Thermal properties of reservoir rocks, role of pore fluids, minerals and digenesis. A comparative study of two differently indurated chalks by Tijana Voake Thesis submitted in fulfilment of the requirements for the degree of PHILOSOPHIAE DOCTOR (PhD) Faculty of Science and Technology Institute of Energy Resources 2020 University of Stavanger NO-4036 Stavanger NORWAY www.uis.no ©2020Tijana Voake ISBN: 978-82-7644-894-8 ISSN: 1890-1387 PhD: Thesis UiS No. 494 Acknowledgements I would like to express my deep gratitude and appreciation to my supervisors Dr. Anders Nermoen and Prof. Ida Lykke Fabricius for their guidance, assistance and support. Thank you for the trust you put in me and encouraged me to present my work at different conferences around the world. I would also like to thank Reidar Inge Korsnes and Kim Andre N. Vorland for their guidance and help in the laboratory works. I thank the National IOR Centre of Norway and University of Stavanger for support, allowing me to work with experts within the field of rock mechanics. I sent my big thanks to my colleagues and PhD fellows for technical and moral support. The Research Council of Norway and the industry partners, ConocoPhillips Skandinavia AS, Aker BP ASA, Eni Norge AS, Total E&P Norge AS, Equinor ASA, Neptune Energy Norge AS, Lundin Norway AS, Halliburton AS, Schlumberger Norge AS, Wintershall Norge AS, and DEA Norge AS are acknowledged for their financial support. I would like to thank my parents, Mirjana and Branko Livada, for always being by my side and their full support along my long educational path. I thank my husband Alexander for making me apply for this opportunity, believe in myself, cheering me on, proof reading and making this journey an unforgettable one. And finally, I would like to thank out little daughter Martha for making this experience even more meaningful. iii Summary Carbonate rocks are distressed by temperature fluctuations, most commonly observed in marble monuments and cladding exposed to varying outdoor temperatures. Similarly, reservoir rocks are cooled on periodic basises during oil production by injecting cold fluid, locally cooling the surrounding reservoir rock. The rock will then reheat to its original temperature when the injection stops. These temperature fluctuations can potentially cause deformation to the reservoir and change its mechanical properties, which must be taken into consideration during the recovery. This body of work focuses on chalk reservoirs, where the proposed effect could be of importance because the main building agent of chalk is calcite, which has a highly anisotropic thermal expansion coefficient. Thus, temperature fluctuations could strain the grain contacts between neighbouring particles. To gain a better understanding of stress accumulation at the contact level, two chalks with differing degrees of contact cement are compared; higher indurated chalk from a quarry in Kansas (USA) and a low indurated chalk originating from Mons (Belgium). The interpretation of temperature dependence is based upon the analysis of three different series of experiments, each providing different mechanical parameter estimates relating to: tensile strength, elasto-plastic partitioning during hydrostatic stress cycles, and elastic moduli (Bulk modulus, Young’s modulus and shear modulus). iv The influence of temperature cycling on tensile strength was tested using samples from the two chalk types in dry and water saturated states, and then exposed to 0, 15, and 30 temperature cycles. The dry samples were not influenced by temperature cycling for either of the chalk types. However, in the water saturated state, tensile strength is increasingly reduced with a progressive number of temperature cycles for both chalks. The effect of temperature cycling was further examined by hydrostatic stress cycling in order to compare their mechanical responses. The two types of chalk were saturated by two different fluids to additionally determine the importance of water weakening. During a hydrostatic stress cycle, the total volumetric strain is partitioned into a reversible (elastic) and irreversible (plastic) component. Here, the fraction of irreversible strain during each stress cycle is reported and compared for the two chalks and two saturating fluids. All tests exposed to temperature cycling between each stress cycle accumulated more irreversible strain, the biggest difference was observed for the water saturated, highly indurated, Kansas chalk. Using the samples from the two types of chalks, the dependence of elastic moduli on temperature cycling was investigated. The bulk modulus tested under hydrostatic conditions did not show significant dependence on increasing numbers of temperature cycles for Kansas chalk, but the modulus