The drilling operation focuses on: adding drill pipe to the string; 

 replacing drill bits; ranaging tlie drilling fluid; monitoring the cuttings and 

 making various tests; and carefully watching the extreme pressures often 

 associated with various strata. There are rany supporting activities that 

 take place during drilling, including movenent of supply vehicles and person- 

 nel, normal maintenance of equipment, and disposal of spent equipment and 

 waste material. There are occasional problems on the site, often resulting 

 from breakdowns. Usually in such cases specialized equipment is brought to 

 the site for making repairs or for "fishing" for broken or stuck devices in 

 the well hole. 



After the hole is drilled to project depth, the well is "logged" by any 

 of a number of methods to evaluate the formations. On the basis of these 

 tests a determination is made to either plug the well if it is unproductive, 

 or to "set casing" and bring the well into production. 



If casing (large-diameter pipe to line the inside of the hole) is to be 

 placed into the bore, the drilling rig lowers it to the proper position. Then 

 cement trucks with high-pressure pumps push cement down through the casing and 

 back up between the outside of the casing and the sides of the well hole. 

 This anchors the casing \/ery firmly and excludes material and pressure from 

 other strata. (During the drilling operation, certain types of casing may be 

 placed to protect strata from contamination or to help support layers that 

 tend to slump. ) 



When the casing has been cemented, it is perforated to allow oil and gas 

 to enter the wellbore. Special treatments such as acidizing or fracturing may 

 be required in certain petroleum-bearing strata to facilitate the movement of 

 fluids to the wellbore. 



The rig is moved away and a "Christmas tree," an assembly of valves, 

 pipes, and fittings to control fluid flow at the surface, is installed. The 

 well may then be connected to a network of flowlines that lead to tank batteries 

 or treatment devices. 



Production. While drilling itself is a short-term operation typically 

 lasting one to three months, production is long-term with a time scale often 

 measured in decades. Production equipment generally consists of numerous 

 separating, treatment, storage, and disposal devices. Petroleum is often 

 associated with sedimentary formations and salt water Thus the fluids rising 

 through the wellbore may be a combination of oil, natural gas, sand, mud, salt 

 water, and in some cases, hydrogen sulfide. Because natural gas and oil exist 

 as two different chemical phases (gas and liquid) and are transported and 

 stored in different manners, it is necessary to separate them completely near 

 the wellsite. 



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