By ensuring the containment of CO₂ in the storage reservoir, we can optimise the storage capacity and maximise the number of suitable reservoirs. These goals can only be achieved by addressing the remaining research gaps identified in the near-well area (well integrity, CO₂ injectivity) and at reservoir borders with the caprock (especially faults). Therefore, Task 10 looks at selected issues of relevance for filling these gaps, putting a special emphasis on the understanding of geomechanical aspects and developing testing methodologies to be used when considering new storage sites.

Near-well area
Derisking of CO2 storage is very important in CCS. E.g. environmental scientists have expressed concerns with the risks related to CO2 storage offshore. Thus, the purpose of the geomechanics part of our research is to maximise storage capacity with minimum risk of a significant leakage to any layer under the sea bed, as the presence of CO2 can constitute an unacceptable hazard if it leaks then to the sea.

To be able to realize the above mentioned risk mitigation, improved geomechanical models for making predictions of fracturing near the injection wells are needed. The ambition is to create simple enough models so that these can easily be implemented in the codes the industry already possesses. The Task will work on this in cooperation with Task 9, as their main focus is structural derisking of rock faults.

The Task will also investigate the development of thermal stresses in proximity to the well and injection conditions (pressure, temperature), as they can lead to tensile stresses around the well. This can in turn develop into fracturing in the cement sheath or sealing shale layers. Tensile stress near the well and their consequences on reservoir containment will therefore be closely investigated, both in the short- and long-term perspectives. Also, the effect of exposure to CO2 on creep (shale or salt) will be researched, as creep could be a healing mechanism for initial leakage close to wells.

Well integrity
Although avoiding compromise in the near-well area remains unquestionably important, the wells themselves remain the most likely pathways for potential leakage of stored CO2. This is because wells make artificial short-cut routes all the way from reservoir to topside. Avoiding well leakage will be addressed by this Task by ensuring there are multiple barriers between the storage reservoir and the surface.

Through our research on CO2 well integrity, we will develop an atlas. The atlas will contain a check list of well integrity issues compromising CO2 storage success. E.g. the atlas will include an overview of cement bonding both to rock and to metal casing to put in industry cementing software when planning new wells or evaluating legacy wells.

In addition, the industry has reacted positively to proposed research on injectivity problems, as such issues have arisen in pilots such as Snøhvit and Ketzin. A methodology of which laboratory tests to perform in order to qualify injection wells for CO2 storage will accompany this atlas.

The activities above have been chosen in accordance with the preferences of Statoil, Shell and Total as expressed in the meetings with them

Results 2021

Last year we completed the second stage of well cement qualification in terms of the bond strength to rock. This involved moving away from a simple neat cement formulation that is easy to analyse due to its simple formulation, and where the test matrix did not involve investigating the weight of many components and additives. However, relevance to field practice was limited when not considering real cement compositions used in the Norwegian offshore, and therefore constituting the main risk elements in legacy wells towards loss of CO₂ containment in the intended reservoir. In the work performed in 2021 and reported in deliverable DT10-2021-1A, the effect of adding micro-silica was investigated in the presence of carbonated brine, simulating exposure to CO₂ seeping up from the storage reservoir. This additive is used in oil and gas well cement to hinder the strength loss for high temperature conditions. The results in our study showed that strength reduction after CO₂ exposure did occur but was less than 10 % for the chosen conditions.

Figure 1: A tomography reconstruction of macro-porosity and crystal precipitation on the surface of micro-silica containing cement plugs exposed to CO₂-brine mixture: (a) After exposure for four weeks; (b) Reference (unexposed) sample. Red – voids, blue – precipitated particles on thesurface.
Figure 1: A tomography reconstruction of macro-porosity and crystal precipitation on the surface of micro-silica containing cement plugs exposed to CO₂-brine mixture: (a) After exposure for four weeks; (b) Reference (unexposed) sample. Red – voids, blue – precipitated particles on thesurface.

