Simulations of Hydraulic Fracturing and Leakage in Sedimentary Basins
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Simulations of hydraulic fracturing and leakage in sedimentary basins by Ane Elisabet Lothe The thesis has been submitted to Department of Geology University of Bergen in partial fulfilment of the requirement for the Norwegian academic degree Doctor Scient January 2004 Summary Hydraulic fracturing and leakage of water through the caprock is described from sedimentary basin over geological time scale. Abnormal pressure accumulations reduce the effective stresses in the underground and trigger the initiation of hydraulic fractures. The major faults in the basin define these pressure compartments. In this Thesis, basin simulations of hydraulic fracturing and leakage have been carried out. A simulator (Pressim) is used to calculate pressure generation and dissipitation between the compartments. The flux between the compartments and not the flow within the compartments is modelled. The Griffith-Coulomb failure criterion determines initial failure at the top structures of overpressured compartments, whereas the frictional sliding criterion is used for reactivation along the same fractures. The minimum horizontal stress is determined from different formulas, and an empirical one seems to give good results compared to measured pressures and minimum horizontal stresses. Simulations have been carried out on two datasets; one covering the Halten Terrace area and one the Tune Field area in the northern North Sea. The timing of hydraulic fracturing and amount of leakage has been quantified in the studies from the Halten Terrace area. This is mainly controlled by the lateral fluid flow and the permeability of the major faults in the basin. Low fault permeability gives early failure, while high fault permeabilities results in no or late hydraulic fracturing and leakage from overpressured parts of the basin. In addition to varying the transmissibility of all faults in a basin, the transmissibility across individual faults can be varied. Increasing the transmissibility across faults is of major importance in overpressured to intermediately pressured areas. However, to obtain change in the flow, a certain pressure difference has to be the situation between the different compartments. The coefficient of internal friction and the coefficient of frictional sliding, used as input in the failure criteria, have minor impact on timing and amount of leakage. However, high values result in some time-delay, and thereby less leakage from overpressured compartments. Sensitivity tests of the Poisson’s ratio and Young’s modulus of the caprock, which controls Biot’s constant, show an effect on the overpressure. Defining Biot’s constant to 1, gives too early pressure accumulation. Lower values of Biot’s constant (>0.85) give a present day pressure distribution closer to the one observed in wells. How to obtain a pressure difference across faults is tested on the Tune Field data set. Two different fault maps are used, one with only large faults interpreted, and one with both large and small faults included. High-pressure differences are difficult to obtain when the transmissibility across one individual fault is reduced. The transmissibilities of a network of small faults have to be reduced to match the pressure measured in wells. Then, either our transmissibility fault models are wrong, or a relay zone between larger faults is more sealing than expected. In addition, the fluid flow path can be more complex than expected, and can only be understood with a multi-layer model. The secondary oil migration modelling carried out using different pressure history cases; show that overpressures have a major impact on the migration pathways. The resolution in the fault interpretation is important for the simulation results, both for pressure distribution and for hydrocarbon migration. Minor deformation bands are described from experiments and from onshore fieldwork in Brumunddal. These are small faults situated in a sandstone reservoir that will have influence on lateral fluid flow. Samandrag Hydraulisk oppsprekking og strømming av vatn gjennom takbergarten, er skildra frå ulike sedimentære basseng på geologisk tidsskala. Dette grunna abnorme overtrykk som fører til ein reduksjon i effektivspenningane i undergrunnen. Dei store forkastningane i bassenget definere dei ulike trykkcellene. I denne oppgåva blir simuleringar av hydraulisk oppsprekking og lekkasje på bassengskala presentert. Ein trykksimulator (Pressim) er brukt for å kunne kalkulere trykkgenerering og dissipering mellom cellene. Trykkendringar mellom cellene og ikkje strømming internt i cellene er simulert. Griffith-Coulomb brotkriterium vert brukt for a finne når forkastninga startar å utvikle seg frå toppen av cella med overtrykk, mens