Well cementing in high temperature and hydrogen sulfide (H2S) corrosive environment presents challenges in preventing cement compressive strength retrogression and selecting weighting agents inert to H2S. This paper presents the development of a cementing system for high pressure high temperature (HPHT) well with bottom hole static temperature in excess of 165°C, a drilling fluid density of 2.19 SG and a high concentration of H2S. A major operator in the Caspian Sea region accepted the cement design and successfully used it on the production liner section of the HPHT well. Cementing of the production liner was complex due to the requirement for a high-density cement system, narrow margin between the pore pressure and frac gradient, HPHT conditions and 18% H2S concentration in the formation fluid. Comprehensive laboratory testing was performed to evaluate the properties of the cement system including measurements of thickening time and compressive strength evaluation using a UCA and destructive method using ultra-HPHT curing chamber for cube sample curing. The presence of H2S limited the use of conventional weighting agents such as hematite and hausmannite, and the high temperature environment dictated the need for quartz silica. These factors required a nonstandard approach to cement blend formulation and flowability assessment. During cement system optimization, the target slurry density was achieved using barite which has a lower density compared to other common weighting agents and significantly reduces cement content in the blend but also is inert to H2S corrosion. A further challenge encountered during cement system optimization was strength retrogression that could not be prevented by the conventional approach of adding 30-40% quartz silica by weight of cement into the system. To overcome strength retrogression, much higher concentrations of silica were required. As a result, the low cement content led to insufficient compressive strength development at liner hanger depth. A solution was found by adding a Vinylamide/Vinylsulfonated polymer (VA/VS) polymer in a certain proportion to the slurry design. Thus, at elevated temperatures, it was observed that the VA/VS polymer tended not to delay compressive strength development while still extending the slurry thickening time. The developed heavy weight cement system was successfully implemented to isolate the 7-in liner on HPHT well. All the stages of job planning, design and execution, along with the slurry optimization process are presented.
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.
Zonal isolation for primary cementing is generally of concern when there is potential for gas migration. The challenge for the industry is to achieve a long-term annular cement seal and prevent gas migration. This paper focuses on the problem of ensuring sufficient bulk expansion of set cement without access to external water and optimizing the cement slurry formulation. The approach to solving this problem is creative and simple within the industry. One of the reasons for wellbore gas migration and inter-connected flows can be due to cement shrinkage over time. This study focuses on laboratory testing of an expanding cement system in the absence of water and analysis of test results of novel the cement system in terms of its implementation on well with high gas migration potential. The cement system behavior will also be described in terms of rheological, filtration and mechanical properties and compared to conventional expanding cement slurries. This approach can be used to improve cement bonding with the aim of minimizing future remedial jobs. Several approaches were implemented to achieve noticeable expansion in anhydrous media. One of the methods showed it was feasible to achieve 1.27% linear expansion in set cement without external water contact, while linear expansion in the presence of water was 0.78%. This method uses the addition of sodium chloride (NaCl) and while it has been previously described in literature, no practical design/testing directions have been given. The study identified the most effective concentration of sodium chloride required for set cement expansion without water availability. The study described how other cement system properties permitted better results in terms of placement quality of highly salt-saturated cement. Overall, complex laboratory test results provide evidence of effective linear expansion in set cement in the absence of external water. The optimization of cement slurry properties was focused on obtaining optimal thickening time, rheology and compressive strength, which was complicated by the presence of a high concentration of sodium chloride. An expanding cement system was successfully tested in the absence of water showing positive linear expansion. A new approach for testing expanding cement systems in the absence of water was introduced how excessive linear expansion could be compromised with compressive strength development. The research results have shown that the use of NaCl additive in high concentrations in high SVF self-healing systems provided improved performance when aiming for effective linear expansion in set cement in the absence of water.
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