Surface casing   Surface casing is set to Protect, water sands, case unconsolidated formations, provide primary well control, support other casing. and case off lost circulation zones. Such a string would be run and cemented back to surface. It is normally the first casing to support some for of secondary well control equipment. Equipment such as a diverter or 21 1/4 bop. Characteristics: Casing size range from 7 5/8 on a shallow holes to 20 inch on deep holes. Hole may have sever hole erosion Shallow string may be pumped out easily. Drilling fluid often viscous with little water loss control or drilled with water. Casing may stick easily in unconsolidated formations. Lost circulation or water flows could be a problem. Most area require that it be cemented to surface. Guide shoe and float collar are commonly used. Packing off can cause high pressure and can burst the casing. Often run thought the bent section of the hole. Often run to depths of 3000 feet plus. Often need top cement job to complete cementing to surface. Drilling this hole section is normally fast and little time is wasted. If spud mud is use it would only be at the start. Formation clays would be use to continue with minimum chemical added. In some area a pilot hole may be drilled then opened up to a larger size. Drilling tools are big and cumbersome and the fewer BHA changes the better. It is often in this section of the hole the start of directional  work begins. Running  into other wells could also become a problem. On multi well platform there can be as many as 36 hole all going in different directions, guiding the bit past some of the other wells can be a problem In the middle east one of the main problem is salt water flows. Holes are drilled using a float and in some cases the slug may be pumped down the annuals in an attempt to control the flow while coming out..  Shallow gas can also be a problem. For such area a diverter should be used. In such a case the well can never be closed in. Diverter of to-day have been vastly improved. and if rigged up properly is an asset. Such a rig up would consist of the diverter package with 12 inc out let, but just as important are the dump lines a dump line dropping 90 feet to the water line on a jack up has a siphoning effect and will suck both gas and water away from the rig floor.