Abstract

Most of the onshore oil fields in south Mexico are characterized by complex geology in Tertiary and Mesozoic rocks. This paper presents a field case where a well was drilled in a very highly fractured and faulted zone with a nonproductive time due to wellbore instability issues of 182 hours and a high potential to execute a sidetrack. A 1D Mechanical Earth Model was built, followed up with Logging While Drilling tools, cavings morphology monitoring and 3D caliper analysis. Once the main failure mechanism was identified as planes of weakness, was recommended to maintain the mud density and improve the mud sealing capacity to reduce the invasion and pressure diffusion into the fractured shale. After mitigation measures were implemented was possible to run in hole to continue drilling up to the 7 inches casing point at 5250 meters. Logs were run, and it was evaluated the borehole shape that allowed to prove the effectiveness of proper mitigation measures, avoiding continued mechanical degradation of the borehole stability in the overlying section.

1. INTRODUCTION

Many of the oil fields onshore southern Mexico are characterized by complex geology in Tertiary and Mesozoic rocks. This require the drilling of high complexity wells through highly faulted and fractured zones, overpressured sections and depleted reservoirs. The presence of tight hole, torque and drag, excessive cavings production and stuck pipe events are typical. Therefore the drilling processes present nonproductive time (NPT) due to additional wiper trips, excessive reaming and back reaming operations and undesirable side tracks with high impact in the final operation costs.

This is a field case where a well was drilled in a very highly fracture and faulted zone, which after drilled to 4500 meters and pulled out of hole for a wiper trip, was very difficult to reach the bottom hole due to several high torques, tight hole events and stuck pipe attempts in the interval from 4300 to 4000 meters. The NPT due to wellbore instability issues, reached 182 hours with an operation cost of USD 132,000 and high potential of execute a sidetrack.

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