Drilling activity has increased rapidly in Iraq over the last few years. With the lack of reliable downhole data to develop a solid strategy to optimize drilling operations, the drilling performance has been inconsistent with frequent failures of drill stem components. Without the aid of quality downhole data it is very difficult to identify the root cause of these failures, and more importantly, provide a means to identify a solution to improve performance. This paper showcases a project in South Iraq where modeling of the drilling BHA's using a unique Finite Element Analysis software package identified critical natural frequencies of the drillstring. The integration of high frequency downhole vibration data collected in multiple positions of the string provided the evidence to validate the vibration road map delivered by the BHA analysis.The objective of the project was to identify the primary vibration mode, evaluate the vibrations severity, cause and source in the 12 !" vertical section and provide recommendations to achieve performance improvement through controlled step changes in bit & BHA design, drilling parameters and operational procedures. The pre-well BHA model analysis identified that the premature bit failure was a direct result of lateral vibrations induced by operating in a critical lateral harmonic zone. This was validated with the downhole vibration data.A well-established optimization process including expert detailed analysis of the high frequency downhole vibration data provided a clear understanding of the relationship between the pre-well BHA model, the drilling parameters planed and the associated downhole drilling dynamics.The impact on performance was clear. Implementation of the new recommended drilling parameters derived from the BHA analysis applied to the same bit and BHA used in Well #1 resulted on a 26% increase in drilling performance in well 2. 2-IntroductionThere are multiple drillstring dynamics modeling software packages available in the industry that enable any drilling team to do a pre-well analysis of each BHA design and model the downhole behavior of the system. Most these packages provide the similar approach of using Finite Element Analysis of the entire string to calculate the combination of drilling parameters based on a proven, scientific approach that will most likely initiate downhole vibration and high impact loading that leads to premature bit and/or downhole tool failures.It should be a requirement to run such modeling analysis for every BHA for two main reasons. First it allows the drilling engineer to twick the BHA design to minimize the impact of the natural resonant frequencies on the system performance. And second, and in case the BHA design remains the same, it ensures that the drilling parameter combination selected (weight and surface RPM) will not initiate disruptive harmonics generated from the natural frequencies of the system itself.

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.

Drilling challenges in the Hassi Messaoud (HMD) oilfield have proven to be equal to the magnitude of its production. Historically, wells drilled in this field were vertical, moderate borehole diameters having a long 12 ¼"section. In order to increase production and to gain a better understanding of the reservoir, Sonatrach Drilling engineering moved to moderately deviated large diameter boreholes, delivering 1000 meters of drain in the reservoir. Instead of the conventional 12 1/4’’ vertical section, 1800 meters was drilled in 16’’ diameter. This section consists of abrasive sands, inter-bedded evaporate/carbonate and sand/claystone formations prone to instability and collapse. In addition, there is a 30 meter band of extremely hard dolomite. Rock strengths vary widely from 5 Kpsi to 30Kpsi. Since early 2000, Sonatrach and National Oilwell Varco (NOV) have been working as a team to rise to the challenges presented by the larger hole size. Most significantly drilling vibrations, slow penetration rates, increased trips to TD, severe bit and BHA damage and NPT due to stuck pipe while pulling out-of-hole. This article describes first, the process by which significant performance improvement was achieved as a result of combining innovative bit design techniques and technologies, BHA optimization, optimization of parameters through BHA dynamics modelling, and formation Rock Strength Analysis. Secondly, the article discusses optimized techniques introduced to enhance performance for the 16’’ section on rigs that have been recently equipped with VFD - AC top drives and properly deal with torque limitation issues. This enabled Sonatrach to repeatedly set ROP drilling records as well as consistently making section TD in a single run. The net result was a 37% improvement in rate of penetration1.