WATER SHUTOFF As mentioned previously, the greatest single 



disadvantage to air drilling is penetrating 

 water-bearing formations. The fluid entering the hole from such formations com- 

 bines with the cuttings to form a gummy aggregate that may stick both to the 

 drill pipe and to the walls of the hole. Eventually a layer is built up that reduces 

 circulation and causes sticking of the drill pipe. 



Many methods of water shutoff have been tried with varying degrees of 

 success. The first method was the conventional cement squeeze, which has often 

 been successful, but it has two disadvantages : ( 1 ) it normally requires 24 hours 

 for the cement to set, after which the water used to displace the cement must be 

 evacuated from the hole, and the hole must be dried, an operation that may 

 take an additional 12 or 24 hours; (2) the water may hydrate some of the 

 formations and cause swelling and even sloughing. Bentonitic shales are par- 

 ticularly dangerous. 



The ideal method for water shutoff is one which requires the minimum rig 

 time and causes no formation wetting. A considerable amount of work is being 

 done to develop some type of plastic material that has a controllable setting time 

 and a sufficiently low viscosity so that it can be forced into the formation under 

 low pressure. It is preferable that the plastic be displaced with air, but if the 

 available air pressure is insufficient, an adequate amount of water may be put 

 in the drill pipe to give the necessary pressure increase. A valve arrangement 

 placed on top of the squeeze tool can prevent the water from entering the hole. 



Several materials that are being used currently on an experimental basis 

 have promised the desired requirements, but at present how successful any of 

 these will be is in question. 



SAFETY Air or gas drilling is not as safe as convention- 



al drilling. When gas is used, there is a con- 

 stant fire hazard due to the possibility of leaks in the supply line or around the 

 stripper head. The gas must be exhausted at a considerable distance from the 

 rig and should be burned at the end of the flow line. As a safety factor, a pilot 

 light should be in continous operation on the flow line. 



Air offers little or no fire hazard, but there is a danger of down-hole 

 explosions and some danger of fire if oil or gas is encountered. If the annulus 

 becomes restricted, or if a flow-line valve is closed, allowing a pressure build-up 

 in the hole, conditions may develop that would cause an explosion. From 

 practical experience it has been found that when the pressure in the annulus 

 exceeds 600 pounds per square inch, the proper gas-air mixture may explode. 

 There have been several instances where both drill pipe and casing have been 

 damaged. One method of overcoming both the fire and explosion hazard is 

 the consumption of the exhaust gases. The primary drawback to this procedure 



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