In deep well drilling operations, borehole stability causes severe problems that requires excessive rig time to solve them. The not productive times are about the thirty percent of the total drilling time. The wells located at the south region of Mexico are characterized by being of high pressure and high temperature with surface and intermediate length zones (3500 meters approximately) formed by sands and clays. The incidence of this problem is explained by the rock-fluid interaction "mud - rock". An immediate solution has been the use of oil based muds; however, sandy layers present another problems. An additional problem is the strong ecological restrictions due to Well's geographical location. From three years ago, Pemex has studied the phenomenon through a project called "Improvement in the compatibility drilling fluid-formation" obtaining significative advances.

According to the importance of having a water based fluid that faces conditions, it was necessary to develop a drilling fluid system that could be able to maintain, by an extended period of time, the stability in the clays. From vast laboratory studies to core samples, the Luna field was selected to use a polymeric potassium-silicate drilling fluid. As a result, it was obtained a drilling fluid that maintained an excellent borehole stability during 70 days. This was the best result compared with other systems used in the area. It is of equal importance the fact that the fluid is environmentally friendly.

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