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

Abstract

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, where water is consumed causing pore pressure reduction, and shrinkage, which increases the risk of fluid/gas invasion, formation of micro-annuli and even the risk of shear bond strength reduction.This paper presents the application of compressible carbon particles as a cement additive to mitigate cement pore pressure reduction and improve zonal isolation. Cement formulations with compressible carbon were designed and evaluated 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 compressible carbon particles will compress under hydrostatic pressure.When the cement is hydrating, the compressible carbon will expand to mitigate the porepressure reduction.To validate the proof-of-concept, a benchtop high-pressure high-temperature (HP/HT) setup and a pilot-scale test setup were developed. A cement formulation with 9% compressible carbon by volume was found to be stable and have controllable performance properties such as thickening time, fluid loss and free water.A control slurry without compressible carbon was designed and tested for comparison.Both the benchtop and pilot-scale tests demonstrated that adding compressible carbon 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 to 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.Additionally, a better hydraulic seal was found in one test, but further investigation is needed.Similar mechanical properties such as compressive strength and tensile strength were measured for both cements with and without compressible carbon.The data showed adding compressible carbon does not have a negative impact on the hardened cement properties.In fact, adding compressible carbon decreased Poisson's ratio slightly. This paper will present the results of proof-of-concept testing for the novel application of compressible carbon in cement to improve zonal isolation by mitigating pore pressure reduction.It will also present new benchtop and pilot-scale experimental setups to measure pore pressure changes during cement hydration.