WorldWide Drilling Resource

Environmental Monitoring by Thomas Kwader, Ph.D., P.G. The Importance of Surface Casing The purpose of surface casing is to hold the unconsolidated materials from collapsing into the hole around the drill rod while advancing the borehole. You don’t want your borehole to keep “growing” while you are drilling into a funnel-shaped hole, which can eventually upset your drill rig. There is another reason why all types of drill rigs should use a surface casing. If you are drilling with fluids (water, drilling mud, etc.) and you encounter a void (crack, crevasse, cavern) you could lose all your fluid and loose sands down the borehole into the highly permeable zone. If this void begins to “mine” out large amounts of ma- terial from beneath the drilling rig, there is a good chance the rig will sink into the new hole growing under it. If the cavity or void is relatively close to land surface, the surface depression will likely be “small” and cause little damage. Depending upon the type of strata near the surface, i.e., clays, cemented sandstones, etc., and their thickness and depth, the void could “grow” to a void larger than the drilling rig itself, and the damage could be catastrophic. I personally know of three rigs lost to an unretrievable depth, and four drill operators who lost their lives when the collapse was sudden and deep. If surface casing would have been set into a competent rock or dense clay lay or the circulation of drilling mud and cuttings would have been maintained, it is likely little or no damage would have occurred. The depth, diameter, and number of the surface casings depend on where the lost circulation zone is located, how many zones may be present, and the anticipated depth of the completed well. If a lost circulation zone is believed to be at 10 feet and 50 feet below land surface and the final well diameter is intended to be 4 inches, the first casing should be a 12-inch casing and seated into the com- petent layer below 10 feet; and the sec- ond surface casing set below 50 feet into a low-permeability zone. The next boring will be about a 7½-inch bit to set the final 4-inch casing inside the 8-inch casing with grout. Surface casings are a wise invest- ment in areas where lost circulation zones exist. Tom Tom Kwader may be contacted via e-mail to michele@ worldwidedrillingresource.com 38 SEPTEMBER 2016 WorldWide Drilling Resource ® What’s YOUR story? Do you have a story or article idea? We want to hear from you! Call Bonnie 850-547-0102 or e-mail bonnie@ worldwidedrillingresource.com Ideas Welcome