Shale as a Permanent Barrier after Well Abandonment
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During the lifetime of petroleum wells, the purpose of the well barrier elements is to ensure well integrity within the short and long time frame while it is exposed to its surrounding environment. Leakages of the sealing material outside the casing can cause serious environmental damages and it may require a new annular sealing operation which can be both time consuming and expensive. The prediction of the natural creeping process of shale to seal off the annulus is a complex and time-delayed process influenced by rock properties, stress configuration and temperature. The understanding of the behavior of creep in the borehole geometry is crucial. Under certain conditions, an annular shale seal around the wellbore creates a barrier element increasing its chances to maintain its functionality and efficiency, better than cement over time. Understanding of underlying physics to predict the creep process and establish a footprint of creeping formations, is necessary. Based on published scientific papers and a survey among the operators, creeping formation is commonly used as an annular well barrier element at the Oseberg field, the Troll field and the Brent field. Several wellbores have verified creep at the Valhall field, while there is still no indication of displacing shale at the Ekofisk field. Verification has to occur through two independent logging measurements and one pressure test to ensure sufficiently bonding and formation strength for the shale well barrier element. Useful indicators for predicting a potential creeping shale are Young s modulus and the presence of the minerals; clay, quartz, CaCO3, organic materials, illite and smectite. However, the minimum level of smectite is still uncertain. In terms of observations at The Brent Field, there was found that smectite do not control the creep process alone. By increasing the casing size, expanding existing casing strings or using an annular fill material to reduce the annular gap, it is possible to achieve sealing earlier for low creep rates. The time-dependent behaviour of creeping shale as a self-sealing annular barrier around the borehole was studied through a relaxation scheme by reducing the contact force as a function of time in order to control the stress in the material. The thermal gradient induced on creep was explored by considering the temperature at the individual contacts. The creep scheme was applied to seven wellbore temperature configurations. The scheme was calibrated against data from the core sample Haynesville-1V. Based on that, the relative annular gap varies only with 2% among the temperature scenarios after 1000 days, and the formation near the borehole seems not to be sufficient affected by the implemented thermal induced creep scheme. However, the creep process under the production- and the injection conditions deviate from the base case. In the beginning, the annular gap closure rate reduces rapidly before the sealing rate decreases when the formation influence the casing. The process may continue to seal off the wellbore. Otherwise, it will stop because of the pressure caused by the casing and/or the arching effect from the elastic part of the formation. The radial deformation of shale in terms of creep may be a reason for casing ovaling and casing collapse in wellbores worldwide. This may emphasize the importance of predicting creeping formations for both annular sealing elements in the permanent plug and abandonment and during the construction of casing design. Further investigations are recommended in several shale samples to evaluate these observations. Extended research in the field of improving the numerical scheme for predicting thermal induced creep to seal off the annulus is encouraged.