Bentonite is a naturally-occurring clay material, largely composed of the mineral, sodium mont-morillonite. The dominant property of bentonite is that it swells to many times its dry volume when hydrated by water, forming a viscous fluid with high gel strength. This makes bentonite an excellent product when used as a sealant, for preventing the influx of surface water into a well, sealing abandoned wells, or building slurry walls to contain contaminated groundwater.Bentonite is also frequently used as the base for drilling muds, including drilling fluids for horizontal wells. In this application, the characteristics that make it such a great sealant also make it a great choice for a drilling fluid, but bad for the ultimate well performance.The swelling property of bentonite is known as “yield.” Premium quality bentonite (usually from Wyo-ming) provides the best yield for a given weight of bentonite powder. When cost is the only concern, high yield is an excellent characteristic, since less powder makes a thicker mud. However, it takes time for bentonite to yield, from hours to days for the individual particles to fully swell to their maximum ex-pansion. Drilling operations seldom have the time to wait for days for mud to fully yield, so the bentonite is injected into the bore before it is fully hydrated. Since bentonite particles are colloidal in size; once mixed in water and introduced into a borehole, the bentonite particles can penetrate into the soil sur-rounding the borehole. Once there in the presence of water, the bentonite continues to swell, sealing off the borehole.