Converting Power to Performance: Gulf of Mexico Examples of an Optimization Workflow for Bit Selection, Drilling System Design and Operation

Abstract

The high cost of deepwater operations places constant focus on the time spent on hole-making and non-hole-making activities. Substantial work has been done to minimize non-productive time, and recently the time spent making hole has received renewed focus. This paper provides an example workflow for the latter for two large hole sections in a recent deepwater well in the Gulf of Mexico (GOM). The relationships between mechanical and hydraulic power input and rate of penetration (ROP) are well known, and recent publications have provided examples of very high ROP in large hole sizes drilled through sediments and salt in GOM. Optimization efforts have driven improvements in bits, directional drilling systems and drillstring components to the point that the performance limiter is the rig equipment; for example, solids handling equipment and top drive power. Top drives on some deepwater rigs deliver 2,600 horsepower (HP), or 55 klbf-ft of torque at 250 RPM. Fully exploiting this has yielded ROP over 350 ft/hr in 16½-in. salt sections. The rig that drilled the well in this study utilized a smaller top drive capable of delivering approximately 1,150 HP. The challenge was to optimize bit, drilling system, drillstring design and operating parameters to deliver the highest possible ROP given the available horsepower. The goal was to make cuttings handling the active limiter. A key consideration for bit design was efficiency; that is, the ability to convert power into volume of rock removed. Equally important were lateral and torsional stability, which affect power wasted while drilling oversized, rugose hole and peak torque for a given mean torque, and thus the window of available operating parameters. These considerations led to selection of advanced hybrid PDC-TCI bits. The desire for minimal torque and drag and precise directional control through sediments and salt led to the selection of rotary steerable technology with continuous proportional steering. A simple model linking drilling parameters and rate of penetration was developed based on offset data and used to establish expected weights on bit and rotary speeds for optimal bit performance. Stabilization of the bottom hole assembly was optimized considering operating parameters and application requirements. The drillstring design maximized torsional stiffness and capacity so the torque and rotary speed could be adjusted as the well transitioned from vertical to directional. The detailed planning and system specification yielded excellent performance. The 26-in. and 16½-in. hole sections were drilled in single runs, on target and with minimal vibration. ROP in the 26-in. interval was 15% faster than the previous best despite drilling a section nearly twice as long. ROP in the 16½-in. section was 38% faster than the rig's previous best. Reductions in drilling time contributed $1M in savings.