Proper planning and front-end engineering, in a Biphasic MPD operation to drill a high temperature depleted reservoir in Mexico south area, demonstrated the real added value of the Constant Bottom Hole Pressure (CBHP) technique to overcome risks associated with a nuisance gas zone, hole cleaning and geo-mechanic issues which caused the loss of the original 5 7/8?? hole by failure of the BHA during a pipe work operation in tight hole.

Annular velocity is considered as the most critical factor affecting hole cleaning condition in a high angle well in the absence of pipe rotation and cuttings bed deposition is an escalating problem to be overcome too. This situation is aggravated when a biphasic system is used, because density, phase and viscosity change with pressure and temperature in multiphase flows. Also, flow regimes in multiphase flow are more complex, and do not lend themselves to simple characterizations such as laminar and turbulent flow and they are dependent upon the angle of inclination as well as the phase behavior of the fluids. On this way, the velocity of the fluids, the temperature of the surrounding formation, geometry, inlet temperature, specific heat of the fluids and surrounding material among others impact the temperature profile.

A complete MPD program that included flow chart decision trees associated with all lessons learnt from the original hole was performed, to improve hole condition through operational practices but also to handle gasification while drilling and tripping. In addition to this, a Hazard Analysis and Risk Control (HARC) was done as well as all contingency plans to mitigate and/or avoid these high potential problems.

Thanks to an elaborated MPD program, state-of-the-art equipment, constant adaptation of the fluctuating conditions of the open hole and experienced decision makers, the side track using low head MPD was successfully performed to drill and complete an un-drillable deeper well with minimum operational problems, increasing the production of this mature field.

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