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.

Ensuring well safety during well construction and abandonment presents a challenge for offshore exploration worldwide. More than ever, successful abandonment of exploration wells is critical. High-quality cement plugs are required to provide a tested isolation barrier. Over the years, the industry has experienced problems in the correctly placing cement plugs. Historically, setting cement plugs in deep open holes has been a difficult task. Often, several attempts are necessary to end with a proven barrier or a competent kickoff plug. Sometimes, minor contamination with synthetic drilling fluid has a detrimental effect on the physical properties of the set cement, which can have severe consequences. This challenging task becomes even more difficult when drilling in deepwater environments because of the nature of frontier exploration, increasing placement depth, high-pressure reservoirs, high-density drilling fluids, salt formations, and high-pressure gas reservoirs. In Brazil, exploration campaigns are moving to presalt reservoirs where operators face a combination of these challenges.An overpressured, presalt gas reservoir in the Santos basin was discovered with an equivalent pore pressure ranging from 18.5 to 18.7 ppg (ultrahigh pressure ~ 20.000 psi) at 6,136 m true vertical depth. During drilling operations in this highpressure, high-temperature (HP/HT) deepwater well, a kick was observed after drilling a few meters below the salt. This led to the application of conventional methods of well control. Once the well was shut in, the pressure readings at the surface were stabilized at 5,600 psi, and the mud weight was increased to 19 ppg to achieve hydrostatic control of the well. Once well control was regained, the well was temporarily suspended with a sequence of cement plugs. These unexpected levels of pressures suggest a discovery. However, further research was postponed because of the current rig limitations and supplies.A total of 392 m of 8 ½-in. openhole section was abandoned using two cement plugs with a special high-density slurry of 21 ppg and high-density spacer of 20 ppg. A third cement plug was set in the previous casing string. The design included a special chemistry for formations with gas and salt. Software simulations and the analysis of placement techniques and practices supported successful operations in this challenging environment.The successful practices demonstrated for these plugs will be useful for other ultrahigh pressure discoveries that may be encountered. These practices may also be useful when setting cement plugs in conventional wells or wells with high density drilling fluids.

The Upper Cretaceous is a challenging drilling environment in Santos Basin, offshore Brazil. The lithology consists of highly intercalated layers of sandstone, siltstone and conglomerate from Santos and Jureia Formations. These formations are drilled vertically in 17 ½" intermediate hole and the length of the section varies from 1600 m up to 2000 m. Optimizing the drilling efficiency in this section is crucial to reducing drilling costs and non-productive time. During a recent exploratory campaign in Santos Basin, severe levels of shocks and vibrations were encountered while drilling this large and intercalated hole section, resulting in a low rate of penetration, premature bit wear, damaged tools, unplanned trips and drill string failures. Three to four bit runs were required to reach section TD. Post job information and drilling data were reviewed by an integrated operator-contractor team. A detailed engineering analysis of bit, BHA design and drilling mechanics was conducted to address the challenges of this section and propose a new approach to the drilling process. The key was the determination of the best combination of WOB and RPM to drill the different lithologies, together with more stable PDC bits, use of downhole motors and combined with specific operational procedures to mitigate shocks and vibrations. The proposed changes in bit selection and BHA configuration, drilling parameters and procedures, real-time monitoring and training have been successful. For the first time in the drilling campaign in Santos basin, it has proved possible to drill the 17 ½" hole in one bit run. The section was finished without any incident, under budget and under time. The improvement in drilling performance achieved with the implementation of this engineering study helped the operator to mitigate shocks and vibrations and save money, thus ensuring a more efficient drilling operation and preventing further drill string failures.