In 2012 ZADCO commenced drilling operations from artificial islands in the Upper Zakum field. The field development is based on drilling extended reach wells up to 35,000 ft measured depth in three different reservoirs. More than twenty wells have been drilled from these islands so far, with the longest well reaching ~31,000 ft MD. Longer than 35,000 ft MD wells are planned to be drilled in the near future. The 9-5/8″ casing in 12-¼″ open hole is set at a measured depth of ~9,000-17,000 ft with cement designed to reach surface. There are several challenges experienced while cementing these wells, due to the narrow pore and fracture pressure gradients. Key challenges include: ECD management, maintaining fluids density and rheology hierarchy, proper centralization, lost circulation, use of NAF drilling fluids and limitations to pipe movement opportunities. Based on ultrasonic imaging of initial wells, the overall desired cement bond was not as good as desired on initial wells. Several improved practices were applied to enhance the cement bond across reservoir sections. However, the log quality was still below expectations. Hence, a more robust solution was required to successfully cement the long maximum reservoir contact (MRC) wells. These challenges were successfully addressed and mitigated through a step-wise approach. The cement slurry designs were optimized by adjusting rheologies and static gel strength, lowering fluid loss values, adding lost circulation material and using trimodal cement designs. Mud removal was further enhanced by using better cleaners in the spacer system to provide efficient cleaning and de-emulsification. Updated mud conditioning procedures and better centralizers was also implemented. Cement bond log and interpretation techniques were improved by using ultrasonic measurements and flexural attenuation measurements and imaging the annulus through these tools to determine actual casing centralization. The log showed a significant improvement especially across the horizontal reservoir sections. The use of these techniques has improved cement quality and enhanced zonal isolation of the producing zones in these horizontal MRC wells and will assist in maintaining the quality for future development of the Upper Zakum field.

This paper presents a case of study of cementing operations in extended reach drilling (ERD) wells on two artificial islands in UAE. The cementing objectives involved covering and isolating the shallower oil or water-bearing zone, sealing any potential crossflow interval between various reservoirs, and mitigating communication between the 13 3/8-in × 16-in. and 9 5/8-in × 12 ¼-in. annulus. Additionally, the cement will become a secondary barrier planned for the 9 5/8-in. casing to prevent potential exposure of the 13 3/8-in. casing in the event of injection gas percolation. For this case was necessary to design the cement with the necessary mechanical properties to extend the well life expectancy. To accomplish the operation objectives, the formations, well design complexity, and possible complications were considered. Understanding these factors facilitated the improvement in the approach and design to obtain better cementing operations results. To achieve all of the targets, various enhancements were implemented systematically over time, which included adding fiber in the cement spacer to mechanically enhance mud removal, adding corrosion inhibitor and bactericide to protect the casing if fluid remained in the well. Various lead cement slurries were designed with tailored rheology to remain within the established narrow margin between the pore pressure and fracture gradient. A flexible and expandable tail cement slurry system was implemented to increase the likelihood of proper isolation. The expansion of the tail cement slurry after the cement sets and the tailored mechanical properties used to achieve the necessary resilience, provide support for the stresses encountered during the life of the well. Upgraded properties, such as fluid loss, reduced permeability, and static gel strength (SGS) development, were used to mitigate possible influx between formations. Both cement slurries were loaded with resilient fiber to enhance the cement ductility. These strategies combined with software simulations enabled equivalent circulating density (ECD) management, contamination avoidance, friction pressure hierarchy, discernment of the top of cement (TOC), determination of possible channels, and appropriate stand-off design. The application of the solutions combined with outstanding consistent field operational performance enabled the following: Fine-tuning various practices, improving isolation across critical zones, achieving the planned TOC, sealing the formations that could create potential future issues, and reducing the probability of interzonal communication or crossflow. A systematic approach was necessary to achieve all the objectives in these challenging wells and determine which practices and technologies provide the appropriate results. Cementing ERD wells with the challenges previously described is not a standard industry practice. This case study presents the staged application of the enhancements that improved the cementing results. These were inferred by evaluating the operational parameters (density, pumping rate, pressure, volumes, and surface returns) in conjunction with the availability of cement logs (CBL, VDL, ULTRASONIC). The results demonstrated the capability to achieve isolation; Furthermore, the continuous annulus surveillance showed no undesirable sustained casing pressure.

