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11 Pore-scale modelling of WAG: impact of wettability Rink van Dijke and Ken Sorbie Institute of Petroleum Engineering Heriot-Watt University WAG Workshop.

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Presentation on theme: "11 Pore-scale modelling of WAG: impact of wettability Rink van Dijke and Ken Sorbie Institute of Petroleum Engineering Heriot-Watt University WAG Workshop."— Presentation transcript:

1 11 Pore-scale modelling of WAG: impact of wettability Rink van Dijke and Ken Sorbie Institute of Petroleum Engineering Heriot-Watt University WAG Workshop FORCE, Stavanger, 18 March 2009

2 2 3-phase (immiscible) flow processes, e.g. –water-alternating-gas injection (WAG): improved oil recovery –NAPL in unsaturated zone: ground water remediation modelled with Darcy’s law: capillary pressure and relative permeability functions –difficult to measure –pore-scale modelling Introduction

3 33 Pore-scale modelling: –pore space structure: connectivity (topology) geometry (pore sizes and shapes) –flow mechanisms: capillary forces conductance (viscous forces) –wettability (contact angles) –incorporated in idealized network models (quasi-static “invasion percolation” or dynamic) capillary bundle models water, oil, gas

4 44 Introduction Capillary forces: –invasion of a single tube (cylinder): –‘rule’ for displacement of water by oil: with capillary ‘entry’ pressure according to Young-Laplace:

5 55 Introduction Wettability: –wettability of pore surface defined in terms of oil- water contact angle (measured through water) water-wet if oil-wet if SOLID SURFACE wateroil

6 66 Introduction Wettability: –in 3-phase flow contact angles: related by Bartell-Osterhof equation: constitute capillary entry pressures for gas-water and gas-oil displacements, e.g. determine presence of wetting films and spreading layers

7 77 Introduction Micromodel experiments: –understand flow mechanisms –validate pore-scale network models –Sohrabi et al. (HWU) pore cross- section: wide and shallow 250  m 50  m

8 88 Outline: effects of wettability Saturation-dependencies of three-phase capillary pressures and relative permeabilities Intra-pore physics: –fluid configurations –capillary entry pressures and layer criteria –non-uniform wettability Network displacement mechanisms: –phase continuity and displacement chains –WAG simulations –comparison simulations and WAG micromodel experiments Concluding remarks

9 9 Saturation-dependencies Traditional example (Corey et al., 1956) Curved oil isoperms Straight water and gas isoperms

10 10 Saturation-dependencies Traditional assumptions for saturation-dependencies Water-wet system: water wetting to oil wetting to gas  water in small pores, gas in big pores pore occupancy (number fraction) pore size r wateroilgas

11 11 Saturation-dependencies Wettability distributions in porous medium often correlated to pore size: –mixed-wet with larger pores oil-wet (MWL): may occur after primary drainage and aging (similarly MWS) r 0 1 water-wet oil-wet r wet

12 12 Saturation-dependencies Paths in saturation space: gas flood into oil, followed by water flood into gas and oil capillary bundle model oil water gas gas flood water flood I III II water-wet oil-wet

13 13 Saturation-dependencies Regions in saturation space: iso-capillary pressure curves II gas is “intermediate-wetting”

14 14 Saturation-dependencies Regions in saturation space: iso-relative permeability curves II gas is “intermediate-wetting” II

15 15 numerical example FW capillary bundle Saturation-dependencies

16 16 Intra-pore physics Films and layers: –water-wet micromodel: WAG flood water wetting films around both oil and gas possible oil layers separating water and gas

17 17 Intra-pore physics Fluid configurations in angular pores: –water-wet pores, e.g. strongly water-wet: all close to 0 water wetting films around both oil and gas possible oil layers separating water and gas: affected by oil spreading coefficient –oil-wet pores, e.g. weakly oil-wet: close to 90 degrees, close to 0 no oil wetting films around water only oil wetting films around gas –ensures phase continuity along pores

18 18 Intra-pore physics true 3-phase capillary entry pressures (improved Y-L) –gas-oil entry pressure depends on water wetting film pressure –determined by free energy calculation (MS-P) –also criterion for (oil) layers bulk displacement layer displacement

19 19 Intra-pore physics consistent relation 3-phase pressure differences and occupancies gas-oil bulk displacement (true varying) oil-water bulk displacement gas-oil bulk displacement, with layer (constant) layer displacement

20 20 Intra-pore physics mixed-wet bundle of triangular pores: –small pores strongly water-wet –large pores weakly oil-wet:

21 21 Intra-pore physics water injection –no difference true (3-phase) and constant (2-phase) during invasion of water-wet pores –huge differences during invasion of oil-wet pores –true: simultaneous w->o and w->g –volume effect oil films

22 22 nonuniform wettability: –after primary- after imbibition drainage –strongly affects water flood Sor (Ryazanov et al., 2009) Intra-pore physics surface rendered oil-wet: aging (Kovscek) water oil oil layers (2-phase)

