The problematic drilling environment of soft, balling, and swelling formations that are common in the Elk Hills Field of California required a novel design for the eccentric bits used for the hole-enlargement procedures. This new eccentric bit design met and exceeded the drilling requirements with an improved overall drilling performance of 32% increased penetration rate on average, reduced bottomhole assembly (BHA) damage through enhanced downhole tool life, and improved drilling curve cost benefits. The design also achieved the world record for the operator for the most footage drilled in 24 hours. This was achieved through better directional control, reduced bit balling, and reduced non productive time (NPT) due to an improved BHA design that mitigates the described drilling issues.
Given the directional and drilling application, the task was to create an eccentric hole-opening tool capable of superior directional control and cleaning capability in soft formations with known positive displacement motor (PDM) limitations. This required revisiting existing eccentric hole-enlargement requirements, including bi-center and two-piece eccentric technology, and implementing improved drilling features for increased performance. A value-driven tool was designed for use with motor applications in swelling formations. This assembly would meet and exceed the drilling requirements for the field.
The authors of this paper will review and discuss the design, development, and run history of this new eccentric bit and hole-enlargement tool. Run data performance improvements, time savings, and reduced BHA damage of existing one-piece and two-piece eccentric tools will be identified and compared with the performance of offset data for bi-center bits and two-piece eccentric tools. The application benefits discussed will include improved directional control, enhanced cleaning effectiveness, and enhanced pilot bit and hole-enlargement tool blade designs compared with that of current bi-center bits for this type of drilling environment.
Introduction
Ten years ago the operator began drilling the shallow oil zones (SOZ) in the Elk Hills Field in Kern County, west of Bakersfield, California (Fig. 1). The geological zones drilled are of Pleistocene, Pliocene, and Miocene ages, consisting of the Tulare, San Joaquin, Etchegoin, and Monterey formations. Many of the shales in this sequence are time and water sensitive due to a 10 to 35% illite/smectite clay content; hence, it is difficult to maintain gauge hole due to swelling from the water-based drilling fluid. In addition to the shallow wells, deeper wells were drilled through the Stevens sandstone (Fig. 2).
Because of the formation swelling, extensive backreaming and time were required to wipe the hole when using standard roller cone bits. Thus, eccentric technology using polycrystalline diamond compact (PDC) cutters was introduced, thereby reducing the number of wiper trips required. By eliminating wiper trips, the cost per foot was reduced by up to 42%. These savings were a direct result of the use of a new one-piece eccentric bit developed to work with a PDM in the soft formations while drilling vertically to a prescribed kick-off point and then building to high angles.
