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.

Cement placement plays an important role in the primary cementing process. There are several best practices in place which are believed to have significant impact on the quality of the overall cement job. Previous investigations suggest that a combination of multiple placement techniques, such as density and rheology gradient coupled with proper displacement rates, pipe rotation or reciprocation, conditioning of drilling fluid prior to cement job, pipe centralization, and bottom plugs, improves the chances of a successful cement job. However, there is little quantitative analysis available to demonstrate the importance of each technique independently in the field. In the past 15 years, operations in offshore Atlantic Canada have cemented 140-mm and 178-mm liners in 216-mm openhole sections in two different reservoirs. The cementing designs for the liners are similar, especially in terms of flow rate, centralization (type or placement), and spacer train, but differ in pipe rotation, mud conditioning, and bottom plugs. Once the cementation process is executed, it is evaluated by an ultrasonic imaging tool, which measures the acoustic impedance to calculate the cement bond index. The average of the cement bond index for the entire liner is then used to quantify the quality of the cement job for each well. The average cement bond index obtained from 53 wells was used to evaluate various cement placement techniques. The average cement bond index is proportional to the amount of cement bonded to the pipe and is inferred to be proportional to factors related to mud removal and cement placement. Factors that affect mud removal, such as mud conditioning, annular velocity, pipe movement, wellbore characteristics, and the presence of a bottom plug, are investigated. Statistical analysis of the cement bond index indicates that some of these cement placement techniques affect mud removal significantly more than others. A comprehensive analysis of these results and an assessment of potential benefits are presented in this study. The results of the study were used to improve the cement job design.

Drilling into fractures and fissures in carbonate reservoirs with lateral pore pressure variations is a major challenge. Accurately predicting and managing fracture initiation and fracture closure pressure is not practically feasible in naturally fractured carbonate rocks. Local fracture closure pressure (FCP, minimum in situ stress) around fractures are much lower than the mean value of FCP for the area, as observed in the Al Shaheen field. Drilling operators are experiencing massive lost circulation resulting in potential formation damage; and cost overruns. To address these lost circulation instances, modern lost circulation pills must be implemented. Drilling through the natural fractures within parts of the Shuaiba limestone reservoir usually results in significant drilling fluid losses, up to 50 m3/h. Lateral pore pressure variation causes dynamic inflow conditions within the wellbore, which make plugging fractures difficult. In addressing these lost circulation challenges, various approaches were previously used, notably reducing the hydrostatic pressure of fluids column, utilizing lighter weight drilling fluids, reducing the penetration of fluids into fractures with the use of various lost circulation materials, and the use of thixotropic cement slurries and lightweight high-solids-content cement slurries. Conventional lost circulation treatments provided very limited success under these conditions. An engineered composite fiber-based lost circulation pill with an innovative blend of fibers and sized solids to bridge and plug thief zones has been developed to address these lost circulation challenges. This pill was designed to be pumped through either a dummy bottomhole assembly (BHA) or through bypass circulation ports above the BHA with total flow area of 1.571 in2. These pills have been successfully used to mitigate losses while drilling as well as to achieve an incremental equivalent static density up to 144 kg/m3 to drill and cement the section. An impermeable grid created by this system was able to withstand the additional pressures. As a result, all the wells treated with these pills in the field were successfully drilled and cemented. After establishing the field specific guidelines over 2 years, continuous success was replicated in other wells for all the operator's rigs in Qatar.

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.

Achieving zonal isolation has always been a challenge when drilling and cementing gas-producing wells in a high-pressure/high-temperature (HP/HT) environment. A generally accepted definition of a HP/HT well is one in which the undisturbed bottomhole temperature at prospective reservoir depth or total depth is greater than 149°C [300°F] and either the maximum anticipated pore pressure of any porous formation to be drilled exceeds a hydrostatic pressure gradient of 97 Pa/m [0.8 psi/ft] or pressure control equipment with a rated working pressure in excess of 68.94 Mpa [10,000 psi] is required.In the past, HP/HT wells were drilled and cemented in the UAE using conventional cementing design and techniques. Those wells had zonal isolation concerns for cemented production strings, proving that cementing was a critical factor in the HP/HT field. In addition, the wells must bear the stresses generated from injection pressure and temperature changes and frequent cycling of injection during well testing and production. Trapped gas and oil between production and intermediate casing (abnormal annular wellhead pressure) has been globally recognized as one of the serious challenges faced by the Industry from well security and isolation perspective. To tackle the potential safety and environmental hazards of abnormal annulus pressure, better cementing practices and solutions are introduced to improve well life cycle and minimize the frequency of workover operations.This case describes the use of self-healing cement and flexible and expandable cement to prevent cement failure due to induced stresses and optimize the isolation and ultimately deliver on the objective set for critical exploration well by the Operator.