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Slide 16

Sometimes there can be “voids” (think of caves) in the ground.  In that case the cement instead of filling up the annular space, fills up the void, and does not make it to the surface.  The annular space is not filled with cement.  This is not uncommon.  And it can cause problems to occur. 

If the driller has to pass through a mined out coal seam while drilling down to the gas bearing formation there will be a huge "void" problem.  It is probably larger than a naturally occurring void, but the driller should know about it in advance.  The solution is not all that relaible.  A larger hole is drilled down through the void.  A larger metal casing is fitted into that larger hole.  However it is not possible to insert cement between the outside of the casing and the inside of by hole (the "annular space") by pushing it down the center of the casing and then back up into the annular space from the bottom up to the surface ("circulating"), as is usually done.  That is because the cement will get as high as the void and then flow into the void instead of on up the annular space to cement in the surface ground water formations.  Instead of "circulating" the cement through the groundwater formations, the driller places a "basket" on the larger casing at a depth above the mined out coal seam.  The driller then squirts cement down into the annular space from the top using a hose or "tube".  Hopefully  the cement will drop all the way to the basket and completely fill up the annular space from the basket to the surface.  However, this method relies on gravity which is less than the pumping pressure of circulating cement from the bottom.  In addition there is no good way to confirm that the cement filled the annular space through all of the groundwater strata before it hardened.  This is called "grouting".

If a cave is the problem, the driller might know there is a cave as they drill down through and the drill bit suddenly drops a few feet instead of drilling steadily down (if they notice). If they notice they should put the larger metal casing mentioned above (sometimes called a sleeve). But if they do not notice, the cement can disappear into the cave instead of cementing the pipe the rest of the way to the surface. The driller will have to try the unreliable grouting process. This is a particular problem in Karst formations where there are many caves, small and large.