Energy Transition by Employing a Self-Healing, Reduced Carbon Dioxide Footprint Sealant in a Strategic Underground Gas Storage Project

Abstract

The Tuz Gölü underground gas storage (UGS) project is a strategic venture in Turkey's energy program. This gas storage facility will be the largest in Europe, having multibillion m3 capacity, by taking advantage of the optimal gas storage conditions offered by subterranean salt caverns. Upon reaching the reservoir, one of the important goals is to obtain hydraulic isolation between the surface and the casing. Inadequate downhole isolation may well result in interzonal communication, gas migration, casing corrosion, and sustained casing pressure. Furthermore, gas flow to surface formations and/or to the atmosphere, could impact the environment and health along with an underlying economic impact. Wellbore isolation was introduced in the form of fully salt-saturated gas control and self-healing cement systems. When drilling into salt caverns, the foremost challenge is to minimize the dissolution of the in-situ salt formation by means of contact with water-based cementing fluids, which can lead to the creation of new flow paths. This occurrence must be prevented at all costs; otherwise, stored gas might leak through these microchannels. Unlike typical salt formations, this candidate field also contains carbon dioxide (CO2). Most wells in the field had a prognosis toward low CO2 content, so cement exposure to CO2 was not deemed an elevated risk; however, if the CO2 exposure risk increased, it would potentially generate an additional challenge both in terms of gas migration control and long-term cement integrity. Currently, more than 100 cementing operations have been performed in the candidate field. After pumping 3,500 metric ton of cement and blending 750 metric ton of the tailored self-healing cement, more than 300 laboratory tests were performed. More than 15,000 staff-hours of testing supported construction of 32 UGS wells, fully cemented with zero health, safety, and environment (HSE) or service quality incidents and, importantly, with outstanding bond log results. Completion strings in 15 wells have already been run where wells are prepared to store gas; the ongoing project is now expanded to 50 UGS wells. Furthermore, an intrinsic benefit of the self-healing cement system is reduced CO2 footprint vs. conventional class G cement, which can be nominally 40% less CO2 per unit volume. With involvement of local laboratories and technical experts in the region, salt-saturated gas-control and self-healing cement slurry systems have been developed and successfully deployed. Information regarding these system's liquid and set properties will be presented, along with techniques used to enhance certain cement properties. The field cases that will be presented describe how challenges were overcome in successfully sealing UGS wells in a highly saline environment, and how the self-healing technology applied in these wells is being extended to include salt-saturated systems and CO2-resistant versions elsewhere.