No abstract

Offshore cementing poses many challenges across the world as drilling oper ations move towards deep-water and ultra -deep-water. As a new initiative of continuous improvement, a deep-water cementing peer review process was started early 2011. To this date, th is team has reviewed more than 12 00 deep-water cementing jobs in more than 30 countries worldwide.

Industry successful cases and lessons learned have shown the challenges of setting cement plugs in open hole. Using industry best practices, it is possible to successfully set cement plugs in deepwater HPHT wells drilled with non-aqueous fluids. However, it is not uncommon to see cases where a single cement plug job has to be repeated more than once in order to achieve a competent barrier. Among different reasons that would cause a cement plug to fail, are: contamination with non-aqueous drilling fluid, insufficient cement and spacer volumes, density hierarchy, friction pressure hierarchy and cement plug slumping. These challenges become more critical as offshore drilling moves towards new frontiers: deepwater, salt formations, deeper measured depth reaching higher bottom hole temperatures and pressures. In Brazil, drilling operators do not face one single challenge in their operations; they are facing a combination of them in most of the new wells. A successful case will be discussed, which describes the placement of a cement plug isolating an over-pressured gas reservoir and allowing the plug and abandonment operation to continue on a HPHT pre-salt offshore well in Santos Basin, in Brazil. The bottom hole pressure was over 137.8 MPa [20,000 psi] (ultra-high pressure well) and bottom hole temperature 171 degC [340 degF], required 2300 kg/m3 [19.2 lbm/gal] mud weight to maintain the well overbalanced after a gas influx happened when the mud had only a density of 2230 kg/m3 [18.6 lbm/gal]. In addition, the heavy-weight and non-aqueous fluid (NAF) added an extra challenge to this operation. On this case, high density, high performance system (HDHPS) with engineered particle size distribution (PSD) was the selected cement slurry in order to overcome ultra-high formation pressure. Special chemistry was combined to the HDHPS to place the cement plug across the gas reservoir and salt formation, maintaining its stability and assuring isolation of the gas in the reservoir. The cement plug placement was designed with dedicated cement plug placement software, which brought a superior value in analyzing the risks involved, determining the placement technique and confirming best practices, thus aiding to define the pre-job conditions and consequently assuring the success of the P&A. These practices can be successfully extended to other operators plugging and abandoning their wells in deepwater, HPHT wells and even in conventional environments. Introduction Placing cement plugs for sidetrack, remedial work, temporary or permanent abandonment is the highest cement activity offshore. It presents many challenges, including well cleaning, mud removal, minimizing contamination, cement slurry slumping, wait-on-cement sufficient time and more (Bogaerts et al. 2012). By adding to this condition a high pressure reservoir with high density drilling fluid to control the well and maintain it overbalanced, a set of new challenges are introduced to the cement job design and execution. In the following paragraphs the reader will understand why HPHT offshore wells present new challenges for placing successful cement plugs, as well see a thorough discussion on the design, execution and evaluation (DEE) on setting the first cement plug, responsible for isolating the gas reservoir, in the plug and abandonment (P&A) operations of the well 1-OGX-63-SPS drilled in Santos Basin, offshore Brazil.

As drilling muds evolve to satisfy well requirements, cementing preflush technologies need to change to ensure proper mud removal during cementing jobs. A new component—engineering-designed fiber—was added to a preflush fluid and tested in the laboratory, with promising results. The system was then implemented in Latin America. Obtaining proper mud removal is very important for achieving zonal isolation at cementing jobs. The new technology consists of the addition of an engineering-designed fiber to cementing preflush fluids to significantly improve the removal of nonaqueous fluids from the well during cementing operations. The fibers are compatible with both cement slurries and mud. They work by removing the mud from the casing or formation through two mechanisms: by mechanical cleaning and by attracting the nonaqueous compound of the mud toward itself by hydrophobic-hydrophobic interaction. Two different methodologies were used to evaluate the fiber's ability to enhance the chemical wash and spacer capabilities to clean and demulsificate the nonaqueous mud fluids. The laboratory tests were performed with cementing preflush fluids with and without the fibers. Results indicated that the preflushes with the fibers were able to clean and demulsificate the drilling mud much more efficiently than preflush without the fibers. Indeed, it was possible to optimize the amount of the preflush surfactants and still obtain excellent results. Some successful cases of field implementation of this technology corroborated the laboratory findings. In both cementing jobs, results indicated very efficient mud removal, and, consequently, zonal isolation and well integrity were achieved. The fibers were successfully pumped in a field in Latin America. This innovative technology is able to enhance cement bonding in both casing and formation and reduces potential remedial job costs in a wide range of challenging environments.

Four deepwater wells in the Gulf of Mexico were identified for permanent abandonment in accordance with local regulations. The abandonment plan called for multiple cement plugs to isolate the production zones from seabed in each well. The most challenging cement plugs in each well were the ones directly above the production packer isolating the casing-tubing annulus and the production tubing. To avoid cement left in the Christmas tree at seabed and potential plugging off any valves in the manifold, cement plugs could not be placed the direct way, pumping down the production tubing. An unconventional approach was proposed to address the challenge. It involved reverse placement of cement plugs, which is not common in deepwater even in these days. Using this technique, cement was pumped down the casing-production tubing annulus and, through perforations, back up the production tubing. Risk analysis indicated very low likelihood of plugging off any valves in the tree. However, this configuration did not allow use of mechanical separators between fluids to prevent intermixing. Additional challenges in placing the plug were the high deviation of the section and the completion brine in the wellbore. Simulation of reverse placement is not possible with existing software. Therefore, the jobs were designed using experimental software, which enabled the design engineer to accurately reconstruct field conditions. Additional attention was given to the job procedure to minimize contamination with the brine and optimize cement placement. Viscous spacer was pumped ahead and behind the slurry to displace the brine. The slurry was designed with low fluid loss to be squeezed through perforations in the production tubing without plugging them off. A total of seven plugs were placed using the reverse placement technique. Specific requirements with regards to the top of cement and plug integrity had to be met before any of the cement plugs could be accepted by the operator. All plugs were tagged and pressure tested successfully in the annulus as well as inside the production tubing on the first attempt. As a result of the campaign, the four wells were abandoned as per schedule and within budget.