Senior Personnel


Professor Johan Sjöblom
Johan Sjöblom has been Professor in surface and colloid chemistry and the Head of the Ugelstad Laboratory at the Norwegian University of Science and Technology (NTNU) in Trondheim from January 1st 2002. He was earlier Professor at the University of Bergen during the years from 1988-1999, and Chief Researcher at Statoils R&D Centre in the period 1998-2001. Sjöblom has served as Adjunct Professor of Colloid Chemistry and Materials Science at Clarkson University, Potsdam, New York, and at the Technical University of Helsinki from 1992-1999.

Sjöblom is the editor of 10 books, and the author or coauthor of more than 300 professional papers. He has educated 50 PhD's and 35 MSc's.

Sjöblom is Editor-In-Chief (from 2005) for Journal of Dispersion Science and Technology, and previous Editorial Board Member of Colloids and Surfaces during the period 1993-2003. Sjöblom is also a member of the Advisory Board of Microporous and Mesoporous Materials. He is a member of the American Chemical Society, the Norwegian Chemical Society, and the Finnish Chemical Society. He is also elected Member of the Norwegian Academy of Technological Sciences and the Swedish Academy of Engineering Sciences in Finland. Sjöblom received his PhD degree (1982) in physical chemistry from the Åbo Akademi, Finland.

Sjöblom has been an industrial consultant in colloid chemistry since 1980.


Professor Gisle Øye
Gisle Øye started as a research scientist at the Ugelstad Laboratory March 1st 2002 and was appointed Professor in 2009. His research interests are focused on colloid chemistry and interfacial phenomena within Produced Water Managament, Enhanced Oil Recovery and synthesis of nanostructured materials.

Dr. Øye received his doctoral degree at the University of Bergen in 1999. In the period 2000-2002 he worked with plasmachemical modification of porous polymers at the University of Durham (England).


Associate Professor Brian A. Grimes
Grimes has a background in mathematical modeling of electrochemical transport phenomena and transport phenomena in porous media. He obtained his Ph.D. in 2002 from the University of Missouri-Rolla under the supervision of Professor A.I. Liapis. Upon completion of his Ph.D., he obtained an Alexander von Humboldt Research Fellowship and began his post-doc in Mainz, Germany under the supervision of Professor K.K. Unger. In 2007, he joined the Ugelstad laboratory. His current research involves constructing a multi-scale modeling framework for transport processes in colloidal dispersions which incorporated molecular and Stokesian dynamics simulations along with population balance modeling.


Associate Professor Kristofer Paso
Kristofer Paso joined Ugelstad as a Post-Doc in July of 2006. His current research interests involve developing an integrated approach to interfacial and surface processes in crude oil systems. Example systems include emulsions, catalysts, and reservoir minerals. Laboratory techniques of atomic force microscopy (AFM) and thermal gradient quartz crystal microscopy (TG-QCM) are used in the investigations. Other interests include interfacial rheology of complex colloidal and polymer systems, as well as developing an understanding of flow start-up processes in pipelines filled with gelled fluids such as wax, emulsions, and dispersions.


Adjunct Professor Hans Oschman
Hans-Jörg Oschmann joined the Ugelstad Laboratory in October 2011 to focus on research related to flow assurance challenges, specifically the interaction between paraffin, asphaltenes and hydrates in the presence of natural and synthetic surfactants at low temperature conditions during multiphase flow transport. Dr. Oschmann joined the industry in 1998 from the German Petroleum Institute with research focused on the influence of asphaltene on paraffin crystallization. Until 2002 he was in charge of the flow assurance product line of TR Oil Services (now Clariant Oil Services) focusing on paraffin, asphaltene and hydrate mitigation strategies. After this period he joined Champion Technologies where he became section manager for flow assurance where he is responsible for all flow assurance related projects including research and development and novel technologies in the Eastern Hemisphere.

Dr. Oschmann is member of the society of petroleum engineers (SPE), the American Society for Testing and Materials (ASTM) and has been serving as elected chairman of board of Ugelstad Laboratory in the period between 2002 and 2008.


Associate Professor II Martin Foss
Martin Foss has been working with the Ugelstad laboratory since 2010 and will continue until 1 January 2013 with the current funding. His work is related the multiphase flow assurance innovation Centre (FACE) where he contributed to the improved cooperation between the Ugelstad laboratory and the Institute for Energy Technology (IFE).