for Mons chalk demonstrated a decreasing trend with increasing numbers of temperature cycles. Young’s modulus and compression modulus measured under confined constant overburden v stress showed no effect with increasing numbers of temperature cycles. Thermal linear expansion coefficient had a decreasing trend with increasing number of temperature cycles. In response to temperature variation, the anisotropic thermal expansion of calcite crystals induces thermal stress that resulted in the accumulation of irreversible strain and decrease of tensile strength. This was especially pronounced in water saturated samples. Water is a polar fluid and has an electrical charge, so an electrical exchange with the calcite surface occurs. This creates a so called electrical double layer, whose thickness is assumed to be directly proportional to the Debye length. The thickness of Debye length is seen to depend on temperature, increasing with increasing temperature. When the temperature is increased, due to the increase of the double layer, the area of repulsion between two neighbouring particles is also increased. It is proposed that fluctuating the length of charged double layer has a permanent damage on chalk through increasing the number of micro fractures. Another contributing factor as to why temperature cycling enhances weakening in water saturated chalk samples rather than dry or Isopar H saturated samples, is that the surface energy �" of the main calcite cleavage plane #1014' depends on the presence of water. The surface energy of dry and fully hydrated calcite surfaces are 0.32 J/m2 and 0.15 J/m2 respectively. Hence, the energy necessary to form a dry surface (fracture) is double the energy required for a formation of wet surface. vi List of publication Papers: Paper I: Voake, T., Nermoen, A., Ravnas, C., Korsnes, R.I., Fabricius, I.L. (2019). Influence of temperature cycling and pore fluid on tensile strength of chalk. Journal of Rock Mechanics and Geotechnical Engineering 11(2): 277-288. Paper II: Voake, T., Nermoen, A., Korsnes, R.I., Fabricius, I.L. (2019). Temperature cycling and its effect on mechanical behaviours of high- porosity chalks. Journal of Rock Mechanics and Geotechnical Engineering 11(4): 749-759. Paper III: Voake, T., Nermoen, A., Korsnes, R.I., Fabricius, I.L. (In review). Elastic moduli dependence on temperature cycling in high porosity chalks. Journal of Rock Mechanics and Geotechnical Engineering Paper IV: Voake, T., Nermoen, A., Korsnes, R.I., Fabricius, I.L. (2017) Induced shear failure by temperature reduction at uni-axial strain conditions. EAGE - 19th European Symposium on Improved Oil Recovery/IOR Norway, Stavanger. Paper V: vii Voake, T., Nermoen, A., Korsnes, R.I., Fabricius, I.L. (2016). To what degree thermal cycles affect chalk strength. SCA annual symposium. Snowmass Colorado. Conference contributions: Meireles, L., Nermoen, A., Voake, T., Welch, M., Fabricius, I.L. (2018) Compaction driven IOR in chalk reservoirs. 39th Annual workshop & symposium, IEA-EOR. Copenhagen. (Presentation) Voake, T., Nermoen, A., Korsnes, R.I., Fabricius, I.L. (2018). Temperature cycling and its effect on stress-strain relationships in high porosity chalks. EGU, Vienna. (Poster) Voake, T., Nermoen, A., Korsnes, R.I., Fabricius, I.L. (2018). Temperature cycling and its effect on stress-strain relationships in high porosity chalks. IOR Norway 2018, Stavanger. (Poster) Voake, T., Nermoen, A., Korsnes, R.I., Fabricius, I.L. (2017) Induced shear failure by temperature reduction at uni-axial strain conditions. EAGE - 19th European Symposium on Improved Oil Recovery/IOR Norway, Stavanger. (Poster) viii Voake, T., Nermoen, A., Korsnes, R.I., Fabricius, I.L. (2016). To what degree thermal cycles affect chalk strength. SCA annual symposium. Snowmass Colorado. (Poster) ix Table of Contents Acknowledgements .......................................................................................... iii Summary .......................................................................................................... iv List of publication ........................................................................................... vii 1 Introduction ............................................................................................... 1 1.1 Physical properties of chalk ....................................................................... 1 1.1.1 Calcite anisotropic thermal expansion .................................................... 1 1.1.2 Chalk water weakening .......................................................................... 3 1.1.3 Calcite surface charge and the adsorption of surface-active ions ........... 4 1.2 Mechanical properties and how they are measured ...................................