A special form of clay or other material is used for the next layer of plugging the borehole. This clay is often bentonite clay, a clay that expands as it soaks up water. It is very similar to kitty litter. It has other additives sometimes to do the best job. Probably because it is cheaper than cementing all the way to the top, most of the vertical height of the plug in the borehole is clay as we shall see in the next slides. It is shown as yellow in these slides.
After this next clay layer, a new cement bridge is then the next layer on top of the clay in the plugging process.
This slide show is simplified and shows the next cement bridge as starting below and going up through the coal seam. As a later slide will show, often there are other horizontal gas and other formations between the bridge at the top of the gas producing zone and bridge at the bottom of the coal seam. And those should have cement bridges above and below, or even through, again with bentonite clay making the layer below and above each cement bridge.