Miscellaneous Subsurface Methods 749 



of the set cement is obtained. A better flow characteristic results. A lighter 

 weight slurry which is desirable in areas subject to loss of returns can be 

 produced. Less shattering of the set cement may occur during gun per- 

 forating. 



Various materials may be added to cement slurry to prevent loss of 

 circulation. Chopped cellophane flake is frequently used for this purpose. 

 Caution must be exercised with certain other lost circulation materials in 

 cement slurry as some organic fibers will eff^ect the pumpability and setting 

 times of the cement. 



Remedial cementing includes all type of jobs, other than casing ce- 

 menting, to correct difficulties encountered in drilling and completing wells. 

 Of greatest importance among such operations are squeeze cementing, plug 

 backs, lost returns, and liner jobs. These have as their more frequent pur- 

 poses the shutting off of water or excess gas, correction of lost circulation, 

 changing of hole depth, and lengthening of the cased section of the hole. 

 These features may be sought after either separately or in various combin- 

 ations at any one time. Geological data is always involved in connection 

 with all such work. 



Exclusion of bottom water which has come into the well by rise of 

 the water table is a comparatively simple procedure. In open hole, usually 

 a plug-back job is involved. This procedure of setting a plug of cement 

 slurry from bottom upward across the oil-water contact. Generally, this is 

 accomplished by pumping a quantity of cement to bottom through tubing 

 or drill pipe and spotting it in place by measured displacement with 

 mud or water. In soft formations the ordinary plug back does not always 

 have a very long life because water soon comes in around the cement plug. 

 This restricted longevity gave rise to the practice of putting pressure in 

 excess of the hydrostatic head of the fluid in the well on the liquid slurry 

 to force the slurry against the face of the formation and to dehydrate the 

 slurry against the formation, a procedure which developes an improved 

 bond. This practice was enlarged upon and is known as squeeze cementing 

 — the forcing of slurry out into, and into contact with, a formation under 

 high pump pressure. 



It is incorrect to assume that the solid particles of cement in a slurry 

 actually penetrate the interstitial spaces in a sand body. Actually these par- 

 ticles will not enter the pores of even a coarse sand for more than a frac- 

 tion of an inch. In spite of this fact a large amount of cement can be forced 

 out into a sand formation. This injection is due to the splitting of the for- 

 mation by subsurface pressure which exceeds the overburden load. It is 

 also true that a considerable portion of the mixing water is forced out of 

 the slurry into the permeable media with which it is in contact, and action 

 which creates a tight cement-to-formation bond and leaves a dense mass of 

 cement within the well bore or casing. 



Squeeze cementing is the most effective means now known for exclud- 

 ing water from production either from open hole or from production 