Professor Emeritus Per Stenius
Professor (emeritus) Per Stenius was born in 1938 in Helsinki, Finland. He graduated from Åbo Akademi (University), Turku (Åbo), Finland. He received hid Ph.D. degree from the same university and worked there as a teacher and acting professor from 1966-1977. He was the director of the Institute for Surface Chemistry (YKI), Stockholm 1977-1991, and Professor of Forest Product Chemistry at Helsinki University of Technology (HUT) from 1991 to April 2003. Professor Stenius was also Associate Professor at the University of Bergen, Norway, from 1997-1999. He retired as chair at HUT in 2003, but retains research activities at HUT in addition to holding an Adjunct Professorship at the Ugelstad Laboratory, Norwegian University of Science and Technology (NTNU) in Trondheim from September 2003.

Professor Stenius is a member of the Finnish and Swedish Academies of Engineering Science as well as the Swedish Academy of Science. He has been a member of the board of several international associations in surface and colloid chemistry, and he has also served as President of the International Association of Colloid and Interface Science (1986-87), and of the European Colloid and Interface Society (1993-94). Professor Stenius has published about 320 scientific papers and edited several books in the field of applied surface and colloid science. His present research interests are in the field of surface and colloid science in forest products technology, in particular fibre surface properties, recycling of fibres, adhesion and surfactant/polymer interactions.


Dr. Sébastien Simon
Sébastien Simon has joined the Ugelstad Laboratory as a post doc in February 2006. He is now a researcher currently involved in 3 different research projects: JIP-1 (Study of petroleum emulsions), JIP-2 (determination of formation mechanism of calcium naphthenate deposits) and FACE (de-emulsification of crude oil emulsions).

He has previously worked with the study of solution and interfacial properties of amphiphilic polysaccharides under the guidance of Dr. Guy Muller, Pr. Didier Le Cerf and Dr. Luc Picton and received his doctoral degree on this subject at the University of Rouen (France) in 2003. After a research assistant position in the University of Rouen, he worked for 16 months in the French Petroleum Institute (IFP, Rueil Malmaison, France) on the determination of rheological, emulsifying and adsorption properties of the higher molecular mass components of crude oils (asphaltenes) under the guidance of Research Engineer Loïc Barré.


Dr. Bartlomiej Gawel
Bartlomiej Gawel started as a research scientist at the Ugelstad Laboratory in March 1st 2009. He is currently working on interfacial characterization of crude oils/produced water systems and studying influence of inorganic particles on the interfacial properties in the frame of the research project: Produced Water Management – fundamental understanding of the fluids.

Dr. Gawel has previously worked at Ugelstad laboratory with preparation and characterization of inorganic porous monolithic catalysts for liquid phase glucose oxidation in the frame of the TE-RENE-RES project financed by Polish-Norwegian Research Funds. He received his doctoral degree on structural characterization of the crystalline materials form X-ray powder diffraction at the Jagiellonian University in Krakow, Poland, in September 2008.


Dr. Galina Rodionova
Galina Rodionova completed her master's degree in the field of Biotechnology (Wide Profile Chemistry) from the Arkhangelsk State Technical University, Russia in June 2008. She obtained her Ph.D in Chemical Process Technology (NTNU, 2011) under the supervision of Professor Øyvind Gregersen. The PhD study has been carried out within the scope of the joint project "Fiber based packaging materials: Development of innovative and sustainable barrier concepts (SustainBarrier)" initiated by the PFI, SINTEF and NTNU.

Galina joined the Ugelstad laboratory as a post-doctoral fellow in February 2012. Her research focuses on preparation and characterization of the synthetic reference fluids for crude oils and their emulsions.


Dr. Meysam Nourani
Meysam Nourani has a PhD in petroleum engineering from Sharif University of Technology, Iran, and studied chemical and petroleum engineering at Sharif University and Tehran University. He has been in the oil industry since 1998. Also he was the director of MAPSA's core analysis laboratory since 2010 before joining Ugelstad laboratory as a post-doctoral fellow in October 2012.

The title of his PhD thesis was "Investigation of Rheological Behavior of Polymer on EOR in Carbonate Reservoir" and right now he is involved in the project titled "A Combined Surface-Colloid Chemical and Rock-Fluid Interaction Approach towards more Efficient Enhanced Oil Recovery Strategies" which he is focused more on surfactant-surface interactions.


