Minimizing the Effects of Cement Slurry Bleed-Water on Water Quality Samples

Abstract

Some groundwater monitor wells produce water quality samples with anomalously high pH measurements. In some of these wells it is obvious that these water quality samples are affected by the bleed-water from the cement used to seal the annuli. To gain an understanding as to why cement bleed -water occurs and how it can be controlled, literature from both the cement and petroleum industries are reviewed. Cement is a very alkaline material. When too much water is used to prepare the slurry, alkaline bleed -water can drain through or along the cement sheath surrounding the casing. This results in an increase in the pH measurements of groundwater samples. This bleed-water can separate from the cement in-three ways: it can move into the formation during cementing, it can accumulate within the cement forming pockets and channels behind the casing, and it can remain within the interconnected capillaries that exist throughout the cement sheath. The drainage of alkaline bleed -water from the cement can be greatly reduced by controlling the amount of water used in the preparation of the slurry. The amount of water added can be monitored during well construction by measuring the slurry density. By implementing this quality control procedure during well construction along with specifying the correct amount of mix-water for the slurry, the elevated pH levels in groundwater samples should be greatly reduced if not completely eliminated.