Drilling Across a Highly Unstable Zone in an Exploratory Well in South Mexico

Abstract

Abstract The Comalcalco Fault is a regional discontinuity that can present highly unstable zones when crossing it or its influence zone. This instability can lead to notorious increase in drag forces, cavings, stuck pipe events and ultimately loss of sections or the well. In the 12 ¼" hole of this exploratory well, this fault was present and despite some of the wells in the field managed to cross it without consequences, the influence was more notorious, and the problems lead to catastrophic results. While drilling the 12 ¼" section, the team was able to drill up to 4368 m without any relevant problems. The fault was intersected and drilled pass to section TD with the previous casing shoe at 2966 m. However, when a trip to surface was necessary, abnormal drag forces that avoided further movement of the string were experienced. This event led to the loss of the section and the need to perform a sidetrack, right across the influence zone of the fault at 3200 m. The risk was reassessed, and numerous preventive and mitigation measures were put in place, including drilling the section with a 12 ¼" pilot bit, enlarging the pilot hole to 13 ½" to cover the influence zone and having a contingency plan to run the 11 ¾" liner. Additionally, the formations were reinforced through drilling fluids design, and drilling and tripping strategies were optimized to minimize zonal invasion and influence zone disturbances that could lead to additional cavings. After the successful sidetrack, the section was drilled to 3600 m, 200 m below the fault’s influence zone. Good ROPs and low levels of stick and slip were observed despite several reaming runs while drilling. The contingency liner was run to TD, achieving the objective of isolating the influence zone of the Comalcalco Fault. With the problematic zone placed behind pipes, drilling the remaining of the original 12 ¼" sections with a 10 5/8" × 12 ¼" reamer resulted in the longest and fastest enlarged section to be realized in the field. With all the preventive measures put in place, the original 9 5/8" casing was set at its planned depth, avoiding an additional casing string and a change of the design of the completion.