Drilling sludge is a mixture of crushed rock, wastewater, oil reservoir fluids and drilling mud residues containing lime, potassium and soda ash. Due to the large amount of water-soluble salts and high alkalinity, drilling sludge poses hazard to the environment. Its toxicity can be reduced via chemical reagents that displace sodium ions and thereby reduce salt concentration.
One of such reagents is gypsum, a white sedimentary material which is common and has been used in construction since ancient times. In order to test its efficiency in reducing salt concentrations, the scientists from Perm Polytechnic University used several sludge samples, on which different types of drilling fluids were used. They were mixed in certain proportions with gypsum, after which an ion-exchange reaction occurred and sodium cations were replaced with calcium cations.
The greatest efficiency was observed at a gypsum concentration of 15%–20%: the content of phosphate ions was reduced by half, and the content of carbonate ions was lower than under other experimental conditions. The results confirmed the mathematical model that was developed at the start of the study and took into account uncontrollable factors of salt dissolution, including density, granulometric composition and filtration coefficient of drilling sludge.
“Based on the results, we can classify the soil samples as non-saline (content of readily soluble salts below 0.5%) and slightly saline (0.5%–1.0%). The use of the reagent significantly increases the water permeability of drilling waste, improving its filtration properties,” Elena Gaevaya, candidate of biological sciences, is quoted as saying by Perm Polytechnic University.
