First Implementation of Self-Healing Cement Systems in H2S/CO2 Aggressive Environment Across Pay-Zone
Authors: Sozonov Alexander Sergeevich , Sukhachev Vasilii Yurievich , Olennikova Olesya Vladimirovna , Akhmetzianov Ilshat Ildarovich , Bakhareva Elizaveta Sergeevna , Burkenya Alexandr Alexandrovich , Doronina Tatiana Valerievna 
Published: September 2020

Abstract
Carbonate reservoirs are often characterized by high pressure and high content of H2S and CO2. For these reasons, drilling the reservoir is the most challenging activity for such fields and long-term zonal isolation across the reservoir section is one of the primary requirements. The work describes the development, laboratory evaluation, implementation and long-term assessment of self-healing properties for H2S/CO2 resistive, low Young's modulus, expandable cement system. The 2.05 SG cement system was specially designed and implemented on 12 wells at N oil field for guaranteed long-term zonal isolation of production reservoirs containing high concentration of H2S and CO2.
The paper includes rationale and details of CO2 and H2S resistive cement system design. Self-healing was tested by pumping simulated reservoir fluid composition, through induced fracture in cement sample at downhole conditions. Observed fracture closure leading the gas flow stoppage is a result of cement matrix in the crack swelling when in contact with hydrocarbons. To address risk of perforations closure and ensure cement integrity in perforation zone, when in contact with hydrocarbons, cement samples were exposed to aromatic oil for twenty months. Long-term zonal isolation was assessed by monitoring the cemented wells for the six years period.
To overcome these problems, self-healing cement system and durable cement systems were developed to ensure well integrity during the life of the well, providing competent annular pressure seal. Extensive laboratory work was undertaken to engineer the slurry to the required specifications. Self-healing cement is based on a responsive material with intrinsic self-healing properties automatically activated upon hydrocarbon exposure to rapidly seal the damage; within hours the downhole well integrity is restored. This reduces the potential health, safety and environmental risks and the extra costs associated to remedy these problems including loss of production.
Self-healing system includes expanding additives and expands after setting, improving cement bonding and sealing micro annuli that otherwise can cause unwanted gas migration. Due to its improved mechanical properties and low Young's modulus it can withstand cement sheath stresses during well operations due to changes in the operating temperature and pressure. For a formation where hydrocarbons are not present to trigger the self-healing mechanism, the durable cement system is the solution to use. It has suitable components to optimize its mechanical properties to withstand pressure and temperature cycling. The slurry had a fit for purpose thickening time and rheology accomplished with excellent fluid loss control, static and dynamic stability for proper placement. Yard trials confirmed designed slurry compatibility with surface field equipment.
Self-healing was achieved in the reservoir fluid composition, when tested at 60 degC and 34.5 MPa. Micro gaps or fissures, which might occur in the cement sheath along life of the well, would be efficiently closed and sealed. Cement also demonstrated ability to preserve integrity after exposure to aromatic oil for twenty months. This confirmed system applicability for placement across reservoir perforation zone.
Mechanical properties of the cement were tested. Cement developed compressive strength of 25 MPa, tensile strength of 3.3 MPa, young's modulus of 7 GPa compared to 10 GPa for conventional systems same density. Lowered Young's modulus in conjunction with 1% cement self-expansion post setting provided additional guarantee of high-quality zonal isolation.
Experience of pumping the system for 12 wells showed excellent mixability and pumpability in the field. Cement logging tools confirmed high quality of zonal isolation. Wells have shown absence of sustain casing pressure for six years.
The CO2 self-healing and resistant cement overcomes the deficiencies of conventional cement slurry in carbon dioxide environments. Using of self-healing cement systems historically showed superior long-term zonal isolation efficiency. However, there is no available information on efficiency and performance of such systems in high H2S/CO2 environment. Application of such system is limited for production zones. This work shares unique experience of design execution and performance evaluation of self-healing system in described complicated conditions.
Moreover, the paper presents correlation of advanced laboratory testing methods with long-term cement performance in real well conditions.