MECHANISMSOF SEDIMENT COMPACTIONRESPONSIBLE FOR OIL FIELD SUBSIDENCE. A thesis submitted to the University of London (University College London) for the degree of Doctor of Philosophy in the Department of Geological Sciences. by Michael Anthony Addis October 1987 -1- to my parents for all their encouragement -11- Abstract During the production of a hydrocarbon reservoir, the compaction of weakly cemented sedimentary materials can result from increases in effective stress, and lead to surface subsidence. Such a phenomenon has recently been observed in the oil and gas bearing chalk fields in the Central North Sea. In order to evaluate the compaction potential of sedimentary materials during exploitation of a reservoir, laboratory experiments- were performed on chalks and clays. These experiments were predominantly'Ký (zero lateral strain) tests. The tests were performed in a high pressure triaxial cell, the development of which continued throughout the experimental 'programme. Tests performed on chalks from the Central North Sea, and from two in onshore sites southern -England showed similar deformational trends. The analyses of these results concentrated on the variables of testing and the possible errors resulting from the use of laboratory data in the modelling of field situations. The analyses of the tests also include a comparison between the experimental methods and the interpretation of the results of this study and those of other workers on the subject of reservoir compaction. A parametric description of the compaction of chalk is presented as a summary to these tests. w Two compaction tests on clay samples from the Central North Sea were also undertaken. The clays were uncemented and show contrasting behaviour to the chalks. These tests were performed to evaluate the amount of compaction likely to occur in the overburden of a hydrocarbon reservoir during production. The results of the deformational trends obtained from this study -111- are compared to those obtained by other workers, with particular reference to the changes in physical parameters during compaction. This follows a literature survey into hydrocarbon reservoir compaction. The K0 tests performed in this study are thought to represent the condition of sediment burial in a tectonically inactive basin. This subject is briefly reviewed, and the relevance of the results presented earlier discussed in terms of the prediction of stresses existing within differing lithologies. The analysis of the results have been performed using parameters commonly in used soil mechanics, this seemed to be appropriate for the deformations undergone by the materials used in this study. -1V- Acknowledgements During the period of research the author has been in receipt of a Natural Environment Research Grant, whibh is duly acknowledged. During the three years of research the author has been associated with three colleges in the University of London. The first two years of the research were performed while a student at the Geology Department of King's College, now sadly "mergered" in outer London. The final year was spent as a student of University College. During the three years, the experimental" work was performed in the soil laboratories in the Department 'of Civil Engineering, at Imperial College. As such I am indebted to many people from the three institutions to numerous to mention. I would however like to thank the chief technicians from King's and University College, Mr. J Congram and Mr. R Dudman respectively, for ensuring relatively few hickups along the way. I would especially like to thank the technical staff of the soil laboratories who have provided excellent support to this study, with much input into the equipment design and construction, as well as many other subject areas. I am sure that without their expertise, help and patience this study would not have been possible. Mr. S. Ackerley, Mr. L. Spall, Mr. G. Keefe, Mr. A. Bolsher and Mr. R. Hare are gratefully acknowledged. I would also like to thank Mrs. E. Gibbs for all her help, and for her forgiveness during times of oil leaks. Particular thanks are given to Mr. S. Ackerley whose enthusiasm, general willingness to do the impossible and his patience with an ignorant geologist is gratefully appreciated. Technical help was also provided by Mr. T. Osbourne and Mr. S. at -V- University College and Mr. R. Giddons in the Department of geological sciences, Imperial college who are duely thanked. During the initial set up of the apparatus the help of Mr. B. Clarke in the Engineering Geology section in Imperial College was sought and willingfully obtained on many occasions; Iwould like to express my thanks for his help and advice. My period of work at the soils laboratory, Imperial College was a true education and I must thank my colleagues in this group for answering my many questions patiently., regardless of the elementary nature of these, and for their encouragement during this study. My many thanks *go to Mr