2021 also saw the end of our well integrity atlas activity, with a publication in the International Journal of Greenhouse Gas Control and the organisation by our partners in the US of a well integrity workshop in May. The journal paper combined the results from our survey, a detailed questionnaire sent to all active CO₂ injection operations worldwide, with our own and published experience from research in well integrity. The atlas provides a unique insight into which difficulties are most encountered, and where future research efforts are needed.

Collaboration with Task 9 was further strengthened in 2021, with additional funds being made available to increase common research on shale creep deformation. This is seen as an important part of the mechanical response of many caprock formations to constant stress conditions, which may play an important role in healing fractures underground, and thus further increase the confidence in the long-term soundness of geological CO₂ sequestration. The collaboration culminated in the submission of an NCCS-supported KSP application to the Research Council of Norway, in collaboration with UNIS, on the theme of shale creep laboratory testing and modelling. A second proposal was chosen by NCCS involving Task 10 and Task 12 on well integrity, with a focus on laboratory testing using the ECCSEL well integrity research infrastructure and the newly installed industrial CT scanner. The theme of this second proposal was the experimental investigation of fracture network development in the near well area and early warning geophysical methods of fracture and leakage detection. In addition, collaboration with University of Pittsburgh and Colorado School of Mines, as well as Queen Mary University of London, will make for additional laboratory and modelling contributions, not the least formulation of remediation fluids that can be tested in the ECCSEL rig.

Results 2020

The report on the well integrity survey (Atlas of Well Integrity) was completed in 2020. The number of teleconferences with our partners across the Atlantic, Lawrence Livermore National Laboratory in California and the National Energy and Technology Laboratory in Pennsylvania, doubled as the reporting deadline approached. The learnings from the approximately 30 replies from operators in charge of CO2 injection wells all around the world were compared to published scientific literature. We have planned a joint USA-Norway workshop in 2021, where we will present the results and discuss the prioritisation of remaining research gaps.

Confident from the benchmarking of previous fault reactivation and hysteresis work using two different computer codes, we tackled shale creep modelling as a way to self-heal micro-annuli potentially developing between well cement sheath and surrounding caprock. A finite element simulation was set up where we looked at the competition between fluid pressure pushing apart the crack faces and shale creeping back in place through constant stress creep.

In our model, a behaviour law for the shale was derived from laboratory measurements done on small Draupne shale samples, exposed to different fluids including brine and super-critical CO2. Thus, our model extrapolates from the time-dependent stiffness uncovered in the fluid exposure tests to predict rock deformation up to 10 000 years after injection. The interesting results show a cross-over point from fracture closure to remaining open, depending on stress conditions and fluid pressure.

Computer model investigating creep of caprock shale exposed to CO2 towards a well's cement sheath. In the case presented here, the contact stress between cement and caprock relaxes with time and the crack closes.
Computer model investigating creep of caprock shale exposed to CO2 towards a well's cement sheath. In the case presented here, the contact stress between cement and caprock relaxes with time and the crack closes.

Main results 2019

Last year we completed the elaboration of a suggested laboratory methodology to measure the tensile strength of the cement to formation bond. Crucial tests were successfully performed on cement and shale composite plugs, where cement was cured under relevant field conditions.

The bond's tensile strength is the weakest link in maintaining well integrity and this is exacerbated when bonding to shales. This work led to the publication of two peer-reviewed papers, a presentation at the ARMA conference in New York. Many also acknowledged that finally there is a method to measure much needed input parameters for computer simulation work. The methodologies developed in Task 10 will be useful for recommended guidelines when evaluating storage capacity and derisking new considered sites.

In the event of large-scale CCS taking off, this will enable oil and gas or mining service companies equipped with good geomechanics laboratories to offer site survey studies in terms of expected well integrity performance.