Dr. Roar Skartlien
Roar Skartlien holds a degree of electrical engineering from Gjørvik Ingeniørhøgskole (1991), a Masters degree in Astronomy from University of Oslo (1994) and a PhD in solar physics (with computer simulations of turbulence and waves in the solar atmosphere) from the University of Oslo (1998). After postdoctorates at the University of Colorado (JILA), and at the National Center for Atmospheric Research in Colorado (HAO) on similar subjects, he moved to his present position as Senior Scientist at the Institute for EnergyTechnology, IFE. He has co-operated in research on emulsion physics and chemistry with the Ugelstad Laboratory (UL) at NTNU since 2007, with focus on computer simulations of hydrodynamic systems, including surfactant effects and emulsions.


Dr. Duo Wei
Duo Wei received her PhD in Physical Chemistry under the supervision of Prof. Rong Guo from Yangzhou University in December 2012. The title of her thesis was "The interaction between hydrophilic modified ibuprofen and water-soluble polymer". Her research was focusing on the Colloid and Interface Chemistry.

Duo Wei joined the Ugelstad laboratory as a post-doctoral fellow in June 2013.


Dr. Jens Norrman
Post doc with VISTA. He obtained his PhD from Lund University in 2007. Jens was a visiting Researcher in Ugelstad laboratory in 2012-13 and is currently working as a Post doc in collaboration with VISTA on additives to heavy oils.


Ph. D. students


Ph.D. student Sulalit Bandyopadhyay
Sulalit Bandyopadhyay completed his MSc in Chemical Engineering from NTNU in August 2012 under the Erasmus Mundus Scholarship programme. His master thesis project on "Biodegradable nano-clusters as Drug Delivery Vehicles" was carried out at ETH Zurich in Prof Morbidelli's group and in collaboration with Ugelstad Laboratory. He received his BChE(Hons.) degree from Jadavpur University,India in 2010. Recipient of JBNSTS and DAAD fellowships,Sulalit joined the Ugelstad Laboratory as a Doctoral candidate in August 2012. He is currently working under the supervision of Associate Professor Wilhelm Robert Glomm on "Smart and Multifunctional Core-shell Nano-particles (NPs) for Drug Delivery". His fields of interest include synthesis and characterization of nano-particles, drug delivery and molecular dynamics simulation. Topic of his Thesis : Smart and Multifunctional Core-shell Nano-particles (NPs) for Drug Delivery.


Ph. D. student Thomas Tichelkamp
Thomas Tichelkamp studied chemistry and completed his master's degree at the Department of Organic Chemistry, Heinrich-Heine-Universität Düsseldorf in May 2012, synthesizing organic dyes for the research on nano electronics. He joined the Ugelstad Laboratory in August 2012 as a Ph.D. student under the supervision of Prof. Gisle Øye. In the context of a project on surfactant improved low salinity EOR (enhanced oil recovery) he focuses on the physical and chemical properties of crude oil/water interfaces.


Ph. D. student Birgitte McDonagh
Birgitte finished her master of science in Biotechnology at the department of Biotechnology, NTNU June 2011. The title of her Master's thesis was: "Optimalised Carbodiimide Chemistry for RGD-coupled Alginate". She started working as a PhD-candidate at the Ugelstad laboratory August 2011. The main work on her PhD is nanoparticles and modificating proteins with nanoparticles of gold. The construct's interaction with membranes will be studied in vitro and in vivo at the dept. of Circulation and Medical Imaging. Associate professor Wilhelm Glomm is her supervisor at dept. of Cehmical Engineering, and Ioanna and Axel Sandvig are her co-supervisors at the dept. of Circulation and Medical Imaging.


Ph.D. student Diego Camilo Pradilla Ragua
Diego Pradilla studied Chemical Engineering (2009) and completed his masters (2013), also in Chemical Engineering at Universidad de los Andes in Bogotá, Colombia. His work there as part of the Process and Product Design Group, was related to the manufacture of emulsions using a multi-scale approach. He joined Ugelstad laboratory in April 2013 as a PhD student under the supervision of Prof. Johan Sjöblom and is currently working on the study of chemical demulsification of crude-oil emulsions (Interactions between asphaltenes and model de-emulsifiers).


