Channeling behind casing connecting two sandstone reservoirs in well 13 was suspected due to poor cement job, possibly due to the high angle well of 76 deg. Avoiding communication behind casing between two sands is detrimental for field reservoir management where reservoir pressure maintenance with water injector wells is paramount for continuous production. This paper describes the treatment background, engineering approach, laboratory testing and QA/QC procedures, fluid design stage and job execution using the propriety low viscosity polymer system to seal off channeling behind casing. Cement bond log performed in well 13, a 76 deg. directional well drilled offshore Peninsular Malaysia showed very poor bond behind casing. An injectivity test conducted by setting a retrievable packer in between perforation intervals of the two sand bodies near the suspected channeling confirmed communication between the two sands. Repair alternatives were evaluated opting between cement or polymer gel squeeze. Hydraulic calculation based on the injectivity test result, roughly set the equivalent channel diameter as 0.15 in. Cement squeeze was therefore rejected in view of the small and long cement channel of 109 m. An alternative method to squeeze a low viscosity polymer system into the channel behind casing was hence designed for the purpose of sealing off the channel. The procedure developed was to create a single perforation in between the two perforations in both reservoirs and squeezing the polymer. A retrievable bridge plug and a retrievable packer straddled the squeeze perforation interval and a polymer gel squeezed through the said perforation. After several squeezes each followed by a curing time, pressure tight seal isolating the two reservoir sands was obtained. This was confirmed by setting a retrievable packer above the lower most perforation in the reservoir sand followed by injecting brine while monitoring for returns through the upper perforation, which were none. This unique method, never applied before, to repair a 109 m continuous cement channel between two reservoir sands separated by a thick shale layer using cross linked gel was successful. Two years later the seal is still intact with production from this dual completion well continuing trouble free. The proprietary gel applied is a cross linked low viscosity polymer which cures under downhole temperature to form a tough seal. It is learned that running a retrievable bridge plug and a retrievable packer in tandem in high angle well best not be attempted in future, instead, both should be run separately. Acidizing through the squeeze perforation will assist to improve squeeze pressure. Application of cross linked gel to repair cement channel has been proven to be a viable alternative to cement squeeze.
Torsional vibration (also known as stick and slip) is a major contributor to equipment failures and severe damage when drilling the 6 1/8-in. lateral limestone Shuaiba reservoir section in PDO North Oil fields. This paper examines multiple factors that can affect the severity of stick and slip and measures their actual impact. These factors include bit/bottomhole assembly (BHA) design and formation/mud properties. The effect of a software plugin to an automated drilling system that was designed to mitigate the effects of stick and slip was also examined. Initially, drilling dynamics data available for the lateral Shuaiba reservoir were analyzed to evaluate the levels of torsional vibration. Several proposed design changes to reduce the torsional vibration were then modeled separately using finite element analysis (FEA) to predict their dynamic behavior. Trials were conducted, and the impact of independently changing each factor in the overall torsional vibration was assessed. Data were collected from over 40 horizontal wells drilled in the same reservoir. In each set of trials, identical drilling conditions were maintained while changing a single factor. The analyzed legacy set of well data showed high levels of torsional vibration (stick and slip) in the lateral section for different fields that share nearly the same reservoir characteristics and bit/BHA design. Using a similar formation profile, the FEA modeling results suggested that stiffening the drillstring and using heavier sets of PDC bits would greatly reduce the torsional vibrations while maintaining a good rate of penetration. When these changes were applied, actual data were analyzed to measure the improvement. Additionally, the analysis found that specific formation characteristics such as formation density highly contribute the severity of torsional vibration. Modeling also suggested that applying higher torque to the bit reduces its RPM fluctuations and allows for lower surface parameters. This, in return, reduces the amplitude of the torsional vibration. Over eight trials were analyzed, and significant reductions in both the measured torsional vibrations levels and equipment failures and damages were seen. Finally, the effect of utilizing a software plugin to an automated drilling system to mitigate stick and slip when drilling the 6.125-in. lateral limestone reservoir was examined. Like the other proposed solutions, the remaining factors were kept constant. The paper offers a rare case study specific to lateral limestones reservoirs, where interbedded layers are a common contributor to the severity of torsional vibrations. The results and conclusions are based on downhole high-resolution data to calibrate finite element models to provide fit-for-purpose solutions. The results eliminate much of the theoretical explanations about root causes of torsional vibrations in limestone reservoirs.
As conventional drilling learning curves mature from drilling simple vertical wells to deviated wells to complex multi-lateral horizontal wells, the boundaries needed to be broken to reach much deeper depths rather than consuming the time in drilling multiple shorter laterals. Horizontal ERD wells in Qarn Alam cluster were planned to be drilled in four sections where the 17.5-in section is drilled vertically followed by a deviated 12.25-in section and continued by landing in 8.5-in section and finally the 6.125-in horizontal lateral. Many attempts of performance improvement initiatives were executed over many years however there were always flaws and inconsistency in drilling performance delivery. As the need of ERD grew, a detailed offset wells analysis had to be performed where all the deficiencies and issues had to be pin pointed, RCA (Root Cause Analysis) had to be performed and plans for success had to be laid out. From challenges achieving required dog legs in the top sections with increased risks of axial and lateral vibrations, to the difficulties faced in the landing section drilling through unconsolidated and reactive shales, to the difficulties transferring weight to the bit at deeper depths in the horizontal laterals drilling highly porous zones of sticky limestones resulting in severe torsional vibrations. A new approach of drilling had to be executed with a renovated set of drilling parameters envelopes, revised trajectory designs, re-engineered BHA designs, right choice of fit for purpose bits and effective real-time performance monitoring.
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