Investigations of Some Rock Stress Measuring Techniques and the Stress Field in Norway
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INVESTIGATIONS OF SOME ROCK STRESS MEASURING TECHNIQUES AND THE STRESS FIELD IN NORWAY by Tor Harald Hanssen This thesis has been submitted to Department of Geology and Mineral Resources Engineering Norwegian University of Science and Technology in partial fulfilment of the requirements for the Norwegian academic degree DOKTOR INGENI0R December 1997 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. ABSTRACT. The purpose of this investigation is to develop equipment and methods for further rock stress assessment, reevaluate the existing overcoring rock stress measurements done by NTH/SINTEF and relate this information to the present geological setting. The work has been carried out both in the field and in the laboratory. Before going out in the field, the equipment for hydraulic fracturing was constructed, and minor improvements were added to the overcoring rock stress measuring technique. In the field, rock stresses were measured by the overcoring and the hydraulic fracturing technique. An observation technique to assess likely high stresses was developed. In the laboratory, tests were carried out to appraise the present overcoring technique. A new method was developed that incorporates a statistical way of assessing the results from rock stresses measured by overcoring using the NTH cell. The main topics of this work have been: The present procedures using the NTH cell was investigated. A new statistical method to assess overcoring rock stress measurements using the NTH cell have been developed. The method has been implemented in a computer code. Furthermore, an improved code of practice for overcoring rock stress measurements using the NTH cell have been proposed, including suggested changes to the present equipment. The NTH cell has been tested in the laboratory, and its ability to reproduce the applied strains, and thus stresses have been investigated. With basis in these tests and the earlier in-situ overcoring rock stress measurements, a quality ranking system for overcoring rock stress measurements using the NTH cell were developed and applied to all existing complete measurements. All existing data on overcoring rock stress measurements using the NTH cell were retrieved and reevaluated to extract information on the regional stress distribution. A complete hydraulic fracturing rig was constructed, and a code of practice and evaluation was proposed to collect information on the minor principal stress. The effect of leak-off tests on the surrounding wellbore has been investigated, and a revised procedure to get information on the minor principal stress was proposed. Additionally, an integrated approach was used to investigate all three principal stresses. Compound rock stress determination was proposed for increased understanding of the state of stress. Field measurements were carried out to investigate if the minor principal stress measured by overcoring was consistent with results from hydraulic fracturing rock stress measurements. Combination of different "results was also used in the Visund petroleum field. Systematic mapping of the surface exfoliation intensity in the larger Kobbelv area, tunnel mapping and overcoring rock stress measurements were used to investigate the stress regime active in the field. I PREFACE The idea to pursue a doktor ingenior degree first originated when I was engaged in rock stress measurements in the county of Nordland, northern Norway in 1985. Through the measurements we identified subhorizontal high stresses governed the spalling in the tunnels in the region. In 1989 when we were awarded the VISTA project on stress field orientation, I enrolled as dr.ing. student at the Norwegian Institute of Technology while still maintaining a full time job with SINTEF. I continued the part time studies when I moved to be employed by Norsk Hydro and later Statoil. My ideas with this project were to reevaluate the results from existing overcoring rock stress measurements and relate the results to tectonic knowledge. In addition I would further develop the basic hydraulic fracturing equipment that we had already built as a prototype at SINTEF, making it suitable for measurements in vertical and sub-vertical drill holes down to a depth of approximate 250 metres. The objective for doing this was partly that there exists many ground water wells that can be tested with respect to rock stress, i.e. when the wells are already drilled, they can be tested using light equipment with minor cost involved. Another aspect was that both the mining and construction industry had shown a need for rock stress profiling measurements conducted in vertical drill holes. Thus, two objectives would be accomplished with the revised equipment setup. The anticipated hydraulic fracturing field test program was not fully accomplished as intended because I moved to Norsk Hydro and therefore was unable to fulfill the planned field measurements. However, it is my hope that the enormous quantity of readily available test sites in large diameter ground water wells will be explored by my successors at SINTEF, and thus provide valuable information to the shallow stress field assessment. ACKNOWLEDGEMENTS The work presented in this dissertation has benefitted from the support from the Royal Norwegian Council for Scientific and Industrial Research through the project “Kartiegging av spenningsforhold i den norske berggrunnen ” under contract No.: BA 5002.18430, and the joint research programme between The Royal Norwegian Academy of Science and Statoil, aka VISTA, through the project “German - Norwegian Research and Development Programme on Basin Analysis and Reservoir Studies" Project A-2 "Stress Field Orientation" under contract No.: V 7111. The documentation on rock stress measurements by overcoring and hydraulic fracturing has kindly been made available to me by the Norwegian Institute of Technology Department of Geology and Mineral Resources Engineering and SINTEF Rock and Mineral Engineering. Some material on hydraulic fracturing has kindly been released to the author from the Norwegian Geotechnical Institute. The material on leak-off and minifrac measurements and borehole images have kindly been supplied by Norsk Hydro. I am grateful to Statoil for the support I have received during the final preparation of this dissertation. My sincere thanks are expressed to Norsk Hydro Exploration and Production Research Centre for allowing and encouraging me to compile my material. I also acknowledge the support from my colleagues in Norway, Sweden, Germany, Iceland and USA for their willingness to discuss and elaborate on Ideas pertinent to this work. I especially express my gratitude to Lu Ming who expediently translated my ideas on rock stress evaluation methods into clear programming and thus made the first version of DISO available. Thanks also to Helge Ruistuen who tediously collated most of the measured strains and related information from the old reports and commented on the final manuscript, Dag 0ktand who through creative programming transformed the DISO output files into a manageable database and Morten Fejerskov who supplied the geographical coordinates to many overcoring measurement sites. I also thank Jan Hollas who helped to construct the hydraulic fracturing rig, Gunnar Halvorsen who helped to plot the GMT stress maps and Silke Hubinger who helped to plot the FMS data. I am indebted to Leif Fagerli, Viktor Tokle, Einar Breiseth and Hans Karl Lund who showed me the practical side of successful rock stress measurements in the field, and who showed me the bright side of field work. Stein Erik Hansen is also thanked for helping in many ways during the initial work. Professor Fil. Dr. Ove Stephansson are thanked for constructive comments to the final manuscript. At last but not the least I present my sincere gratitude to my mentor and supervisor, Professor Dr.ing. Arne M. Myrvang without whose support and without whom there would not have been any projects. Simen, Sunniva and Kari are finally thanked for their patience and endurance through these years. II CONTENTS ABSTRACT ......................................................... I PREFACE....................................................................................................................... II ACKNOWLEDGEMENTS ............................................................................................................................... II INTRODUCTION............................................................................................................................................. 1 General ................................................................................................................................................1 Purpose and method ...........................................................................................................................2 Thesis organisation .......................................................................................................................... 2 TECTONIC SETTING IN THE WESTERN FENNOSCANDIA......................................................................... 7 Tectonic history and some related observations ....................................................................... 7 Principal structures and lineaments .......................................................................................... 10 Distribution of stresses in the earth 's crust .................................................................•..........