Shallow compaction and fluid flow pattern in reservoirs and caprocks underneath Barents Sea

The Barents Sea has vast oil and gas resources concealed in the sedimentary rocks underneath the sea floor. Seismic exploration in the Barents Sea and many other places offshore Norway has found enigmatic underground features that represent narrow vertical channels of focused fluid flow, the so called 'fluid escape pipes' and 'gas chimneys'. Focused fluid flow features and low reservoir pressure represent challenges for petroleum industry, causing problems during drilling and increasing the risk for seafloor installations. On the other hand, these features are often related to hydrocarbon reservoirs and might indicate the presence of hydrocarbons. Seismic chimneys have also recently gained attention in the context of CCS as it is not entirely clear whether they can affect the safety of CO2 underground storage. However, very little is known about the processes of their formation and their exact relation to reservoirs. Such understanding will help to answer practical questions about the potential risks associated with focused fluid flow features and how to use them in our search for new oil and gas resources. Compaction and decompaction of the rock in the geological past, e.g. during the glaciation and deglaciation periods, significantly affected the distribution of petroleum and other fluids under the ground and might be responsible for formation of the complex fluid flow pattern underneath the Barents Sea. Thus, in this project we have combined experimental and numerical modelling approaches to understand how focused flow structures and fluid pressure anomalies might have formed in the Barents Sea. We show in the lab that common reservoir rocks exhibit viscous properties and non-linear stress dependency of permeability, which might be responsible for formation of chimney structures. Experimental results are incorporated into a constitutive model that provides a simple description of rock behavior under a wide range of strain rates. Our numerical simulations indicate that time-dependent rock (de)compaction yields ascending solitary porosity waves forming high-porosity and high-permeability vertical chimneys that will reach the surface. The size and location of chimneys depend on the reservoir topology and compaction length. Chimneys formation is a fast process, which takes only a few months to fully evolve and ultimately lose their connection to the reservoir. Their formation is accompanied with the self-healing suggesting these may not be long-term migration pathways once fluid flow stops. Our model provides a simple description of rock behavior under a wide range of strain rates. The model predictions agree well with experimental data from triaxial instantaneous and creep tests.

Project contributes to understanding of fluid flow processes that can lead to leakage of hydrocarbons and injected fluids from reservoirs. A better control of rock creep rates provided by the project, makes it possible to put numbers into models and make predictions for real geological environments. Experimental data obtained in the project can be used for validation of various theoretical and numerical models. Our analyses indicate that time-dependent rock (de)compaction yields formation of high-porosity and high-permeability vertical chimneys that will reach the surface. The size and location of chimneys depend on the reservoir topology and compaction length. Chimneys formation is a fast process, which takes only a few months to fully evolve and ultimately lose their connection to the reservoir. Given the fast rates of chimney formation, mitigating measures in case of leakage over CO2 storage sites must be in place as soon as injection start.

Barents Sea has vast oil and gas resources hidden in the sedimentary rock sequences underneath the sea floor. For safe and optimal exploration of these resources the understanding of fluid flow processes in such rocks needs to be gained. Geological sequences underneath the Barents Sea exhibit highly developed fluid flow features such as fluid escape pipes and gas chimneys. There are also indications of underpressure in the few wells. In the proposed project we will focus on the geological processes that could have formed these features. First, we will study the rheological behaviour of carbonates and other reservoir rocks. We will experimentally explore compaction and decompaction creep of selected rock specimens from the Barents Sea in order to determine rheological laws that can be used as input parameters for coupled geomechanical-reservoir simulators and basin modelling. Rock response will be measured under both compressional and extensional stress regimes, under triaxial conditions. Using obtained experimental data we will constrain in-house developed theoretical models for viscoelastoplastic (de)compaction of fluid-saturated porous rocks and basin evolution. Verified models will be further used to study underpressure development and generation of focused fluid flow features in underneath the Barents Sea. The work will be done in close collaboration between IFE (Norway) and Skoltech (Russia).