Introduction:Gachsaran Formation is the cap rock of Asmari oil reservoir located in southwestern of Iran. The formation consists of halite, anhydrite, and tachyhydrite, The most important feature of this formation is the presence of high-pressure fluid.Method:Drilling companies have to use heavy-weighted mud to drill the high-pressure formation. Sometimes the weight of drilling fluid is used, up to 2.65(gr/cm3). Although heavy-weight mud prevents formation fluid to flow into the well, it is difficult to maintain and control its properties. If the hydrostatic pressure is insufficient, the formation connate fluid penetrates into the drilling mud and contaminates it.Result:The study found that the symptoms of this contamination lead to an increase in calcium, magnesium, carbonates, and bicarbonates levels, as well as a decrease in pH. The drilling fluid rheology also affected by the contamination.Discussion:Then, method of curing this event is discussed. Prevention, the best treatment for this event was introduced. It was also found that, as soon as contamination signs appear, immediately increase the drilling fluid weight as much as possible, and then adjust the pH between 10.5 and 11.5. The maintain method described is continued until section drilling ends.
Investigation of Petrographic and Diagenetic Properties of Asmari Reservoir Cap Rock, SW Iran