The surface casing is set to protect your water table, and prevent washouts close to the surface during drilling. Up there the coal seams are another obstacle, and have to be dealt with differently. Normally they will set a larger string of casing such as 13 3/8" to a depth that will seal off any bad shallow formations and water sands that will cause problems during drilling. This will usually be between 60' and 400'. The next string will usually be 9 5/8" or 8 5/8" to a much deeper depth usually between 800'-3000' depending on geographical location. After that drilling will go on until they reach TD and the production casing will be set. A string of 5 1/2" casing is popular for this. I will go to the bottom of the hole in most cases. The bottom hole depth usually ranges from 6000' to 14,000', as a general rule. the well may only have a true vertical depth of 6000', but the well bore may be 10,000' when they are drilling a directional hole. The production casing is the string that brings gas to the well head and into the gathering system. The exact process varies from region to region, but that is a general description.