The Bolontiku field is located offshore on the continental shelf of the Gulf of Mexico, adjacent to the coast of Tabasco state. This field is composed of dolomitized carbonates of the Upper Jurassic Kimmeridgian formations, which yields 39° API hydrocarbons. Exploitation has dropped the bottomhole pressures from 8,159 psi to 5,600 psi and has created an average operating drilling window of 0.07 g/cm3. Such a narrow operating window increases the technical difficulty for continued development in this mature field using conventional drilling techniques. The complexity of effectively controlling the wellbore pressure has resulted in an endless cycle of fluid loss to formation, kicks, and well control events that translate into non-productive time (NPT), which increased operating time and costs, potentially leading to well abandonment. A managed pressure drilling (MPD) technique allows for effective control of the pressure profile throughout the wellbore, identifying the bottomhole pressure (BHP) limits and applying appropriate backpressure accordingly. Owing to its efficiency, this technique has evolved from an innovative technology to become a required application to mitigate the inherent wellbore pressure problems associated with conventional drilling. Therefore, as MPD evolves, different approaches for well control evolve for kick events. This paper describes a well-control application simultaneous to the drilling operation using MPD with a closed-loop pressurized control system. This paper reviews a case history of two wells that were drilled with MPD and compares results against three wells that were conventionally drilled in the Bolontiku field. MPD and simultaneous well control allowed for drilling the Bolontiku 37 well, which consisted of compartmentalized pressure that historically lead to fluid losses and water influxes. Therefore, it was possible to drill through zones that before were not technically possible.

Summary 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. Introduction According to world level statistics, the borehole instability problems costs to drilling industrymore than 1 billion dollars a year. The average cost due to non productive times, resulting from, instability is about 1.5 million of dollars by well and in extreme cases can reach until 16 million in only one well. The borehole instability occurs when the borehole stresses or the interaction between the formation and the drilling fluid interact modifying in one way or in another the hole. The consequences of the instability are differential stuck pipe, excessive reaming time, circulation losses, fishing, sidetracking, inability to reach desired casing setting depth, poor logging and cementing jobs. To avoid the problems by borehole instability and to balance the interaction rock- fluid is the key factor drilling operations and requires fully understanding of the causes and processes that provoke it. The main objective of this project was employing a drilling fluid during the drilling process of the Luna 18 well, that permitted to detain the effects provoked by the rock fluid interaction based on studies accomplished with core and fluid samples by measuring three fundamental parameters: linear swelling, dispersion and capillary suction time. The capacity of inhibition, gotten upon carring out the tests with the most active formation samples permitted to establish a second objective: To get a drilling fluid as substitute of the inverse emulsion systems. The generated cuttings from the silicate-potassium system have a void toxicity level, therefore it is not necessary their treatment or confining. Thus, besides avoiding ecological damage, also are eliminated the costs by this concept. Finally, since the potassium nitrate is a product that favors the vegetables development, the generated cuttings are employed as fertilizing and one of the scopes of this project was to verify the feasibility of using of the cuttings with this purpose.