Lost circulation is a widespread problem in many formations in the United Arab Emirates, including Shallow Unconsolidated, Shallow Vuguler and Limestone. To minimize losses during drilling and cementing, several types of lost circulation solutions such as bridging agents and surface-mixed and downhole-mixed solutions have been used. Still, operators lose huge volumes of mud and cement slurry and expensive rig time. Consequently, many wells also require remedial operations, which can compromise well integrity. Loss-zone diagnosis and characterization helped to tailor the loss-circulation control solution for different requirements. Losses can occur due to unconsolidated formation (surface holes) or induced and natural fractures. Typical loss rates can vary between 150 to over 700 bbl/hr, particularly while drilling the 16-in. and 12 ¼-in. openhole sections. Thus, total losses in shallow vuguler formation require a different treatment than induced losses in shallow unconsolidated or limestone formation. Induced losses in limestone while drilling prevented increase in mud weights required to drill deeper reactive shale formation. A composite fiber-based system based on a novel four-step methodology was designed using advanced software analysis. Prior to the field trial a complete lab-scale and yard-scale testing was done to confirm superior effectiveness. In one of the pilot wells, multiple leakoff tests (LOTs) were performed in the Limestone formation to verify the minimum stress (1.40 SG), which did not change with subsequent LOTs. The formation was then treated with the reinforced composite mat based system, and the following formation LOT showed integrity buildup to 1.51 SG. It proved to be an easy to apply solution that can be mixed and pumped through a qualified bottomhole assembly (BHA). Zone and mechanism specific lost circulation control solutions are most effective in reducing nonproductive time (NPT). It was also demonstrated that fiber-based solutions work well for controlling losses and formation strengthening. The engineered composite fiber-blend system exhibits improved performance and robustness in terms of curing losses and even improves formation integrity.

This paper describes how the unique centralizer requirements for extended reach drilling (ERD) wells can be attained. By continuously evaluating past casing runs in combination with engineering input, the learning curve led from a standard centralizer to a highly customized solution. The necessary flow path target to enhance the wellbore isolation through cement placement is met by achieving the right centralizer performance and placing. A single-piece high-restoring-force centralizer is the best solution for the high inclination well profile to obtain the required 9 5/8-in. casing stand-off for ERD wells. The original centralizer design experienced challenges such as high doglegs in some of the longest 9 5/8-in. casing strings that have been run in the UAE to date. Customize the centralizers for different well profiles was necessary. They were developed and tested according to the latest API 10D (2001) specifications using precision equipment to ensure reliable test results that enable accurate hook load and stand-off simulation. Initially, a standard off-the-shelf design of a 9 5/8-in. x 12 1/4-in. single-piece centralizer was used in two wells with the following results:Friction factor exceeded the expected values across the interval on occasion.Total Depth (TD) was sucessfully reached by washing down to bottom.Good centralization as per software design was attained (tageting 80%) with moderate to good isolation. Due to the performance, while running in the hole (RIH), concerns arose due to the unexpectedly high friction factor which, could lead to difficulties RIH and reaching TD in future wells. The modified centralizer design has led to the following improvements:Reduction of friction factor to an average of 0.24 due to a significant decrease in the centralizer running force even through reduced hole diameter intervals and the high dogleg severities (DLS)Reaching TD successfully.Stand-off remained around 80%, as demonstrated by outstanding cement bond log results across the critical sections. It is important to consider that this centralizer was designed not to lose any performance after being run through reduced hole diameter intervals. The application of enhanced centralization design (i.e., standoff >80%) ensured good quality of the cement job.

Achieving well integrity relies on achieving zonal isolation among narrowly separated sublayers of the reservoir throughout a long openhole section. This requires flawless primary cementation with a perfect match of optimized fluid design and placement. In a UAE field, there are several challenges experienced while cementing production sections, predominantly due to long open holes with high deviation, use of nonaqueous fluids (NAF) for shale stability, and loss circulation issues while drilling and cementing. The need to pressure-test casing at high pressures after the cement is set and the change in downhole pressures and temperatures during well completion / production phases result in additional stresses that can further endanger the integrity of the cement. Breaking of the cement sheath would lead to sustained annular pressure and compromise the needed zonal isolation. Hence, the mechanical properties for cement systems must be thoroughly tested and tailored to withstand the downhole stresses. A systematic approach was applied that used standard cementing best practices as a starting point and then identified the key factors in overcoming operation-specific challenges. In addition to the use of engineered trimodal slurry systems, NAF-compatible spacers, and loss-curing fibers, an advanced cement placement software was used to model prejob circulation rates, bottomhole circulating temperatures, centralizer placement, and mud removal. To enhance conventional chemistry-based mud cleaning and to significantly improve cleaning efficiency, an engineered fiber-based scrubbing additive was used in spacers with microemulsion based surfactant. Furthermore, a real-time monitoring software was used to compute and monitor equivalent circulating density (ECD) during the cementing operation and to evaluate cement placement in real time. Results of cement jobs were analyzed to define the minimum standards/criteria and then to verify the efficiency of the applied solutions. The 9 5/8-in. casing / liners were successfully cemented using this methodological approach, and lessons learned were progressively used to improve on subsequent jobs. Advanced ultrasonic cement bond logging tools along with advanced processing and interpretation techniques facilitated making reliable, conclusive, and representative zonal isolation evaluation. The cement bond logs showed significant improvement and increased the confidence level towards well integrity. After establishing field-specific guidelines over 2.5 years, continuous success was replicated in every well for all the rigs operating in this UAE field.