23 23 non-uniform wettability layers in 3-phase configuration consistent entry pressures and layer criteria Intra-pore physics

24 24 Intra-pore physics high P ow drainage gas injection

25 25 Network displacement mechanisms phase continuity: –connectivity –films and layers (wettability) –water-wet micromodel: WAG flood

26 26 Network displacement mechanisms connected, trapped and disconnected phases –phase cluster map trapped oil cluster invading gas cluster disconnected water cluster water cluster connected to outlet outlet inlet disconnected oil cluster oil cluster connected to outlet disconnected gas cluster

27 27 multiple displacement chains displace disconnected clusters based on “target” pressure difference determining lowest target requires shortest path algorithm Network displacement mechanisms trapped oil cluster invading gas cluster disconnected water cluster water cluster connected to outlet outlet inlet disconnected oil cluster oil cluster connected to outlet disconnected gas cluster e.g. gas->oil->gas->water

28 28 Network simulations 3-phase flow simulator 3PhWetNet: regular lattice, arbitrary wettability, capillary-dominated flow few free parameters describing essence of pore- scale displacements (needs “anchoring”) –coordination number z –pore size distribution –volume and conductance exponents –wettability (contact angle distribution) –film and layers (notional)

29 29 Network simulations Network model: –parameters “anchored” to easy-to-obtain data: network structure and wettability –example mixed-wet North Sea reservoir data water-wet mixed-wet (MWL) gas flood water flood

30 30 Network simulations Network model: –predict difficult-to-obtain data, e.g. 3-phase k r and P c three-phase gas injection displacement paths three-phase gas relperms

31 31 WAG network simulations mixed-wet no films or layers varying coordination number z high residual, but additional recovery during WAG for z=3

32 32 WAG network simulations displacement statistics (chain lengths), z=5 few multiple, many double displacements continuing phase “movement” but no additional recovery

33 33 WAG network simulations displacement statistics (types), z=5 mainly 3 displacement types, corresponding to doubles, e.g. g->o and o->w during gas flood

34 34 WAG occupancy statistics (z=5) after water flood 1

35 35 WAG occupancy statistics (z=5) during gas flood 1 gas intermediate-wetting

36 36 WAG occupancy statistics (z=5) end of gas flood 1 oil moved into water-wet pores

37 37 WAG occupancy statistics (z=5) end of water flood 2 oil moved back into oil-wet pores

38 38 WAG network simulations WAG occupancy statistics (z=5): end gas flood 2 oil and gas in both water-wet and oil-wet pores

39 39 WAG network simulations Chain lengths (z=3) significant number of multiple chains z=5

40 40 WAG network simulations additional types of displacements g->o for water and o->g for gas floods z=5 Displacement types (z=3)

41 41 WAG simulation micromodel experiment weakly wetted: little evidence of (continuous) water and oil wetting films (around water) spreading oil: assume oil layers and oil wetting films around gas water-wet oil-wet mN/m

42 42 WAG simulation micromodel experiment Fractionally-wet –50% water-wet & oil-wet pores –angles distributed between degrees –oil layers and oil wetting films around gas Comparison simulated and experimental recoveries –recovery ceases after WAG 2

43 43 WAG simulation micromodel experiment Displacement chain lengths –many multiples (few films: low phase continuity) –multiples dying out after WAG 3

44 44 Type of displacements –all types of displacements occur –many displacements involving oil movement –after WAG 3 mainly w->g, g->w WAG simulation micromodel experiment

45 45 WAG simulation micromodel experiment fluid distributions after gas flood 1 –narrow gas finger in both simulation and experiment –significant amount of oil displaced –multiple displacements: e.g. gas->oil->gas->water

46 46 WAG simulation micromodel experiment fluid distributions after water flood 1 –water disperses gas –slightly more extensive in experiment

47 47 WAG simulation micromodel experiment fluid distributions after gas flood 2 –different gas finger appears –additional oil production

48 48 WAG simulation micromodel experiment fluid distributions after gas flood 3 –new gas finger in simulation –some additional oil displaced (“jump” in recovery) –after this flood mainly water displacing gas and vice versa

49 49 Conclusions Mixed wettability leads to three types of pore occupancy and corresponding saturation-dependencies of three-phase capillary pressures and relative permeabilities: –difficult to capture in empirical model True three-phase capillary entry pressures and layer criteria essential for consistent and accurate modelling Phase continuity driver for WAG at pore-scale –strongly affected by network connectivity and presence films and layers: precise wettability –multiple displacement chains –new fluid patterns during each cycle (micromodels) –recovery ceases after few WAG floods, oil movement may continue

50 50 Near-miscible WAG: micromodel Continued gas injection in strongly water-wet experiment: Much oil displaced through film flow + mass transfer (?) After 1 hour After 2 hours


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