Backing Off a Free Drillstring: Planning and Execution on a World-Class ERD Well

Abstract

Since the inception of horizontal and Extended Reach wells, operators have faced the deviation-related complications that arise when dealing with stuck pipe and pipe recovery operations. Drillstring washouts in particular create a problematic situation in an ER drilling operation, as it necessary to spend extensive time circulating and rotating at high rates to sufficiently clean a high angle wellbore to a level that allows a safe and trouble free trip1,2,3. When a washout occurs prior to the completion of the cleanup process (which they almost always do), the operator is faced with a difficult choice:continue to circulate until the hole is clean, running the risk of enlarging the washout and potentially creating a twist-off, orattempt to pull out of a dirty hole, which may (and often does) result in tight hole and/or stuck pipe. A case history is presented where a drillstring washout occurred on a long extended reach well prior to the completion of the cleanup process. Attempts to trip the string proved futile, and it became apparent that the string would probably become stuck during further attempts to pull out of the hole. A modified version of the traditional backoff technique was safely used to separate the pipe below the washout without the drillstring actually being stuck. The entire string was subsequently recovered, saving >13MM NOK (2MM USD) in lost time and equipment. This is believed to be the first ever successful backoff of a free drillstring. Introduction High angle and extended reach wells present many technical challenges, most of which require "unconventional" solutions as compared to vertical or low-angle wells6. Fishing operations are no exception. Often, the most favorable option for dealing with stuck pipe in vertical wells is to perform a backoff of the drillstring. The primary advantage of this time-proven technique is that it allows the drillstring to be separated from the stuck section and reconnected with full mechanical integrity. However, in order for a backoff to be a viable option in a vertical well, the drillstring must be rotationally stuck at some point in the well. In unique circumstances, it is desirable to disconnect from the drillstring with the intention of reconnecting at a later time. The most common instance in high angle wells is an untimely drillstring washout prior to cleanup of a "dirty" hole. Unlike vertical wells, the drillstring does not necessarily need to be stuck in a highly deviated well for the backoff technique to be feasible. By applying detailed torque and drag analysis and "energy isolation" measures, fit-for-purpose procedures can be developed to safely backoff a drillstring that is not stuck. High Angle Fishing Challenges To understand the logic behind why a backoff is more favorable than "conventional" disconnection methods, one must first explore the difficulties that arise in high angle fishing operations. These difficulties can be separated into two categories: Separation The first step in separating stuck pipe is to identify where the pipe is stuck. In situations where circulation is possible, pumping down diagnostic and severing charges is a common solution. In the unfortunate circumstance that circulation cannot be established, wireline tractor technology is required for conveyance4 in wells above ±70° inclination. For higher tangent angles (75°-80°), coil tubing intervention also requires tractor technology due to helical buckling. Once the stuck point is determined, the same sophisticated conveyance methods required for detection are also necessary for severing. In these environments, the only separation technique that does not require intervention is the "blind backoff"5. Unfortunately, the blind backoff technique is quite imprecise (accidental backoff of a connection above the intended location), and requires that the pipe be stuck (in vertical and high angle wells).