The extraction of subsurface resources still pose unacceptably high risk for well integrity loss and impacts can be disastrous—especially offshore where the environment is unforgiving. Oil and natural gas exploration and production companies incur considerable operational expense and capital investment mitigating these harsh conditions and reaching the remote locations of the wells. To minimize loss and maximize the probability for long-term well integrity, operators must use materials, tools, and methods that ensure success. Scientific advances gained from this research, helped to achieve this goal by developing innovative materials and methods to improve offshore well integrity by conducting research focused on the critical barrier system stability in a well. The barrier system consists of the casing-cement-rock interfaces and provides mechanical support, acts as a hydrologic barrier, and prevents corrosion of the well system.
The researchers studied how cement protects the well casing when subjected to corrosive subsurface environments under environmental static and flow-assisted conditions. A borehole simulator apparatus was used to study how drilling fluid materials and methods can be improved to ensure a well-conditioned borehole, which is critical for establishing a quality bond between casing and rock.
A novel testing apparatus was constructed that simulated a host of subsurface conditions that impart stresses onto a well system. Experiments were used to verify a numerical model for well system stress analysis. This model was developed into a tool that operators, service companies, and regulators can use to study the stress distribution around well systems.
Corrosion experiments showed that electrochemical testing can simulate corrosion over the long term. These experiments confirmed that when a casing is properly protected by cement, the high pH pore fluids prevent most forms of acid attack on casing. However, corrosion degradation starts if CO2/H2S pore solutions reach the casing surface.
The borehole simulator apparatus was designed and constructed, allowing researchers to study the buildup and removal of mud filter cake on a borehole wall to develop more robust models for filtrate performance.
Proof-of-concept experiments for the scaled well system were completed. The apparatus successfully imaged mechanical failure of cement and casing after a fluid pressure was applied to the inside of a “casing,” giving researchers a better understanding of mechanical failure of wells when subject to cyclic loading.
Simple analytic numerical models have been developed to estimate the stress and strain distribution in a well system under different wellbore orientations and different tectonic stress fields. The results compare well with more advanced Finite Element Models being developed, which are needed to more accurately simulate changes to the system due to temperature and pressure cycling and materials performance issues like corrosion.
Visualizing Well System Breakdown; Experimental and Numerical Analyses. 51st U.S. Rock Mechanics Geomechanics Symposium, San Francisco, CA, June 25-28, 2017; American Rock Mechanics Association.
Ziomek-Moroz, M.; Huerta, N. J.; Beck, J.; Lvov, S.; Tiwari, A.; Kutchko, B.; Hawk, J. A.; Rose, K. Exploration and Production in Arctic – A Report on Assessment of Geomechanical, Corrosion and Drilling Challenges; NETL TRS report, under revision.
Beck, J.; Fung, R.; Hall, D.; Buyuksagis, A.; Ziomek-Moroz, M.; Lvov, S. Effects of H2S and CO2 on Cement/Casing Interface Corrosion Integrity for Cold Climate Oil and Gas Well Applications. Electrochemical Society Transactions 2016, 72, 107–122.
Feng, R.; Beck, J. R.; Hall, D.; Ziomek-Moroz, M.; Lvov, S. N. Corrosion Behavior of 13Cr Casing Steel in Cement-Synthetic Pore Solution Exposed to High Pressure CO2 and H2S. Electrochemical Society Transactions 2015, 69, 27–40.
Sengupta, A.; Beck, J.; Zhao, H.; Schatz, R.; Ziomek-Moroz, M.; Lvov, S. N. Corrosion Behavior of 13Cr Casing in Cement Simulated Pore Solution. Electrochemical Society Transactions 2015, 66, 13–36.
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*Image Source: NETL
*Image Source: NETL
Two X-ray CT images of a scaled well system subject to internal pressurization.
(a) At 3,000 psi the cement has begun to fail (red oval).
(b) At 4,500 psi, internal confining pressure the cement and rock have failed catastrophically.