Oil-based muds have been used widely in Iranian oil fields to drill pay zones and shaly formations. However, their applications have become more restricted because of high initial cost and prices of handling. The other important aspect is the environmental concerns that become more stringent in Iran. Therefore, an environmental friendly mud replacement having a good performance has become Iranian oil companies' demand.

Water-based glycol muds seem to be good alternatives for oil-based muds. These environmental friendly muds can inhibit shaly formations, lubricate drill stem, clean the hole, and deposit a thin and impermeable filter cake which reduce the risk of differential pipe sticking and damage, respectively. Moreover, initial and handling expenses are reduced, and these lead to the reduction in the overall costs of drilling the well.

In this study, rheological property of the water-based glycol mud was assessed and compared with the oil-based one. Cutting integrity test was performed in order to evaluate the performance of shale inhibition. This test was conducted on bottom hole samples of Kazhdumi, Daryan, and Asmari, three different problematic shaly formations of Maroon oil field. Formation damage caused by water-based glycol and oil- based muds was measured and compared.

It was found that the water-based mud has a comparable rheological property with respect to oil-based mud. The cuttings recovery percentage was within the excellent range. Formation damage caused by water- based glycol mud was more than oil-based mud. The results show that this mud system is a very good alternative to oil-based muds in Iranian oil fields.

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