Our collaboration with Lawrence Livermore National Laboratory (LLNL) was strengthened by mutual work on a well integrity problem survey. A detailed questionnaire was sent to all active CO2 injection operations worldwide. A first round of answers was received and analysed, giving unique insight in which difficulties are most encountered, and where future research efforts are needed. This work will feed into the Well Integrity Atlas, a compilation of well integrity issues encountered at CO2 storage operations, their severity, mitigation measures and their success. This will be available to all operators, such that learnings can be more readily distributed, but also naturally lead the research community in identifying and addressing remaining gaps.

Together we have proposed new models

We have collaborated closely with Task 9 and the supporting researcher project SPHINCSS. This has led to a series of conceptual numerical models of simple fault and stress conditions whereby permeability along the fault and hence leakage risk from a storage site increases.

The results, where the structure of the fractured zone on each side of the fault core is explicitly modelled using SINTEF's fracturing tool MDEM (modified discrete element method), are surprising as they predict that depleted reservoirs might be more at risk than virgin aquifers, due to stress history effects. The main conclusions have been confirmed using a commercial finite element tool, FLAC by ITASCA.

On the basis of this work, we have together with Task 9 proposed new models which address reactivation problems and help us understand specific differences between depleted oil and gas reservoirs and aquifers. This is crucial for gaining confidence in where to invest in CCS and how to scale up operations.

New finite element simulations confirming in-house fracturing software results: fault slip towards shallower layers is observed for depletion of reservoir as evidenced by the plastic deformation map (top) with resulting stress distribution map (bottom).

Results 2018

Main Results

Successful measurement of the interface tensile strength between cement and caprock.
A further injection test was carried out with somewhat higher scCO2 inflow rate and the same counterflow of brine as before. The test resulted again in massive clogging of the rock core, this time on a larger volume and without helical instability.
Formulation of Atlas of well integrity questionnaire in the USA.

Impact and innovations

Work in Task 10 addresses face on the most pressing well integrity and near-well effects needing to be addressed in order to open for massive and large-scale sequestration of CO2. These are the issues remaining where especially legacy wells and high rate injection could compromise safe CCS operations.
The innovation resides in the new laboratory methods used to simply isolate the identified weak elements in all researched topics, so as to address the highest remaining risks at the lowest cost and most effective manner.

Precipitated salt in 2 Berea sandstone plugs submitted to different CO2 injection flow rates (increasing from left to right)

Results 2017

Task 10 deals with geomechanics and well integrity, focusing on derisking the well and near well area. The wells by which CO2 is injected in a storage reservoir are identified as the largest risk for leakage from the reservoir. A laboratory activity was initiated to look at injectivity loss due to precipitation of salt near the wells. The first test resulted in massive clogging of the rock core, even though low salinity was chosen to match North Sea conditions.

A small activity also looked at developing geological fault description in geomechanical software, to help predict conditions for a leakage to occur along a fault. This activity relates to work being done in task 9 on faults. The in-house SINTEF software MDEM was used to look at injection conditions where a conductive path would be created in the weak zone alongside a fault.

Publications

Publications listed below are not in registered in Cristin.

See all other NCCS Publications registered in Cristin, the Norwegian Research Information system.

Conference Publications

2019:

  • Top Seal Undrained Pore Pressure Response - Sensitivity to Skempton's A - M. Duda. The 5th EAGE Fault & Top Seals conference
  • CEMENT BOND STRENGTH MEASUREMENTS - Opedal, Nils van der Tuuk; Cerasi, Pierre; Vrålstad, Torbjørn. ASME 2019 38th International Conference on Ocean, Offshore and Arctic Engineering
  • Tensile strength of cement to shale interface - A. M. Stroisz, K. Gawel, R. Bjørge, P. Cerasi. 53rd US Rock Mechanics/Geomechanics Symposium

2018:

  • Modelling faults i the SINTEF MDEM code - Cerasi P., Rongved M., Bauer A. GHGT-14, Melbourne

Task leader

Pierre Rolf Cerasi

Senior Research Scientist
993 40 928
Name
Pierre Rolf Cerasi
Title
Senior Research Scientist
Phone
993 40 928
Department
Petroleum
Office
Trondheim
Company
SINTEF AS