Shale Barrier Toolbox: Designing future wells for efficient completion and simpler P&A

For millions of years, Norwegian oil and gas have rested properly wrapped below kilometres of tight shale layers. Deep wells provide access to these reserves; however, the wells may also provide potential pathways for uncontrolled migration of hydrocarbons to the surface, unless the wells are effectively sealed. An oil well is essentially a hole in the ground, lined with a steel tube called casing. Along some sections of the well, the gap between the casing and the surrounding rock is filled with cement in order to prevent leakage of oil or gas along the well, but in large sections of the well this gap remains open when the well is completed. The usual thing to do when the wells are to be abandoned is to fill critical sections of this gap with cement. This is a time consuming and expensive process. Field experience has shown that some shales have the ability to creep in and close the gap all by themselves, thus creating a "shale barrier" around the well. This represents huge cost savings for plugging and abandonment operations, if it occurs. In addition, shale is much tighter than cement, and less vulnerable for deterioration. There is currently no well established method for predicting the occurrence of such shale barriers, nor for activating the formation of them in case this does not happen naturally. Moreover, there are also challenges related to detection of such barriers since they are hidden behind the casing. This project is addressing these challenges. The goal of the project is to provide the operators with a toolbox for optimizing the use of shale barriers for efficient and permanent sealing of wells. The project work involves both experimental and theoretical studies for better understanding of how formation stresses, borehole trajectory, mud composition etc. affect the shale barrier forming process as well as the tools used to verify the presence of the barriers. The quality of various formations as potential shale barriers will also be assessed. The project has performed petrophysical and rock mechanical characterization of soft field and outcrop shales. The measurements includes also time dependent deformation (creep) and classification of sealing properties from barrier experiments developed to simulate realistic barrier development. Experiments on shale activation with use of chemical additives to the annulus fluid have been performed and development of setup and use of drilling mud in the annulus during barrier formation has been done. This has given information on how the sealing properties of the barrier is affected by these fluids. An extensive study of the pressure test methods used in the field today to verify barriers have been performed over the last years. This study has now been published in the OMAE 2020 conference. The project has also gained new knowledge and verification of analytical models from 3D FEM simulations of creep in borehole geometries. The barrier experiments have given new insights on the effect on pressure variation and heavy drilling mud in the annulus on sealing properties of the barrier. Together with numerical simulations have this supported the development of an analytical model for estimation of probability for shale barrier, sealing properties and load on casing from the barrier forming shale. The model (Toolbox) use input data from petrophysical logs and data that the operators usually have available for their fields. The Toolbox model is verified on laboratory data, tested with good result on field data and is now made available to the project participants.

The project has contributed to the general understanding of time dependency in shale and specifically on shale as a barrier through advanced experiments and modelling of field shale material. This knowledge has been built in SINTEF in collaboration with the eight industry participants in the project and distributed not only to the participants but also through multiple conference presentations and journal articles. The models and software developed in the project enables the users to plan for use of shale barriers in old wells ready for P&A or how to design well paths to optimize the probability to have shale barriers and also to select minimum casing weight sufficient to hold the expected in situ stresses. Using shale barriers enables the operator to leave the casing in hole and hence reduce the cost of P&A while still ensuring an efficient seal between the hydrocarbons and the surface. The knowledge on shale barriers is also applicable in the new emerging field of CO2 storage on the Norwegian Continental Shelf. New storage sites may use the knowledge from this project to design CO2 injector paths to be optimized for using shale as an annular barrier and hence both reduce cost, cement used and ensure good sealing between the CO2 and the surface. Furthermore, the understanding generated in this project on of time dependent properties of soft shales may be used to improve borehole stability models and tools which again may reduce the drilling cost of new wells.

Oil and gas wells constitute open passageways for hydrocarbons, from the perfectly sealed reservoirs to the free surface. The operators control flow within the wells, however there is a risk that the annulus between the casing and the rock may become undesired channels for leakage. Such leakage may represent a safety hazard as well as environmental problems and considerable costs for the operating company. In critical sections of a well, the annulus is filled with cement injected at the casing shoe, in order to provide proper sealing and avoid leakage outside the well. This procedure may leave long sections of the well un-cemented, which may provide leakage paths along the annulus through otherwise sealing formations. In a number of cases it has been observed that the surrounding shale creeps into the un-cemented annulus and eventually forms an efficient sealing barrier around the well. The mechanisms involved when a shale barrier is formed are currently under investigation, and active implementation of such barriers is therefore limited. However, active use of this technology has a huge environmental as well as economic potential. The main goal of this project is to contribute to reduction of climate gas emissions and environmental impact by providing scientifically based tools to optimize the use of shale barriers. This will be achieved through better assessment of the available means to control the mechanisms, construction of dedicated tools for decision-making support, and education of experts and training of petroleum engineers.