1 shows locations of selected wells from oil fields in the Lower Tuscaloosa sand. The Lower Tuscaloosa sand trend extends for about 135 miles from south-central Mississippi northwest into eastern Louisiana and for about 100 miles from north to south. The study included assembly of geologic and engineering data to formulate numerical models to simulate the range of flow conditions through abandoned wells in the Lower Tuscaloosa trend. The final step in the study was the actual numerical simulation of flow conditions for these abandoned wells. The simulations performed are believed to represent conditions throughout the Lower Tuscaloosa sand and the conclusions should be applicable to all abandoned Lower Tuscaloosa sand wells. The study is intended as an example that can be replicated in other oil- and gas-producing areas with similar conditions. To conduct such a study, sufficient information concerning the subsurface geology and hydrology, oil production history, and historic well construction and abandonment practices must exist or be possible to obtain to develop a realistic computer model. Also, geologic and hydrologic conditions that are sufficiently consistent over some area or distance are needed for the model to be represent enough cases to make modeling worthwhile. One would not be likely to conduct such modeling where every injection well required formulation of anew model.

Previous Work.
Ward et al. 2 performed the first numerical Previous Work. Ward et al. 2 performed the first numerical modeling work on the movement of fluids through an abandoned well. Ward et al. modeled the leakage of injected contaminants through an abandoned unplugged borehole. The work demonstrated the capability of the SWIFT III model for such investigations, but has no direct application to the problem studied here. Ward et al.'s investigation involved tracking wastes moving from an injection well to and up through an abandoned well. The problem here involves tracking the movement of native saline water from a saline-water-bearing aquifer in response to the pressure created by an injection well. Although significant difference exist in the details of the two problems, most procedures in setting up the models and data input are the same. In the modeling of injected waste movement, the injected fluid was tagged and tracked and no attention was given to the native saline water residing in the injection reservoir. In this problem, the injection fluid was assumed to the same as the native saline water and all the water that moved up the borehole was considered a potential contaminant to USDW. Warner modeled the response of a specific existing abandoned well in the West Mallalieu oil field to injection through a nearby water-injection well. Javend el et al. developed an analytical model of the abandoned well problem. While this model cannot be used to determine the effect of an abandoned well on the quality of groundwater in a USDW, it allows calculation of the gross amount of water that can flow into an abandoned well and thus provides a means to compare the results of numerical modeling of the flow into an abandoned well with a much simpler analytical calculation. The Javendel analytical model was used to verify the accuracy of flow results obtained from the numerical model during calibration.

Abandoned Well Problem
Thousands of wells have been drilled and abandoned during the 130-yearhistory of the U.S. petroleum industry. Regulations for plugging these wells, nonexistent in the early days of the plugging these wells, nonexistent in the early days of the industry, have evolved over the years to their present effective level. Thus, an unknown but large number of abandoned wells exist that may be unplugged or inadequately plugged by today's standards. As a result of incidents in which abandoned wells were implicated as sources of groundwater contamination, such wells are often considered indiscriminately to be potential pathways for contamination of a USDW. Contamination can result from interaquifer flow of natural formation water or by transmission of injected fluids from the injection reservoir to a USDW.
JPT
P. 874