Application of Compressible Carbon in Cement to Mitigate Cement Pore Pressure Reduction and Improve Cement Bonding

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Summary A quality cement job is essential to ensure long-term zonal isolation in oil/gas and carbon storage wells. This may be affected by the cement hydration process, during which water is consumed, causing pore pressure reduction and shrinkage, which increases the risk of fluid/gas invasion, formation of microannuli, and even the risk of shear bond strength reduction. In this paper, we present the application of compressible carbon (CC) particles as a cement additive to mitigate cement pore pressure reduction and improve zonal isolation. We designed cement formulations with CC and evaluated them using API standard tests and industry-wide recognized tests to ensure they met functional requirements for well cementing, such as mixability, thickening time, rheology, compressive strength development, fluid loss, and free water. The concept is as follows: When cement is pumped into an annulus, the CC particles will compress under hydrostatic pressure. When the cement is hydrating, the CC will expand to mitigate the pore pressure reduction. To validate the proof of concept, we developed a benchtop high-pressure, high-temperature setup and a pilot-scale test setup. A cement formulation with 9% CC by volume was found to be stable and have controllable performance properties such as thickening time, fluid loss, and free water. We also designed and tested a control slurry without CC for comparison. Both the benchtop and pilot-scale tests demonstrated that adding CC into the cement formulation mitigated the pore pressure reduction during cement hydration. This may reduce the risk of fluid/gas invasion that could result in migration in the set cement. Comparing the cement containing carbon with the control system without carbon, the gas permeability was reduced. The bond strength obtained from shear bond tests improved significantly, which may reduce the risk of debonding and be an indication of the reduction of shrinkage. In addition, a better hydraulic seal was found in one test, but further investigation is needed. Similar mechanical properties such as compressive strength were measured for both cements with and without CC. The data showed that adding CC does not have a negative impact on the hardened cement properties. In fact, adding CC decreased Poisson’s ratio slightly. In this paper, we present the results of proof-of-concept testing for the novel application of CC in cement to improve zonal isolation by mitigating pore pressure reduction. We also present new benchtop and pilot-scale experimental setups to measure pore pressure changes during cement hydration.