Miscellaneous Subsurface Methods 747 



For a successful operation of this type, some of the results sought are: 

 (1) sealing of the producing zone from encroachment of water from other 

 zones penetrated; (2) prevention of migration of fluids between any zones 

 covered by the cement; (3) support and protection of the casing against 

 collapse, corrosion, and other damage; and (4) prevention of blow out on 

 the outside of the casing. 



From a mechanical standpoint and without regard to any geological 

 consideration, the following points are vital to any good casing cementing 

 job: 



(1) A properly drilled and conditioned hole, which is as true a bore 

 as possible for the area with regard to both vertical drift and dia- 

 metral trueness to gauge. Free circulation with as low a fluid loss 

 as can be obtained. Mud of low water loss, low viscosity and suf- 

 ficient weight to hold back subsurface fluids. A final clean up of 

 the bore to rid it and the mud system of as much suspended or 

 deposited cuttings, debris, and gelatinized mud as possible. 



(2) A proper string of casing to suit existing well conditions, equip- 

 ped with a guide shoe and at least one float collar, all correctly 

 tallied and checked. 



(3) Cementing equipment of high reliability to suit the conditions of 

 the area of operation and well depth as to pump pressures and 

 volumes desired. Strong, reliable, and speedy casing-head con- 

 nections. Two plugs, a bottom and a top one, of the best possible 

 design to separate mud from cement slurry, wipe the casing walls, 

 and produce a positive shut-off when the slurry is landed in place. 



(4) Capable, well-trained, alert, and reliable cementing crew and 

 drilling-crew personnel. 



(5) A type of cement, as portland, slo-set, of high early strength to 

 suit time and temperature conditions of the hole being cased. 



(6) Ample supplies of water and mud to effect mixing of the cement 

 and pumping it into place. 



The above items are basic; without them no casing-cementing job can 

 be successful; yet, even with all of them, many such jobs are unsuccessful 

 because water or gas is not properly sealed in place. Remedial measures 

 are necessary later to correct the original failure. Several things may be 

 done to better insure success on any casing job in practically any area of 

 operation, and it is with these added features that the geologist should be 

 especially concerned. 



(1) A study of the various well logs in regard to oil and/or gas 

 zones, water zones, water-oil contacts, and gas-oil contacts. Zones 

 of weakness which might cause or have already caused lost cir- 

 culation. Size of the final bore at various depths and these depth 

 relations to formations which must be properly covered with 

 cement. Temperature existing in the well. 



(2) From a study as in (1) above, select the best position for the 