Ph.D. student Aleksandar Mehandzhiyski
Aleksandar Mehandzhiysk finished his Msc in computational chemistry in the Sofia Univeristy in October 2012 with master project titled “Theoretical study on the Claisen-Schmidt reaction for synthesis of chalcone”.Before starting his PhD at Ugelstad Laboratory he was employed by the Bulgarian academy of sciences, working on organics synthesis of biologically active compounds. He started his PhD in January 2013 under the supervision of Professor Brian Grimes. His project involves molecular modeling of carboxylic acid – metal ions interactions and selectivity, and molecular dynamics simulations of interfacialy active compounds at the water-oil interface.


Ph.D. student Silje Molnes
Silje Molnes is a PhD candidate working on the NORCEL project. Silje completed her M.Sc in Biopolymer Chemistry at NTNU in 2013. She started her Ph.D work at Ugelstad Laboratory in April 2014, and is supervised by Adjunct Professor Kristin Syverud and Associated Professor Kristofer Paso. The research area is enhanced oil recovery in sandstone reservoirs, with main focus on developing nanocellulose as a "green" additive in fluids for reservoir flooding


Ph.D. student Muh Kurniawan
Muh Kurniawan is a researcher at LEMIGAS, Indonesian Research Center for Oil and Gas Technology. Muh completed his B.Sc. in Chemistry at Bandung Institute of Technology (2001) and M.Sc. in Chemical Engineering at University of Indonesia (2013). He started his Ph.D. work at Ugelstad Laboratory in December 2014 under supervision of Assoc. Professor Kristofer Paso. The research area is flow assurance of waxy crude crude oil, and the main focus is development of environmentally friendly wax inhibitors.


Ph.D. student Sreedar Subramanian
Sreedhar Subramanian completed his MSc in Chemical Engineering at the Department of Chemical Engineering, NTNU in 2013. His master’s thesis dealt with the application of interfacial rheology techniques to study the mechanism of chemical demulsification of crude oil emulsions. Sreedhar started his PhD at Ugelstad Laboratory in May 2014 under the guidance of Professor Johan Sjöblom. His doctoral work is focused on improving the knowledge of mechanism of asphaltene precipitation and deposition."


Ph.D. student Sandra Rodríguez

Sandra Rodríguez completed her Master degree in Polymer Science, a joint Master Program of the Free University Berlin, the Humboldt University Berlin, the Technical University Berlin and the University of Potsdam, in October 2014. The topic of her thesis was the synthesis of stimuli-responsive polymers for the fabrication of smart hydrogels for biosensing devices. She joined the Ugelstad Laboratory in May 2015 as a PhD candidate under the supervision of Associated Professor Kristofer Paso. Her project consists in the synthesis of amphiphilic block copolymers for CO2 capture and storage.


Technical Staff


Camilla I. Dagsgård - Laboratory Manager, Senior Engineer
Camilla I. Dagsgård completed her Cand. Scient degree in Biotechnology in 2001 and has years of experience from laboratory work with analytical chemistry and microbiology. When starting to work as an engineer at Ugelstad Laboratory in 2009 she came from the quality control department of GE Healthcare. Camilla I. Dagsgård is now the Laboratory Manager of the lab.


Bicheng Gao, Engineer
Bicheng Gao finished his master's degree in Chemical Engineering at NTNU in 2010 and has worked as a lab engineer at Ugelstad Laboratory since then. He is working on both routine analyses and research projects.


May Grete Sætran, Head Engineer
May Grete Sætran has been working as a department engineer in Ugelstad laboratory since October 2012 and is working with both routine analysis and research projects. She has been working as an engineer at the university for several years.




