Abstract
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.
Highlights
Gachsaran Formation is the cap rock of Asmari oil reservoir located in southwestern of Iran
On the plot of Yield Point (YP), rheological properties increase at contaminant concentrations of 5% and 7%, due to low pH values and accretion of trace clay in the mud at lower formation fluid concentration
Apparent viscosity was seen to be directly dependent on the concentration of the contaminants because all ion contents of the formation fluid were affected by trace amounts of clay in the mud, thereby reducing the pH value
Summary
Gachsaran Formation is the cap rock of Asmari oil reservoir located in southwestern of Iran. Member 7 consists of limestone, gray marl, and anhydrite. Member 6 involves a succession of anhydrite, limestone, red marl and rock salt. Member 5 is composed of an alternation between gray and red marls, anhydrite and thinly limestone layers. Member 4 contains thick salt layers alternating with anhydrite, gray marl, and seldom limestone. Member 2 comprises rock salt, gray marl, anhydrite, and laminated limestone. Member 1 is made up of five different evaporitic cycles of marl, anhydrite, bituminous shale, and limestone, with a typical width of 50 m. The latter member covers the Asmari reservoir in the southwest of Iran [6]. The width of the sedimentary formation varies across the area, as shown in
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