Slimhole Handbook: Procedures and Recommendations for Slimhole Drilling and Testing in Geothermal Exploration

Slimhole well casing design for high-temperature geothermal exploration and reservoir assessment

“…This method is very expensive and puts severe impact on surrounding environment. Drilling production-size holes can also put a large expense during first stages of the project and if well turns out to be non-productive or low in temperature, it can lead to long periods of debt for an investor [2]. In other words, very expensive geothermal wells are drilled with no reservoir knowledge and very high financial risk.…”

“…These criteria amount to much lower costs of drilling wells with 6-inch diameter or less. As for Sandia National Laboratories report on slimhole drilling, intermediate cost of drilling slim wells is around 60% of the large well's overall costs to equivalent depth [2], where some other sources assume even 25 to 35% [5]. On this account, investor can drill three to four slim wells for the cost of one large-size well for better geothermal reservoir evaluation.…”

“…A large number of researches made in Japan and USA confirm that flow testing in slim wells turned out to be equally informative as these in large size wells. Thus, data produced from slimholes can provide good prediction of reservoir productivity [2].…”

“…Top drive with hydraulic motor or chuck is used to turn the drill string. Due to very fine cutting production during coring and smaller hole volume, wireline coring can be carried out without drilling fluid returns ("blind" drilling) [2,6,7]. The core barrel used for drilling can be then left as a casing string, lowering the cost of casing.…”

“…Wells with smaller in diameter can, however, be used for exploration drilling to confirm the resource. Slimhole technology is adopted from already established techniques from deep-water wells, mineral exploration and geotechnical investigations [2]. Slim wells can be utilized not only in high-temperature reservoirs.…”