Studying the Effect of Geological Formation and Formation Water on Drilling Fluid Performance (Case Study)

While drilling an oil well, the bit will penetrate different geological formations, and these formations will contaminate the drilling fluid either through rocky crumbs or formation water. The amount of alternation caused in the drilling fluid depends on the lithological type of the drilled formation and the percentage of ions in the formation water. Water is the main component of drilling mud, especially water-based mud, and it can be fresh water or salt water. One of the most important duties of the mud engineer when preparing drilling mud is to do a water test to measure the salinity: chloride (Cl^–) and calcium (Ca²⁺). This study relied on field information for oil well in southern Iraq, which was adopted as a model for this case study. The study concluded that the presence of some ions in drilling mud such as Cl, sodium (Na) and Ca either reduces or increases the rheological properties of the mud system and in turn affects the rate of penetration, and it could also lead to serious drilling problems. The results proved that one of the most important indicators of mud contamination is a rise in the proportions of magnesium (Mg), Ca, carbonate (H₂CO₃) bicarbonate (HCO₃), in addition to a low potential hydrogen (H). The rheology of the mud was also affected by contamination, depending on the type of ion polluting the drilling mud and the amount of its percentage. On the other hand, neglecting the treatment and monitoring of mud properties lead to serious problems. The results showed that adding silica dust contributed to treating drilling mud contaminated with calcium ions by 20%, as an increase in the value of the mud’s viscosity was observed after adding silica. When the concentration of sodium chloride (NaCl) was about 8% or less, an increase in the rheological properties of the clay was observed, and the opposite was observed at concentrations of 10% or more.