Professor Emeritus


Preben Cato Mørk
Professor Emeritus from 2007


Arvid Berge
Professor Emeritus from 2004


Post Docs and Researchers


Iva Kralova
- Laboratory Manager/Senior Engineer 2006-2011

Fan Yanru
- Post Doc 2007-2011

Erland L. Nordgård
- Researcher 2009-2010
- Career after NTNU: Research Scientist, Resman

Helene K. Magnusson
- Researcher 2005-2008
- Career after NTNU: Research Scientist, SINTEF Materials and Chemistry

David Arla
- Post Doc 2006-2007
- Career after NTNU: Research Scientist, SINTEF Petroleum Research

Pål Viggo Hemmingsen
- Researcher 2002-2006
- Career after NTNU: Senior Engineer, StatoilHydro ASA

Judit Adam
- Post Doc 2006
- Career after NTNU: Research Scientist, SINTEF Energy Research

Maria Häger
- Post Doc 2004-2006
- Career after NTNU: Sandviken, Gävle, Sweden

Marit-Helen Glomm Ese
- Researcher 2003-2006
- Career after NTNU: Research Manager, SINTEF Energy Research

Tom-Nils Nilsen
- Senior Researcher 2005
- Career after NTNU: Research Scientist, BYGGFORSK

Gisle Øye
- Researcher 2002-2005
- Career: Professor, NTNU

Harald Kallevik
- Post Doc 2001-2002
- Career after NTNU: Senior Engineer, Statoil

Øystein Sæther
- Post Doc 2001-2002
- Career after NTNU: Senior Engineer, Statoil

Linn Bergflødt
- Post Doc 2001-2002
- Career after NTNU: Research Scientist, Statoil


Ph.D. students


Albert Barrabino

  • Title of thesis: "Phase Inversion, Stability and Destabilization of Model and Crude Oil Water-in-Oil Emulsions"
  • Defense of thesis: December 18th 2014

Karina Kovalchuk

  • Title of thesis: "Multiscale Modeling of Interfacial Mass Transport in Liquid-Liquid Dispersions"
  • Defense of thesis: May 14th 2014

Lalit Kumar

  • Title of thesis: "Multiscale Modeling of Interfacial Mass Transport in Liquid-Liquid Dispersions"
  • Defense of thesis: May 22nd 2014

Mona Eftekhardadkhah

  • Title of thesis: “Interfacial Properties of Dissolved Crude Oil Components in Produced Water” 
  • Defense of thesis:  September 13th 2013

Mehdi Benmekhbi

  • Title of thesis: “Properties of Surfactant-Stabilized Liquid-Liquid Interfaces and Relation to Emulsion Stability”

  • Defense of thesis:  October 22nd 2013

Sina Maria Lystvet

  • Title of thesis:"Emergent properties of protein-gold nanoconstructs for biomedical applications"
  • Defense of thesis: March 14th 2013

Andreas Lynn Nenningsland

  • Title of thesis:"Extraction, Quantification and Study of Interfacially Active Petroleum Components"
  • Defense of thesis: December 17th 2012

Nils van der Tuuk Opedal

  • Title of thesis:"NMR as a tool to follow destabilization of water-in-oil emulsions"
  • Defense of thesis: September 30th 2011

Klodian Xhanari

  • Title of thesis:"Nanosized Cellulose Fibrils as Stabilizers of Emulsions"
  • Defense of thesis: January 28th 2011

Serkan Kelesoglu

  • Title of thesis:"Flow Behaviour of Water-in-North Sea Acidic Crude Oil Emulsions and Preparation of Synthetic Reference Acidic Oils and Their Emulsions"
  • Defense of thesis: December 20th 2010
  • Career after NTNU: Post Doc, Ugelstad Laboratory

Asal Amiri

  • Title of thesis:"Rheology of silica-based dispersions and Cross-sectional modelling of settling slurries"
  • Defense of thesis: October 7th 2010

Umer Farooq

  • Title of thesis:"Characterisation of crude oil-water and solid-water interfaces and adsorption/desorption properties of crude oil fractions. The effect of low salinity water and pH"
  • Defense of thesis: September 30th 2010
  • Career after NTNU: Research Scientist, SINTEF Materials and Chemistry

Erland Nordgård

  • Title of thesis:"Model Compounds for Heavy Crude Oil Components and Tetrameric Acids. Characterization and Interfacial Behaviour"
  • Defense of thesis: September 7th 2009
  • Career after NTNU: Research Scientist, RESMAN

Martin Smestad Foss

  • Title of thesis:"The Effect of Oil on Carbon Dioxide Corrosion Inhibition on Carbon Steel - Potential for Improved Corrosion Protection"
  • Defense of thesis: January 30th 2009
  • Career after NTNU: Research Scientist, Institute for Energy technology (IFE)