friksjonsglide brotkriteriet er brukt for å determinere reaktivering langs forkastninga. Minste horisontal spenning kan bli kalkulert frå ulike formlar, men ein empirisk formel gjev realistiske resultat for simulert trykk og spenning samanlikna med målte verdiar. I simuleringane er to datasett brukt; eit frå Haltenterrassen og eit frå Tunefeltet, i nordlige Nordsjøen. I studiet frå Haltenterrassen, er starttidspunktet for hydraulisk oppsprekking og lekkasje kvantifisert. Dette er hovudsakleg styrt av lateral strømming og permeabiliteten til dei store forkastningane i bassenget. Lave permeabilitetar til forkastningane gjev tidleg brot, mens høge permeabilitetar gjev sein eller ingen hydrauliske forkastningar eller lekkasje frå område med anormalt høgt trykk. I tillegg til å variere transmissibiliteten til alle forkastningane i eit basseng, kan transmissibiliteten på tvers av enkelt forkastningar endrast. Å auke transmissibiliteten til forkastningar er vesentleg i område med høgt eller relativt høgt trykk, men for idet heile tatt få endra flømmingsmønsteret, må det være ein viss trykkdifferanse mellom cellene. Koeffisienten for intern friksjon og koeffisienten for friksjonsgliding, som er brukt som inngangsdata i brotkriteria, har vesentleg mindre påverknad på starttidspunkt for forkastninga og mengde lekkasje. Sjølv om høge verdiar gjev noko forsinkelsar, og dermed mindre lekkasje frå celler med høgt overtrykk. Sensitivitetstestar for Poisson ratio og Young modul til takbergarten, som kontrollerar Biot konstant, viser over tid ei effekt på overtrykket. Biot konstant sett til 1, gjev tidleg trykkakkumulering. Mindre verdiar av Biot konstant (>0.85) gjev ei trykkfordeling i dag, som er nærmare det som er observert i brønnar. På datasettet frå Tunefeltet har vi testa korleis trykkdifferansar over forkastningar kan simulerast. To ulike kart over forkastningane er brukt, ein med berre store forkastningar, og det andre med på store og små forkastningar inkludert. Det er vanskelig å oppnå store trykkskilnader, berre ved å redusere transmissibilitet over ei forkastning. Transmissibiliteten til eit nettverk av mindre forkastningar må reduserast for å same trykket som er målt i brønnane. Det tyder, at enten er modellen som er brukt for transmissibiliteten til forkastningane feil, eller so er ei ”relay” sone mellom store forkastningar meir forseglande enn venta. For å kunne fullt ut forstå væskestraumen, bør ein gjere simuleringar med ein modell med fleire lag. Ved å modellere sekundær oljemigrasjon med ulike trykkhistorier, ser ein vesentlege endringar på migrasjonen med ulike trykkscenario. Detaljerte tolkingar av forkastningar er viktig både for trykkfordeling og hydrokarbonmigrasjon. Deformerte band er skildra både frå eksperiment og frå feltarbeid i Brumunddal, Noreg. Dette er små forkastningar utvikla i sandsteinreservoar, som vil vere viktig for lateral strømming. Acknowledgement First of all, I would like to thank my supervisor Prof. Roy H. Gabrielsen, UiB for his advices and encouragement in the research work. I would also want to thanks Hans Borge, at SINTEF Petroleum Research for a lot of help doing the coding in PRESSIM and for always looking at the bright side of life. Without him, this thesis would have not been a reality. Also Idar Larsen contributes with coding in the final work. Øyvind Sylta has been very nice to discuss with. My other colleagues at SINTEF Petroleum Research have contribute with ideas and discussion, mainly versus rock mechanics; Prof. Rune Holt, Erling Fjær, Angela Maria Pillitteri Gotusso and Olav Flornes. Norsk Hydro ASA is thanked for their funding of the thesis, providing data and giving permission to publish. I would specially like to thanks Christian Zwarch for having the idea to the thesis together with Øyvind Sylta. Then I would like to thanks Michel Erdmanns, Arnt Williams, Olav Lauvrak, Susanne Sperrevik, Geir Mørk and the Halten Terrace group in Oslo for interest discussions and useful help. The SMIFF2 steering committee and work group (IFE, NGI, SINTEF) is thanked for nice input, especially in the first years, when everything was difficult. I would also like to thank the structural geology group in Bergen, for never forgetting me, though I have been emigrating to Trondheim. Special thanks to Silje Berg, Tore Skar and Rune Kyrkjebø for fun and nice discussions. Finally, I would like to thank my parents and my sisters for support and encourage in many years. Table of contents Summary Samandrag Acknowledgement Chapter 1 Introduction p. 11-24 Chapter 2 Modelling of hydraulic leakage by pressure and stress p. 25-56 simulations and implications for Biot’s constant: An example from the Halten Terrace area, offshore Norway: Lothe, A.E., Borge, H. & Gabrielsen, R.H. Chapter 3 Effect of different formulas of the minimum horizontal p. 57-80 stress