This project reviewed literature in order to: - evaluate any downhole environmental risks associated with drilling fluids and completion practices in the Cooper and Eromanga Basins; and - assess the effectiveness of cementing practices in achieving long term zonal isolation between reservoir formations penetrated by a well in these basins. Although an area of concern, possible environmental impacts of drilling fluids were found to constitute only a small portion of downhole problems. The use of drilling fluids was considered to be an issue because of the potential to invade freshwater aquifers, particularly those of the Great Artesian Basin (GAB). In order to determine the significance of fluid loss to the formations, mud recap reports were reviewed. These reports record drilling fluid losses and provide an estimate of the depth and formation where this occurs. Other potential impacts of drilling fluids include: - microbial contamination in aquifers; - contamination from biocides used to control microbial activity; - poor mud cake removal. From the review of the mud recap reports, the most significant areas of fluid loss were in the first 4000 ft of the well. The most likely reasons for these losses were: unconsolidated sands; and clay swelling. In any event, it was determined that due to the presence of the Bulldog Shale and its low permeability, an effective seal would be present to prevent the migration of drilling fluids into more environmentally significant formations. Although in deeper wells the fluid losses were difficult to quantify, it was concluded that the majority of the fluid loss was most likely to occur in to the producing zones of the formation. The main consequence of this is reservoir formation damage rather than an irreversible contamination of the aquifer. Once brought 'on-line', a well will produce the majority of drilling fluid lost to the formation. Therefore, it is concluded that potential drilling fluid contamination of Cooper and Eromanga Basin aquifers, in particular the Great Artesian Basin, is not a significant risk. The major area of concern was the effectiveness of cements in achieving long-term isolation in the wellbore of individual formations penetrated by the well. Cements were identified as crucial because they are currently the only means for zonal isolation in the wellbore. Zonal isolation is deemed necessary in the wellbore because it provides a means for the prevention of cross-flow through the wellbore. Such cross-flow is considered to be a major risk for aquifer contamination. The effectiveness of cement zonal isolation in the wellbore was reviewed by investigating the potential mechanisms of cement failure. The mechanisms identified were: - high temperature; - sour conditions/sweet conditions; - bacterial presence; - cement shrinkage; - formation damage; - poor mud cake removal; - high cement permeability; and - cement carbonation (chemical reactions). The findings of this investigation concluded: - cement carbonation and deterioration due to hostile environments was the major mechanism for cement failure; - it is necessary to establish a cement time-scale for which the cement should continue to provide zonal isolation to the formations isolated in the wellbore; - current cement technology may not be able to provide long term zonal isolation and new technologies need to be considered; and - wells drilled through a successful drilling program will not always be accompanied by a competent cementing job. A system employing the use of logging equipment was devised in order to evaluate whether the cement was meeting the objectives it was designed to achieve. Examples of criteria that can be used to evaluate the integrity of a cement job are postulated in this report.