QUALITY CONTROL OF BENTONITE/CEMENT GROUTS FOR ENVIRONMENTAL USE

Beginning in the early 1930s, the petroleum industry used bentonite as an additive in Portland cement grouts. Advantages of bentonite/cement grouts include: lower slurry weight, increased slurry volume, reduced grout viscosity, lower heat generation during cement curing and reduced cost. Disadvantages include: lower strength, higher porosity and increased fracturing. Bentonite/cement grouts have recently been accepted in environmental and other near surface well applications, because of a misconception that the addition of bentonite to cement reduces the amount of cement shrinkage during curing and the lowered curing temperature is necessary to prevent collapse of PVC and other plastic well casing. In spite of the higher environmental and public safety risks associated with shallow drilling operations, most state laws do not include specifications for the application and mixing of bentonite/cement grouts and some states actually encourage their use. There are three methods, developed by the petroleum industry, to properly mix cement and bentonite: 1) dry mix the materials, 2) hydrate the cement first then add bentonite, and 3) a method patented by D.K. Smith. However, a common mixing procedure, used by many monitor well drillers, is to pre-hydrate the bentonite to produce a clay-slurry and then attempt to blend dry cement into the slurry. This procedure results in a hard, moist to dry cement “blob” that cannot be pumped without adding additional water. This wet bentonite/cement grout is unsuitable, because it has high porosity, high permeability, very low-strength and fractures easily. Unfortunately, freshly mixed wet grout appears similar to grout mixed by the other methods. A simple quality control procedure is required. Mud weight, measured in a driller's mud balance, provides a diagnostic means to determine the quality of a freshly mixed bentonite/cement grout. Over-hydrated bentonite/cement grout has a “mud weight” that is about 2 lbs/gal lighter than muds mixed following the petroleum standards. Determining the mud weight prior to placement provides a quality control procedure to ensure proper bentonite/cement grout mixes.