A. J. Bond, Dr. G. T. Dounias, Mr. D. French, Mr. J. Hellings, Mrs. G. N. Georgiannou Lesas, Dr. M. Maccarini, Mr. M. Mahmoud, Dr. J. Maswoswe, Mr. D. L. Nyaoro, Dr. E. Ovando-Shelley, Mr. D. Schreiner, Dr. S. Shibuya and Mr. D. Toll, Mrs. T. Tika Vaffilikos. I would also like to thank Dr. M. Maccarini for our many discussions on the behaviour of cemented materials and his ready exchange of ideas. The help of Mr. D. Toll and use of his computer software. expertise and for the calculation of results is gratefully appreciated. Use of software developed by Mr. A. J. Bond for the 3-dimensional plots is also gratefully appreciated by the author. I am very grateful to have been able to work at the soil mechanics laboratories, and as such I must thank the head of the soil mechanics section, Prof. J. B. Burland for this opportunity. I would also like to express my gratitude to the academic staff of the section, Dr. R. J. Chandler, Prof. J. N. Hutchinson, Dr. R. Jardine, Dr. S. Leroueil (visiting academic), Dr. D. M. Potts, Prof. A. W. Skempton, Dr. P. R. Vaughan and particularly Dr. A. E. Skinner, for many stimulating and enlightening discussions. Many thanks go to the Norwegian Petroleum Directorate for allowing -vi- me the opportunity of working on samples obtained from the North Sea. I would especially like to thank Mr. K. Tonstad, Mr. 0. P. Berget and Mr. P. E. Overli, for many fruitful discussions. Colleagues at King's College and University College are also thanked for all their support and help in very many ways, my thanks go to Dr. H. Chen, Mr. M. J. Leddra, Mr A. Goldsmith, and Miss N. A. Yassir. I am especially grateful to Miss N. A. Yassir for correcting my grammar -a most unenviable task, many thanks. At this point I would like to express my gratitude to my numerous co-tenants/ colleagues for their patience with the author during the period of research, as well as for their welcome distractions and excellent company, these include Mr. H. Davies, Miss K. Jeffr*ey, Miss K. Lewin, Mr. J. Morris, Mr. A. Pritchard, Mr. M. Shaw, Dr. J. Williams, and Miss M. White. It only remains for me to thank my supervisor, Dr. M. E. Jones for his help and encouragement, and for directing me towards a very interesting area of study. His support is acknowledged and appreciated especially in times of critisism from the author. I would finally like to express my appreciation to my family and especially-to my parents, who have always encouraged and supported me, not only through this period of research, but through my whole education, the good and not so good times. My thanks are insufficient to express my gratitude. -vii- Contents i Title ............... 111 Abstract ............... Acknowledgements Contents .............. viii List-of figures ............... Xl List tables of ............... xvi Glossary terms of ............. xviii Chapter 1 Introduction 1 ............... Chapter 2 Aspects 8 of compaction ............... 2.1 Introduction 8 ............... 2.2 The 10 principle of effective stress ............... 2.3 Undrained pore pressure response ............... 18 2.3.1 Sample 20 saturation ............... 2.4 One dimensional 21 consolidation ............... 2.5 Representations in 24 of stress states sediments ........... 2.5.1 Stress paths ............... 24 2.5.2 Critical state model ............... 28 2.6 Mechanical behaviour of chalks ............... 34 2.6.1 Behaviour bonded of particulate materials ........ 34 2.6.2 Stress-strain characteristics of weakly cemented sedimentary materials ............... 39 2.6.3 Compaction 42 of chalk .............. 2.6.3. i Normal 42 consolidation ............... 2.6.3. ii Secondary 49 consolidation/creep ............. 2.6.3. iii Summary 50 of salient points ............... 2.7 Strain 50 rate effects ............... 2.8 Effects temperature 59 of on consolidation ............... Chapter 3 Experimental 66 methodology and materials .............. 3.1 Introduction 66 ............... 3.2 The 68 coefficient of earth pressure at rest ............... 3.2.1 Introduction 68 ............... 3.2.2 Relationships 71 of Ko ............... 3.2.2. i Ko 0' 71 relationship ............... 3.2.2.11 K° 73 --V relationship ............... 3.2.3 Laboratory determination 74 of K ............... 3.2.3.1 Requirements 74 for the K° test .............. 3.2.3. ii Methods of measuring K° during 75 zero lateral strain tests ............... 3.2.3. iii Recommendations for lateral strains 77 during K tests ............... 3.2.4 K high 78 at stresses .. "............ 3.2°. 4. i Previous K 78 high pressure experiments ..... 3.2.4. ii K derived from angles o? internal friction ............... 79 -viii- 80 3.3 Constant rate of strain tests ............... 80 3.3.1 Introduction ............... 81 3.3.2 Interpretation of the CRS test ............... 90 3.4 Compaction tests in the high pressure oedometer ......... 91 3.5 Materials used in this study ............... 91 3.5.1 Introduction ............... 91 3.5.2 The geology of chalk ..............