Numerical Simulation of an Ekofisk Single Well Chemical Tracer Test Mari Mikalsen Petroleum Geoscience and Engineering Submission date: June 2014 Supervisor: Jan-Åge Stensen, IPT Co-supervisor: Robert Moe, ConocoPhillips Norwegian University of Science and Technology Department of Petroleum Engineering and Applied Geophysics ___________________________________________________________________________ ___________________________________________________________________________ Numerical Simulation of an Ekofisk Single Well Chemical Tracer Test ___________________________________________________________________________ Master Thesis by Mari Mikalsen June 2014 I ___________________________________________________________________________ I ___________________________________________________________________________ Acknowledgements This master thesis is the final part of the Master of Science program in Petroleum Engineering at the Norwegian University of Science and Technology (NTNU). The thesis was written in co-operation with ConocoPhillips Scandinavia AS. I would like to thank them for the motivating and interesting project. Next, I would like to thank Dr. Robert W. Moe for being my mentor at ConocoPhillips. I would like to thank him for his excellent technical guidance, constructive support and interesting discussions. Also, I would like to thank Arvid Østhus for taking a special interest in my thesis; giving me constructive feedback and guidance all the way through. Thanks to Ole Eeg for his full support and assistance. Further, I would like to thank my supervisor at NTNU, Jan Åge Stensen, for his support during this thesis. A special thanks to all the graduates at ConocoPhillips. Without all the lunch breaks, good conversation and countless coffee breaks, I would have lost my mind. Finally, I would like to thank my family and friends for the support and understanding during the course of this thesis. II ___________________________________________________________________________ III ___________________________________________________________________________ Abstract The Ekofisk field in the North Sea has been undergoing waterflood since 1987. It has proved to efficiently recover oil by means of spontaneous imbibition. The plan is to continue waterflooding until the end of the license in 2028. The challenge lies in how to recover the residual oil left immobile after the waterflood. The average oil saturation in the flooded parts of the reservoir is approximately 30 %. Surfactant flooding has now been proposed as an option, and is showing promising results in the laboratory. An enhanced imbibition study by injection of low concentration surfactants was conducted in the mid-1990s. The study was terminated in 1997 due to lab measurements of unsatisfactory high adsorption, making it un-economical. Further studies on surfactants were not done until 2011. The most attractive feature of the surfactant this time around is its ability to lower IFT enough to free immobile oil from the pores. Both the economics and understanding of the surfactant process have improved significantly over the last 20 years, making it an option once again. Current plans involve the implementation of a single well chemical tracer test in 2015 to confirm the lab results on the effect the surfactant flood has on the residual oil. Several simulation studies were conducted in this thesis to determine the expected injection rates and the volumes necessary to execute an efficient pilot test. Particular attention is paid to the influence of surfactant adsorption and the effect of geological features in the test area. Adsorption proved to have a particular large effect on the acting distance of the surfactant slug. Based on history matching and other specific simulation studies, the expected injection rate was determined to be 35 bbl/day for a 20 feet high perforation interval. With this rate a total of 6 months to complete the SWCTT was required with the predetermined slug volumes. IV ___________________________________________________________________________ V ___________________________________________________________________________ Sammendrag Ekofiskfeltet I Nordsjøen har blitt vannflømmet siden 1987. Det har vist seg å være svært effektivt med tanke på å utvinne olje ved hjelp av spontan imbibering. Planen er å fortsette vannflømmingen av feltet frem til lisensen går ut i 2028. Utfordringen ligger i hvordan å utvinne den residuelle oljen som blir forlatt immobil i porene. Den gjennomsnittlige oljemetningen i de flømmede områdene av reservoaret er omtrent 30 %. Surfaktant har nå blitt foreslått som en mulighet, og viser lovende resultater fra kjerneflømminger. Et økt- imbibering studie ved injeksjon av lav-konsentrasjon surfaktant ble utført på midten av 1990-tallet. Studiet ble avsluttet i 1997 etter at laboratoriemålinger viste utilfredsstillende høye adsorpsjonsverdier som gjorde hele prosjektet uøkonomisk. Videre studier ble ikke gjort før i 2011. Den mest attraktive egenskapen til surfaktanten i denne omgang var dens evne til å redusere IFT nok til å frigjøre oljen fra porene. Både det økonomiske aspektet og forståelsen av prosessen bak flømming med surfaktant har økt betraktelig i løpet av de siste 20 årene, og gjør det til et alternativ nok en gang. Nåværende planer er å implementere en ènbrønns kjemisk tracer test i 2015 for å bekrefte laboratorieresultatene om effekten surfaktant har på den residuelle oljemetningen. Flere simuleringsstudier ble utført i denne oppgaven for å bestemme forventede injeksjonsrater og nødvendige volumer for å gjennomføre en vellykket pilottest. Spesiell oppmerksomhet ble rettet mot adsorpsjon av surfaktant og effekten av geologiske egenskaper i testområdet. Adsorpsjon viste seg å ha en spesielt stor effekt på virkningslengden av surfaktanten. Basert på historietilpasning og andre spesifikke simuleringsstudier, ble den forventede raten bestemt til 35 bbl/dag for et 20 fots høyt perforeringsintervall. Med denne raten tok det totalt 6 måneder å fullføre pilottesten med de forhåndsbestemte volumene. VI ___________________________________________________________________________ VII ___________________________________________________________________________ Table of Contents 1 Introduction .............................................................................................................. 1 2 Ekofisk Field History and Background ........................................................................ 3 2.1 Field Discovery ............................................................................................................. 3 2.2 Geology ........................................................................................................................ 4 2.3 Field Development....................................................................................................... 5 2.4 Subsidence ................................................................................................................... 7 2.5 Production from Chalk Reservoirs ............................................................................... 8 2.5.1 Water Weakening in Chalks ............................................................................... 11 2.6 EOR Screening for Ekofisk .......................................................................................... 12 3 Theory .................................................................................................................... 17 3.1 Rock Properties .......................................................................................................... 17 3.1.1 Porosity ............................................................................................................... 17 3.1.2 Permeability ....................................................................................................... 18 3.2 Fluid-Rock Interaction ............................................................................................... 20 3.2.1 Interfacial Tension .............................................................................................. 20 3.2.2 Wettability .......................................................................................................... 21 3.3 Relative Permeability ................................................................................................. 21 3.3.1 Pseudo Relative Permeability Curves ................................................................. 23 3.4 Saturation .................................................................................................................. 23 3.4.1 Spontaneous Imbibition ..................................................................................... 24 4 Literature ................................................................................................................ 27 4.1 Surfactant Flooding ................................................................................................... 27 4.1.1 Types of Surfactants ........................................................................................... 29 4.1.2 Microemulsion Phase Behavior .......................................................................... 30 4.1.3 Optimum Salinity for Ultralow IFT ..................................................................... 32 4.1.4 Surfactant Retention .......................................................................................... 33 4.2 Recent Advances in Surfactant EOR .........................................................................