*Corresponding author: firstname.lastname@example.org
In Kupal oil field, drilling shaly and marly formations (such as Pabdeh and Gurpi) face some challenges such as stuck pipe, tight holes, increased drilling time and cost and in some cases unprecedented hole deviation. Water based drilling muds can be proper substitute for oil based drilling muds. Therefore, mud parameters were studied in this research and swelling of the shale samples were investigated both for water based and oil based muds using the swelling-meter apparatus. Using the experiemental results, water-based muds containing starch and ASAFASOOL showed to reduce the swelling for 11.5%; thus, theywere selected as well stabilizin the chaly-including compounds in Pabdeh and Gurpi formations. The Potassium ions were identified as the main reason for controlling shale swelling.
Keyword:Water Based Muds, Shale Stabilization, Optimal Mud, Pabdeh and Gurpi Formations, Kupal Oil Field
 A. Bushehri, A. Raki, A study of shale behavior in drilling operations, Report No. 4587, Published in National Iranian South Oil Company, 1995.
 J. Down, Formate technical manual, Part B, Compatibilities and interactions, Section, B12, Compatibility with shale, by Cabot Specialty Fluid, 2008.
 E. Van Oort, On the physical and chemical stability of shales, Journal of Petroleum Science and Engineering, 38, p. 213–235, 2003.
 H. Tabatabaei, A. Motamed, Identification of type of sedimentary mineral and interpretation of the sedimentary basin considering the gamma ray spectrometry, Journal of the Earth , Vol. 2, No. 3, p. 17-24, 2007.
 M. Malayeri, R. Matourian, Drilling fluids, Published in Iranian Offshore Oil Company, 2009.
 T. Hamida, E. Kuru, M. Pickard ,“Rheological characteristics of aqueous waxy hull-lessbarley (WHB) solutions”, 2009.
. Y. Saki, N. Dinarvand, B. Habibnia, K. Shahbazi, Experimental Investigation of Possibility of Replacing Oil- Based Muds with Environmentally Friendly Water-Based Glycol Muds in Maroon Oil Field. Paper SPE 132769, 2010.
 M. Abdollahi, A. Varamesh, M. Rekabdar, & A.R. Nasiri, “Synthesis and use of amphoteric carboxymethylcellulose graft copolymers in the environmentally-acceptable water-based drillingf luids as a water sensitive shale stabilizer”, Iranian Journal of Polymer Science and Technology, Vol. 22, No.6, pp., 2010.