How precise can drilling for deep boreholes be? What sort of distance is needed between storage holes, how much control do you have?
Typical steerable directional drilling has a variance of about 0.5 degrees per 1.5 km. This translates into about a 13 meter differential from the target at a roughly 95% confidence level. For greater accuracy, Measure While Drilling (MWD) tools that measure position using accelerometers and magnetometers may be used and combined with gyro-while-drilling (GWD) tools. Positional data is sent continuously to the surface and the data updated roughly every minute so one can spot trends while actively drilling. Corrections can be made as soon as a deviation is spotted. Normally in the oil and gas industry, drillers have a target window that has a left or right tolerance that they have to stay within. It’s common to request them to stay within 10 feet left or right of the plan while drilling a lateral section of an oil or gas well. Once a first borehole is present, subsequent boreholes can be placed even more accurately by measuring its position relative to the first borehole. In the oil and gas industry, there are examples of arrays of parallel wellbores just a few meters apart in regions that require this accuracy for resource extraction.
Our safety case is based on parallel horizontal boreholes that are 100 m apart; smaller spacing can and are being considered. Our current thinking is that perhaps a 30 m spacing would be optimal, but many factors including the geology, waste form, and heat generation effects among others need to be taken into account. We do not expect the spacing between repository sections to be limited by uncertainties in borehole positioning.