Ann-Mari Dahl Hanneseth

  • Title of thesis:"An Experimental Study of Tetrameric Naphthenic Acids at w/o Interfaces"
  • Defense of thesis: January 23rd 2009
  • Career after NTNU: Research Scientist, Statoil

Anne Siltset

  • Title of thesis:"Emulsions (w/o and o/w) of Heavy Crude Oils. characterization, Stabilization, Destabilization and Produced Water Quality"
  • Defense of thesis: November 13th 2008
  • Career after NTNU: Research Scientist, Statoil

Dorota Dudasova

  • Title of thesis:"Characterization of solid particle suspensions with organic coatings in oilfield produced water"
  • Defense of thesis: September 19th 2008
  • Career after NTNU: Research Scientist, Statoil

Simone Less

  • Title of thesis:"Mechanisms of water-in-crude oil emulsion formation, stabilization and resolution by electrostatic means"
  • Defense of thesis: June 25th 2008
  • Career after NTNU: Research Scientist, Saudi Aramco

Ingvild A. Johnsen

  • Title of thesis:"The impact of dissolved hemicelluluses on adsorption of wood resin to TMP fines"
  • Defense of thesis: October 12th 2007
  • Career after NTNU: Research Scientist, PFI

Martin Andresen

  • Title of thesis:"Surface Modification of Microfibrillated Cellulose"
  • Defense of thesis: August 31st 2007
  • Career after NTNU: Research Scientist, Borregaard Lignotech

Martin Fossen

  • Title of thesis:"Aggregation, Interfacial Properties and Structural Characteristics of Asphaltene Solubility Fractions"
  • Defense of thesis: August 28th 2007
  • Career after NTNU: Research Scientist, SINTEF Petroleum Research

Sondre Volden

  • Title of thesis:"Preparation and Characteristics of Novel Silica-Based Materials and Adsorbed Macromolecules"
  • Defense of thesis: December 19th 2006
  • Career after NTNU: Post Doc, Ugelstad Laboratory

Shukun Chen

  • Title of thesis:"Rheological Properties of Water in Oil Emulsions and Particulate Suspensions"
  • Defense of thesis: October 6th 2006
  • Career after NTNU: Senior Engineer, Statoil

Andreas Hannisdal

  • Title of thesis:"Particle-Stabilized Emulsions and Heavy Crude Oils. Characterization, Stability Mechanisms and Interfacial Properties"
  • Defense of thesis: June 15th 2006
  • Career after NTNU: Senior Engineer, Aibel

Torbjørn Vrålstad

  • Title of thesis: "Synthesis and characterization of cobalt-containing mesoporous model catalysts."
  • Defense of thesis: December 2nd 2005.
  • Career after NTNU: Research Scientist, SINTEF Petroleum Research

Øystein Brandal

  • Title of thesis:"Interfacial (o/w) Properties of Naphthenic Acids and Metal Naphthenates, Naphthenic Acid Characterization and Metal Naphthenate Inhibition."
  • Defense of thesis: June 29th 2005.
  • Career after NTNU: Senior Engineer, Statoil

Magne Kawai Knag

  • Title of thesis:"Surfactant Aggregation in Solution and on Metal Surfaces and the Impact on Corrosion Rate."
  • Defense of thesis: June 23rd 2005
  • Career after NTNU: Research Scientist, Origo-Process

Wilhelm Robert Glomm

  • Title of thesis: Preparation and Characterization of Nanosized Structures with Applications in Bioscience and Materials 
  • Defense of thesis: June 23rd 2004
  • Career after NTNU: Associate Professor, NTNU

Trond Erik Havre

  • Title of thesis: Formation of Calcium Naphthenate in Water/Oil Systems, Naphthenic Acid Chemistry and Emulsion Stability
  • Defence of thesis: November 28th& 2002
  • Career after NTNU: Senior Chemist, Origo-Process

Narve Aske

  • Title of thesis: Characterisation of Crude Oil Components, Asphaltene Aggregation and Emulsion Stability by means of Near Infrared Spectroscopy and Multivariate Analysis
  • Defence of thesis: September 23rd 2002
  • Career after NTNU: Senior Engineer, Statoil

Inge Harald Auflem

  • Title of thesis: Influence of Asphaltene Aggregation and Pressure on Crude Oil Emulsion Stability
  • Defence of thesis: August 15th 2002
  • Career after NTNU: Senior Engineer, Statoil