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When control drilling interbedded formations due to lost circulation challenges, Bi-Center bits often fail to open the hole to the expected drill size. Possible causes include inconsistent downhole weight on bit due to formation changes, lateral and torsional vibration. Each leads to the pilot of the Bi-Center bit drilling an oversized hole which results in the bit drilling off-center, giving an undersized hole. To combat this, a patented double profile Bi-Center bit was developed. Its elongated pilot section includes a unique midreamer section which improves lateral stability due to more balanced cutter forces. Also, the mid-reamer provides an additional gauge section immediately below the reamer which in conjunction with the pilot gauge has proven to reduce tilt and keep the pilot bit centralized, providing more consistent full gauge hole. Additionally, vibrations generated by the drillstring are common during the hole-opening process. These affect not only the drilling performance but also the quality of the wellbore. A unique eccentric stabilizer, ideally located in tension in the BHA, provides vibration dampening in the drill-string. The V-Stab interrupts harmonic modes vibration as well as reducing the magnitude of shocks due to its unique geometry. Caliper logs from field trials of a 10–5/8 × 12-/14" CSDX6413S-B1 Bi-Center bit with a 10–5/8 × 12" vibration dampening tool demonstrate excellent hole opening performance and improved borehole quality. In contrast, logs for conventional Bi-Center bits run with the vibration dampening tool showed inconsistent hole opening and poor borehole quality in the same application. This paper describes the benefits of the unique combination of these new technologies, presenting detailed drilling performance data showing how they resulted in significant performance improvement and cost savings in southern Saudi Arabia. Introduction Lukoil Saudi Arabia drills some challenging exploration wells in southern Saudi Arabia. These wells have encountered several difficulties involving severe borehole stability problems. On the well "D", the 13–3/8 casing was planned to be set on top of Sulaiy Formation at around 9,258 ft. However, the 16" phase was unable to reach this depth, having to be stopped at a much shallower depth of 7,625 ft because of lost circulation problems Additionally, the 13–3/8" casing could not be run to 16" hole total depth due to restriction in the hole. The 13–3/8" casing was set at 6,865ft and the rest of the 16" open hole was finally covered by 11–3/4" expandable casing that was set at 7,617ft to ensure that the 9–5/8" casing setting depth could be reached as per the drilling program Fig.1. Since the 11–3/4" expandable casing has 11.286" drift ID after expansion, a hole opening tool was required to open the hole from the casing pass through size to 12–1/4" hole size, to allow the 9–5/8" casing to be set properly.
When control drilling interbedded formations due to lost circulation challenges, Bi-Center bits often fail to open the hole to the expected drill size. Possible causes include inconsistent downhole weight on bit due to formation changes, lateral and torsional vibration. Each leads to the pilot of the Bi-Center bit drilling an oversized hole which results in the bit drilling off-center, giving an undersized hole. To combat this, a patented double profile Bi-Center bit was developed. Its elongated pilot section includes a unique midreamer section which improves lateral stability due to more balanced cutter forces. Also, the mid-reamer provides an additional gauge section immediately below the reamer which in conjunction with the pilot gauge has proven to reduce tilt and keep the pilot bit centralized, providing more consistent full gauge hole. Additionally, vibrations generated by the drillstring are common during the hole-opening process. These affect not only the drilling performance but also the quality of the wellbore. A unique eccentric stabilizer, ideally located in tension in the BHA, provides vibration dampening in the drill-string. The V-Stab interrupts harmonic modes vibration as well as reducing the magnitude of shocks due to its unique geometry. Caliper logs from field trials of a 10–5/8 × 12-/14" CSDX6413S-B1 Bi-Center bit with a 10–5/8 × 12" vibration dampening tool demonstrate excellent hole opening performance and improved borehole quality. In contrast, logs for conventional Bi-Center bits run with the vibration dampening tool showed inconsistent hole opening and poor borehole quality in the same application. This paper describes the benefits of the unique combination of these new technologies, presenting detailed drilling performance data showing how they resulted in significant performance improvement and cost savings in southern Saudi Arabia. Introduction Lukoil Saudi Arabia drills some challenging exploration wells in southern Saudi Arabia. These wells have encountered several difficulties involving severe borehole stability problems. On the well "D", the 13–3/8 casing was planned to be set on top of Sulaiy Formation at around 9,258 ft. However, the 16" phase was unable to reach this depth, having to be stopped at a much shallower depth of 7,625 ft because of lost circulation problems Additionally, the 13–3/8" casing could not be run to 16" hole total depth due to restriction in the hole. The 13–3/8" casing was set at 6,865ft and the rest of the 16" open hole was finally covered by 11–3/4" expandable casing that was set at 7,617ft to ensure that the 9–5/8" casing setting depth could be reached as per the drilling program Fig.1. Since the 11–3/4" expandable casing has 11.286" drift ID after expansion, a hole opening tool was required to open the hole from the casing pass through size to 12–1/4" hole size, to allow the 9–5/8" casing to be set properly.
