This paper outlines Qatar General Petroleum Corporation's use of polycrystalline diamond compact (PDC) bi-center bits, which are functionally a combined bit and undereamer that drills over-gauge hole beneath a restricted hole size. Two bicenter bit trials have been made by Qatar General Petroleum Corporation (QGPC) in 12 1/4" intermediate hole sections through cretaceous carbonate formations from the Halul to the Sulaiy in its onshore Dukhan field. The bi-center bits were run in an attempt to increase clearance between the 12 1/4" open hole and 11 3/4" couplings on the 10%" casing which had to be run and cemented through the section.


QGPC started its Arab "D" gas recycling project to recover condensates from the Arab "D" fractured limestone reservoir (fig. 1) in the Dukhan area. High pressure gas will be gathered from wells located around the periphery of the gas cap, to be processed in a central facility, compressed and re-injected into wells located along the crest of the formation. Plans are to inject gas at 55 mmscfd. The 5 1/2" × 5" tubing required to inject this large volume of gas, necessitated 7 5/8" × 7" and 10 %" × 9 5/8" casing sizes respectively. The 10%" 51 ppf, L-80 Buttress casing with 11%" couplings is run in 12 1/4" hole, where annulus clearance is very tight, especially in areas where the hole is gauge (12 1/4") or less (fig. 2). Repeated instances of held up and stuck pipe whilst attempting to run the 10%" casing through this size hole (fig. 3) instituted the practice of making a timely reaming trip (fig.4), prior to running the casing to insure it made it to bottom.

Conventional under reaming was considered. Although it is fairly reliable, numerous trips, downhole blade breakage and possible fishing jobs were predicted over the 3500' 12-1/4" section.

The 9 5/8" pilot × 12 1/4" pass through diameter × 13.5" drilled hole diameter bi-center bit (fig.5) (sometimes referred to as a speed reamer) was envisaged as a drilling tool which could pass through 13 3/8" OD, 12.459" ID, 54.5 ppf, K-55 surface casing and drill a 13.5" hole beneath it, thereby simultaneously solving the hole clearance and reaming problems.

Bi-Center Bit Technology

The first Bi-center bits were produced more than 20 years ago, but prior to 1994 their use was very limited due to problems with unusual or excessive wear, unpredictable steering tendencies and high reactive torque. In 1994, a new bi-center bit design was successfully tested in the Gulf of Mexico. This new technology that includes force balancing between the pilot and reamer sections using special stabilizing elements has been further refined during the last two years.

The bi-center bit is primarily composed of two parts, pilot bit and reamer section. The pilot is similar to a conventional drill bit and is used to centralize the tool when drilling ahead. The reamer is designed to have all of its cutting elements on one side of the tool and its function is to enlarge the pilot hole to the desired final hole diameter. This unique geometry that allows bi-center bits to pass through a small hole then drill a larger one (fig. 5), places some restrictions on BHA design and hole inclination control. Having all the cutters on one side of the reamer results in an extremely high force imbalance. Force imbalance refers to the situation where the vector sum of all cutting forces is not zero. All bits have at least some imbalance but a high force imbalance usually results in erratic, somewhat unpredictable directional performance.

It is difficult to properly stabilize the string above the reamer since a full gauge stabilizer within 30' would result in the bi-center bit not being able to pass through the restricted hole size. P. 53^

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