Investigation of Compositional Grading in Petroleum Reservoirs

Transcription

1 Investigation of Compositional Grading in Petroleum Reservoirs Zhangxing Chen University of Calgary

2 Outline Importance of the Research Factors Leading to Compositional Variations Compositional Grading Theory Numerical Experiments Conclusions

3 Importance of the Research Initialization of simulation: - Mechanical equilibrium - Chemical equilibrium

4 Importance of the Research (cont d)

5 Importance of the Research (cont d) Accurate modeling of composition variation highly affects: - Reserve estimation - Design of production and development strategies

6 Factors Leading to Composition Variation Gravity: gravity segregation. Thermal diffusion: light components to warm zones and heavy ones to cold zones. Incomplete hydrocarbon migration/mixing: complete mixing takes time. Natural convection: leading to an increase of horizontal compositional variation. Dynamic flux of water aquifer contacting only a part of reservoir: creating a sink for continuous depletion of light components (e.g., methane)

7 Factors Leading to Composition Variation (cont d) Asphaltene precipitation during migration: leading to different layers with different permeability to host different types of oil. Biodegradation varying laterally and vertically: causing significant variation in H 2 S content and API gravity of the reservoir. Reservoir compartmentalization: causing loss of pressure and fluid communication between adjacent fault blocks.

8 Factors Leading to Composition Variation (cont d) Partial barriers: causing limited fluid and pressure communication. Genesis: related to source rocks. Capillary forces: having an effect on fluid distribution in systems with pore radius in the order of 1 micron. Artificial issues: e.g., miscible gas injection

9 Processes and Time Scales Affecting Fluid Compositions Multiple processes that affect fluid properties: Reservoir charge/filling, fluid mixing through Darcy flow/advection/ diffusion, gravity segregation, biodegradation, fractionation, and differential leakage of gas vs. oil. Different time scales (key to understanding the relative significance of fluid data to reservoir segmentation studies): Charge/filling of reservoirs: geological time - several millions of years Biodegradation: thousands to hundreds of thousand of years Molecular diffusion: 1 to 100 million years Pressure diffusion: hundreds or even thousands of years Convective flow: thousands to million years

10 Importance of the Research (cont d): Understanding Reservoir Fluid Compositions Time scale

11 Difficulties in Modeling Compositional Variation We do not have enough physical/chemical understanding of these phenomena. Boundary conditions are changing continually. Mathematical models may be so complex or even unknown.

12 Compositional Grading Gravity Thermal diffusion (Soret effect) Capillary effects

13 Theory Classical theory New general theory

14 Classical Theory Constraint of chemical equilibrium for an isothermal system (Gibbs, 1876):

15 Classical Theory (cont d) Constraint of chemical equilibrium for a nonisothermal system (Faissat, et al., 1994):

16 New General Theory Mass conservation

17 New General Theory (cont d) Diffusive mass flux:

18 Relationship between classical and new theories For an isothermal system, the classical constraint of chemical equilibrium can be obtained from the pressure diffusion. For a non isothermal system, it can be obtained from the thermal diffusion.

19 Simulation Approach R&D Program: Have developed a software package that integrates geological processes (source rock maturation, hydrocarbon generation, migration, charge/ filling, etc.) with reservoir processes (fluid mixing, advection, diffusion, gravity segregation, biodegradation, etc.).

20 Case Study A: A Light Oil A North Sea reservoir The thickness of reservoir: 200m Reference pressure and temperature at 3,000m: 40 MPa and 320 K Temperature gradient: 0.02 K/m Components: C 1 --C 10+

21 Case Study A (cont d) Depth (m) C 1 % C 10+ % P (MPa) dens (kg/ m 3 ) P b (MPa) R s (Sm 3 / Sm 3 ) B o (m 3 /Sm 3 )

22 Case Study A (cont d) P ref (bar) T ref (K) Error OIP %

23 Case Study B: A Black Oil Two-Phase Location: the Azadegan oil, southwest of Iran. Components: C 1 C 7+ Temperature gradient: 0.01 K/m, which is normally considered isothermal.

24 Case Study B (cont d) Depth (m) C 1 % C 7+ % P (bar) Dens (kg/m 3 ) P b (bar) P d (bar) R s (Sm 3 /Sm 3 ) B o (m 3 /Sm 3 ) T (K) MW of C GOC depth (m)

25 Case Study B (cont d) Condition Error OIP % Without Plus Fraction Change with Depth Isothermal

26 Case Study C: Single Phase Location: the Azadegan oil, southwest of Iran. 12 Components: H 2 S, N 2, CO 2, C 1, C 2, C 3, ic 4, nc 4, ic 5, nc 5, C 6, C 7+ Reference pressure and temperature at 3,000m: 175 Bar and 370 K Temperature gradients in x, y, and z directions: 0.003, 0.004, K/m.

27 Case Study C (cont d) Component Mole% H 2 S 0.04 N CO C C C ic nc ic nc C C

28 Case Study C (cont d)

29 Case Study D: Analytical Solution yr yr yr yr yr yr

30 Case Study E: Diffusive Mixing 2 myr 20 myr 80 myr 400 myr

31 Case Study F: n-component

32 Case Study G: Rayleigh Number

33 Case Study H: Reservoir with Baffle for n-component Mixing Kx = 100 md in reservoir Kx = 10, 1, 0.1, md in baffle nc4 mole fraction Kz = 10 md in reservoir Kz = 1/10 of Kx in baffle C1 mole fraction Pressure gradient (atm)

34 Case Study H: Reservoir with Baffle for n-component Mixing (cont d)

35 Case Study I: Effect of Pressure and Thermal Diffusions Equilibrium at t = 17 million years with pressure diffusion Equilibrium at t = 20 million years with thermal diffusion Equilibrium at t = 19 million years with pressure and thermal diffusions

36 Conclusions The effect of compositional grading is magnificent and cannot be ignored; its effect is more pronounced as the fluid becomes near-critical. Ignoring change in composition can lead to huge errors in OIP calculations as much as 50% of the real number. The temperature gradient must be included in calculations as it has a remarkable effect on compositional grading and the change of physical properties with depth. Molecular weight and so all other properties of the plus fraction can change with depth, which cannot be ignored. Gravity causes the fluid and the plus fraction to become heavier towards the bottom while the temperature gradient does the opposite. Pressure equilibration seems fastest.