Technological improvements of drilling and reaming methods continue to be evaluated and introduced to the drilling industry. This paper describes recent, controlled testing of new expandable concentric stabilizers and reamers, performed on a full-scale, highly instrumented drill rig. An inherent problem of drilling and reaming concurrently is that conventional fixed stabilizers run above expandable reamers can be no larger than the pass-through diameter of the restriction above it and thus cannot effectively stabilize the upper BHA, which often results in undesirable vibrations. However, recent controlled tests have been conducted in twin wells drilled from the same casing under a full-scale drilling rig, one well with only a concentric expandable reamer and the other with both expandable stabilizer and reamer. The testing has shown that utilization of this novel stabilizer produces significant gains in performance. BHA modeling predicted lower bending moments above the reamer when a concentric stabilizer was utilized. The well drilled with the stabilizer above the reamer resulted in higher ROP with lower downhole WOB and up to 35% reduction in drilling mechanical specific energy (MSE), as compared to the well drilled without the expandable stabilizer. The stabilized well had significantly better drilling efficiency, which is attributed to reducing buckling and whirl in the drill pipe and upper BHA, and reduced frictional losses against the borehole wall. Additionally, lower levels of whirl, lateral and stick-slip vibrations were recorded with the new expandable stabilizer. The paper describes the novel design features of this expandable stabilizer, which are credited with the step-change improvement in drilling efficiency. Monetary savings from increased drilling efficiencies and improved reliability are anticipated for operators and will be discussed in this paper. Introduction The following three passages from previously written technical papers introduce the subject of this paper and the inherent problem of stabilizing the BHA and collars above the reamer in the enlarged borehole. "Historically, concentric underreaming has been plagued by several challenges. Some reamers, especially earlier models used after drilling a hole, were not rugged and failed downhole, leaving parts behind to fish out. Some current industry concentric reamers have complex functionality, requiring a fine balance of WOB, flow, and pressure drops in order to activate and operate properly. Some have multiple sliding mandrels with close tolerances that have problems closing after use due to accumulation of solids. Some reamers require lower flow rates while drilling out the casing plug and then a higher flow rate to activate the reamer, sometimes failing to open and remaining closed the entire run. Some reamers utilize hydraulic pistons for cutter blades that are difficult to close and pull into casing after drilling with water-based mud. These hydraulic reamers had problems with blades closing under reaming of high-angle holes due to weight of the BHA. Some of the reamers in the market have had small and less-effective cutter blocks that only allowed a few PDC cutters, reducing their life and cutting efficiency." (Radford et al. 2008)
Historically hole-opening and underreaming operations have been restricted to softer formations. This paper describes what is believed to be the first successful drilling and underreaming of a very hard, chert-rich formation.Many producing wells have been drilled into the deep Cretaceous formations of south Mexico. These consist primarily of hard limestones containing dolomite stringers, large chert nodules, with very erratic hardness and some natural fractures; UCS estimates range from 17,000 to 30,000 psi. On average it takes three TCI bits and two or three PDC bits to drill the Cretaceous interval, which is typically from 4,800 m to 5,400 m TVD.After several unsuccessful attempts to drill exploratory wells into Jurassic formations below the Cretaceous, the operator redesigned these wells with a contingency casing string. Rig hoisting limitations required each hole interval between conductor and the Jurassic to be underreamed -larger diameter casing strings could not be handled safely. This meant drilling the Cretaceous formation in 8 ½-in. hole and underreaming to 12-in. diameter in order to accommodate the desired 9 5/8-in. casing.A new eccentric fixed-blade reamer and pilot PDC bit were designed for this application, using latest technology cutters, and profiles, cutter layouts and backrakes, which provided the highest possible durability and thereby gave the best chance of withstanding the high, erratic rock hardness and chert nodules.The entire Cretaceous interval was successfully drilled and underreamed in a total of five runs. In each case, the assembly was tripped for logging, coring or section TD, with the pilot bit and reamer in good condition. Logging confirmed the desired 12-in. diameter hole. This case history confirms the ability to simultaneously drill and underream hard, chert-rich formations, in this instance greatly reducing the operator's risk of failing to reach their target and giving a wider range of completion options.
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