WorldWide Drilling Resource

20 DECEMBER 2016 WorldWide Drilling Resource ® Environmental Monitoring by Thomas Kwader, Ph.D., P.G. Be Careful - Don’t Collapse the Casing or Screen Although it doesn’t happen often, well casing and screens can collapse. Unfortunately, the repair is not easy either. Collapse is often due to a difference in pressure between the outside of the casing, and the inside of the casing. Although steel is relatively strong, the wall thickness and depth at which it is set, also factor in to the pressure differential. Also remember, for any given wall thickness (Schedule 40, SDR-21, etc.) the larger the diameter, the “weaker” the pipe, i.e., a one-inch pipe is more difficult to collapse than a 24-inch pipe of comparable wall thickness. Drilling through consolidated rock (limestone, shale, dolomite, etc.) is likely to maintain a rigid borehole and less likely to collapse, than a borehole drilled through unconsolidated sands and gravels. Loose sand and gravel can collapse; and the deeper the hole, the greater the pressure on the outside of the casing or screen. By the same token, heavy drilling mud or neat cement can exert a very high pressure on the outside of a casing and become an issue beyond a few hundred feet. Well screens are relatively weak and subject to collapse if set very deep in unconsolidated formations or when long sand/gravel packs are placed outside the screen. Also, in some cases, deeply screened wells in low-yielding aquifers which drawdown to deep depths, the drawdown inside the well/screen can create enough pressure differential on the outside to col- lapse the screen. Another scenario to consider is the grouting of PVC well casings. When neat Portland cement is mixed with water, an exothermic reaction occurs where heat is generated. Neat Portland cement with a thick cemented annulus (greater than 2 inches) can reach 160 F (71ºC) or more. If calcium chloride is added to accelerate the curing, the temperatures can reach 200 F (93ºC) or more, making the PVC more soft and pliable and can cause the PVC to become “oval” shaped which may hinder setting the pump. Tom Tom Kwader may be contacted via e-mail to michele@worldwidedrillingresource.com '&+ + & ')% +#'& & ' #&* "'& 1 / % #$ )' #&* +. ')! ... +!. ')! # $ " ! % # !& & ) '* % **0 ,#+ * '& ) & &+ ) & ) '* ! % # " % #! " # " ! +' - $'( ()'%'+ & ()'+ + / * !)',& . + ) I enjoy reading the Wide World Drilling Resource ® publication even though my company is not directly in the drilling business. I look forward to reading Tim Connor's articles. I am happy to see a publication in the industry that is far-reaching to indi- viduals who may also see these arti- cles and be positively influenced. Jennifer Chilcote