Laboratory data are presented showing that lime muds utilizing a polysaccharide deflocculant are very effective in combating dispersion of shale particles when compared to several commonly used types of water based muds. Similar studies using Wyoming bentonite particles are reported showing that potassium hydroxide results in less clay dispersion than sodium hydroxide when used for alkalinity control of a lime mud deflocculated with polysaccharide. While the potassium is polysaccharide. While the potassium is shown to be reactive with clay particles, the presence of lime in the mud leaves more potassium ion available to react With shale exposed in the wellbore and protect against borehole instability. protect against borehole instability. Field results are reported showing that potassium ion concentrations of 1,000 to 4,000 mg/L have been adequate in potassium/lime muds containing polysaccharide deflocculant to provide good polysaccharide deflocculant to provide good borehole stability and low mud maintenance costs.
Lime muds came into use about forty years ago in the Louisiana and Texas Gulf Coast and continued to be widely used for the next fifteen years. Popularity of the system was due mostly to Popularity of the system was due mostly to low gel strength and tolerance for formation solids - resulting in less water dilution and barite consumption when drilling abnormally pressured shales. Improved borehole stability was also noted. Principal components of the early lime muds were lime, caustic soda, and quebracho, a tannic acid extracted from tree wood and bark. Chromelignosulfonate subsequently replaced the quebracho. Starch and CMC were used for added filtration control.
Performance of the lime mud was generally attributed to calcium ion supplied by the lime that served to limit swelling and dispersion of shales. A disadvantage of the calcium ion was that it flocculated bentonite added to the mud and it lessened the efficiency of starch and CMC as filtrate reducers. The role of the caustic soda in the system, therefore, was two-fold. First, it neutralized and solubilized the acidic thinners, quebracho or chromelignosulfonate. Second, it limited the solubility of the lime and lessened the flocculation caused by calcium ion. As an undesirable effect, the caustic soda served as a source of sodium ion that could contribute to swelling and dispersion of shales.
As with lime and caustic soda, the thinner used in the earlier lime muds had both good and bad effects. Deflocculation of clay particles to reduce yield point and gel strength was desired. Not desired was the dispersion of shale aggregates that resulted in increased plastic viscosity of the mud and contributed to erosion of shale downhole. A major contribution to lime mud technology was the development of a polysaccharide deflocculant (PSD) that could be added with lime to lower yield point and gel strength without causing strong dispersion of shale.
More recently a second major improvement in lime mud was made.