green monster question, page-5

If oxygen is used instead of air, the BTU of the resultant gas is generally higher. Oxygen is preferred for deeper wells. In shallower coals (<1,000ft.), both the injection and production wells are vertical and drilled in a grid. For deeper coals, directional wells with CRIP technology are preferred. CRIP refers to “controlled retractable injection point” and is a key innovation developed through US research. It allows for multiple ignition points along the path of a horizontal wellbore. AIR is used in shallower coals, but overseas trials have show that UCG can pose a potentially significant risk to groundwater when the trial is at shallow depth, this will not be the case for Oxygen blown - deeper wells, the risk is MUCH LOWER!