No abstract

Lost circulation (LC) is commonly encountered in drilling and cementing operations and can be a costly problem that increases nonproductive time (NPT). Various methods can be applied to control losses—from applying best operational practices to incorporating LC control materials in treatment fluids and cement slurries. Among the cementing challenges, the loss of circulation can also be associated with poor zonal isolation and becomes more critical when zones between reservoirs should be covered to avoid sustained casing pressure (SCP) in the future and to extend the life of the well. The application for this tailored spacer was the first one globally. Challenges addressed in the largest offshore field loss zone in the UAE included a high likelihood for losses during initial cementing, narrow equivalent circulating density (ECD) gradient, uncertainty in bringing cement to surface in the casing operation, and zonal isolation between reservoirs, formation, and caprock. As part of the best cementing practices, the design and tailoring of the spacers should be considered. Correct use allows the cement to cover zones of interest with less contamination, and cement properties can develop and interact efficiently with the formation. Using a tailored spacer fluid system engineered to effectively and efficiently help prevent LC and maintain wellbore stability while preparing the wellbore to receive cement is discussed. The tailored spacer system uses additive synergies to help prevent LC in porous and fractured formations and enables control of rheological hierarchy and wellbore fluid displacement efficiency. The spacer was designed to optimize cementing operations where losses are observed and, in this case, incorporate additional LC materials to help prevent severe losses and achieve the desired top of cement (TOC). The tailored spacer system was pumped ahead of the cement slurry, to reduce permeability across the formation, with superior properties that help prevent fluid loss of the cement to the permeable formations. Enhancing the cement bond and allowing an effective mud removal differentiate it from a conventional spacer. Using this spacer system enabled cementing goals to be achieved. Cement was brought to the surface, casedhole logs exhibited excellent cement bonding, and no SCP was registered, helping eliminate the need for unwanted remedial operations to secure the zonal isolation, which saves rig time.

Good cementing practices are required to achieve effective zonal isolation and provide long-term well integrity for uninterrupted safe production and subsequent abandonment. Zonal isolation can be attained by paying close attention to optimizing the drilling parameters, hole cleaning, fluid design, cement placement, and monitoring. In challenging extended reach wells in the UAE, different methods were employed to deliver progressive improvement in zonal isolation. Cementing the intermediate and production sections in the UAE field is challenging because of the highly deviated, long, open holes; use of nonaqueous fluids (NAFs); and the persistent problem of lost circulation. Compounding the problem are the multiple potential reservoirs; the pressure testing of the casing at high pressures after cement is set; and the change in downhole pressures and temperatures during production phases, which results in additional stresses. Hence, the mechanical properties for cement systems must be customized to withstand the downhole stresses. The requirement of spacer fluids with nonaqueous compatible properties adds complexity. Lessons learned from prior operations were applied sequentially to produce fit-for-purpose solutions in the UAE field. Standard cement practices were taken as a starting point, and subsequent changes were introduced to overcome specific challenges. These challenges included deeper 12 ¼-in. sections, which made it difficult to manage equivalent circulating densities (ECDs), and a stricter requirement of zonal isolation across sublayers in addition to required top of cement at surface. To satisfy these requirements, several measures were taken gradually: applying engineered trimodal blend systems to remain under ECD limits; pumping a lower-viscosity fluid ahead of the spacer; using NAF-compatible spacers for effective mud removal; employing flexible cement systems to withstand downhole stresses; and modeling the cement job with an advanced cement placement software to simulate displacement rates, bottomhole circulating temperatures, centralizer placement, mud removal and comply with a zero discharge policy that restricts the extra slurry volume to reach surface. To enhance conventional chemistry-based mud cleaning, an engineered scrubbing additive was included in the spacers with a microemulsion-based surfactant. The results of cement jobs were analyzed by playback in advanced evaluation software to verify the efficiency of the applied solutions. This continuous improvement response to changes in well design has resulted in a significant positive change in cement bond logs; a flexural attenuation measurement tool has been used to evaluate the lightweight slurry quality behind the casing, which has helped in enhancing the confidence level in well integrity in these challenging wells. The results highlight the benefit of developing engineering solutions that can be adapted to respond to radical changes in conditions or requirements.