748 Subsurface Geologic Methods 



casing shoe, giving careful consideration to gas-oil contact and 

 nearest upper water. Determine minimum height of fiU-up of 

 cement required. Assist in selecting proper cement for tempera- 

 ture and time requirements and resistance to corrosive waters. 



At this point several modern practices in casing cementing should be 

 considered, selected, and applied as the prior log study may indicate. 



In the past it was believed that cement slurry exerted a powerful 

 scavenging action on the walls of the bore, an action which would free it 

 from all mud cake and allow the cement to bond directly to the face of the 

 exposed formations. It is now recognized that this is not unusually true 

 and that, in fact, generally the flow of cement will not entirely rid the 

 formation face of mud accumulations. This fact led to the adoption of 

 mechanical abrading devices (called wall cleaners or scratchers) and cen- 

 tralizers to assist in completely surrounding the casing with cement and 

 allowing it to contact and adhere to the formation face. 



Two types of wall-cleaning devices are in general use. One requires 

 reciprocation of the casing in the bore to remove the mud cake. The other 

 cleans sections of the hole by rotation of the casing after it reaches the 

 selected shoe position. Both utilize spring-wire wickers in support members 

 set around the casing for the reciprocation type and axially along the cas- 

 ing for the rotation type. 



Both the above wall cleaners have certain advantages and disadvan- 

 tages in regard to operation and initial cost. Only a study of each should 

 guide the prospective user ; however, of one fact he can be assured : Mech- 

 anical wall cleaning at proper positions in the well bore, coupled with cen- 

 tralizers, will vastly improve the possible success of the casing-cementing 

 job. 



Location points of wall cleaners in relation to sections of hole to be 

 cleaned up, and therefore best protected, is a definite geological premise. 

 It is neither necessary nor economically practical to place wall cleaners and 

 centralizers indiscriminately on the full length of a casing string. Place 

 them where they have an opportunity to accomplish their intended work 

 and obtain the best results — for example across oil-water contacts and oil- 

 gas contacts; at the top and bottom of known productive or possible later 

 productive formations; between closely spaced zones which must be acid 

 treated, Hydrafrac treated, or separately produced; and contacts of and 

 across water zones which might cause excessive corrosion or infiltrate into 

 another zone. 



It is unnecessary and definitely uneconomical to apply abrading and 

 centralizing devices in thick nonproductive zones, or in sections having 

 excessively large hole diameters due to wash-out during drilling. 



Certain admixtures to the usual water-cement slurry offer added as- 

 surance to cementing success. Bentonite added to the cement is advan- 

 tageous because it prevents settling of the cement particles which cause 

 water-filled cavities to form behind the casing. Some increase in volume 



