SPE 63086(originally SPE 49269)

Miscibility Variation in Compositionally Grading Reservoirs

Lars Høier, SPE, StatoilCurtis H. Whitson, SPE, NTNU and Pera

BACKGROUND

Miscible gas injection processes are well documented in the literature.

Compositional variation with depth has also been studied the past 20 years.

However, almost nothing in the literature is found on the variation of miscibility conditions with depth in reservoirs with compositional gradients.

PURPOSE

Intuitively, it is difficult to picture the variation of MMP with depth for a reservoir with varying composition and temperature.

This study shows that a simple variation does not exist, but that certain features of MMP variation are characteristic for most reservoirs.

Fluid Systems

• Three reservoir fluid systems, each with significant compositional grading.

• Lean and enriched injection gases.

• Peng-Robinson EOS, typically with 15 components, five C7+ fractions, and no grouping of intermediates.

Calculating Miscibility Conditions

A Multicell Algorithm Developed by Aaron Zick

• Defines “true” minimum miscibility conditions (pressure or enrichment)

• Identifies the developed-miscibility mechanism– Condensing/Vaporizing Drive (C/V)– Vaporizing Gas Drive (VGD)– Condensing Gas Drive (CGD)– First Contact Miscible (FCM)

Calculating Miscibility Conditions

• Zick algorithm is fast and uses an internally-consistent numerical solution.

• Zick algorithm has been verified in this study by numerous 1D numerical (“slimtube”) simulations for a large range of fluid systems, injection gases, and miscible mechanisms.

MMP from Slimtube Simulations

4500

4600

4700

4800

4900

5000

0.00 0.05 0.10 0.15 0.20 0.25 0.30C7+, mole fraction

Dep

th, m

400 425 450 475 500 525Pressure, bara

Reference Sample

Reservoir Pressure

C7+

Saturation Pressure

MMP versus DepthExample 1

MMP versus DepthExample 1 – Lean Gas Injection

VGD

VGD

VGD

MMP versus DepthExample 1 – Enriched Gas Injection

C/V

C/VC/V

VGD

MMP versus DepthExample 1

MMP versus DepthExample 1 – Varying Enrichment

C/V

C/VC/V

C/V

VGD

Oil Reservoirs – Summary

MMP always increases with depth, both for VGD and C/V mechanisms.

• VGD MMP is always greater than or equal to thebubblepoint pressure.

• C/V MMP can be greater than or less than thebubblepoint pressure.

Gas Condensate Reservoirs – Summary

In gas condensate reservoirs, MMP variation with depth follows exactly the dewpoint variation with depth

only

when miscibility develops by a purely VGD mechanism.

Gas Condensate MMP – Summary

For a depleted gas condensate reservoir,

the composition of the retrograde condensate

controls the C/V MMP.

Gas Condensate MMP – Summary

MMP can be significantly lower than the dewpoint pressure.

This requires that the C/V mechanism exists, which usually results from the injection of an enriched gas (or CO2 ?).

C/V Mechanism in Gas Condensatesbelow Dewpoint Pressure

Key features in 1D slimtube simulations:

- An oil bank develops, increases in size, and propagates through the system.

- The miscible front is located on the ‘’back side’’ of the saturation bank, leaving behind a near-zero oil saturation.

C/V mechanism in a depleted system (gas condensate reservoir, 0.7 PV injected)

0.0 0.2 0.4 0.6 0.8 1.0

200300

400500

600700

800900

0.00.1

0.20.3

0.40.5

0.60.7

0.80.9

Normalized Distance from Inlet

Density, kg/m

3

Oil S

aturation

Oil density

Gas density

Oil saturation

Oil bank development(depleted gas condensate reservoir)

0.0 0.2 0.4 0.6 0.8 1.0

0.00.2

0.40.6

0.81.0

Normalized Distance from Inlet

Oil S

aturation

Slug case:

0.24 PV injected

0.48 PV

0.72 PV

0.96 PV

Continuous rich gas:

0.24 PV injected

0.48 PV

0.72 PV

0.96 PV

CONCLUSION,MMP below Dewpoint Pressure

Dispersion has a strong influence on the development of miscibility by the C/V mechanism for lean gas condensates.

Elimination of numerical dispersion (gas condensate reservoir)

0.00 0.02 0.04 0.06 0.08 0.10

0.550.60

0.650.70

0.750.80

0.850.90

0.951.00

N -1/2

RF

at 1.2 PV

injected

1002005001000

Pressure:

(bara)

350

330

312

300

290

275

260

Slimtube displacment STO recoveries (gas condensate reservoir)

250 275 300 325 350

0.50.6

0.70.8

0.91.0

Pressure, bara

RF

at 1.2 PV

inj

Extrapolated to infinity

500 grid cells

200 grid cells

100 grid cells

CONCLUSION,MMP in depleted reservoirs

For a depleted retrograde condensate reservoir, the composition of the retrograde condensate at the start of a cycling project controls the C/V MMP.

CONCLUSION,MMP in depleted reservoirs

Simple 1D slimtube simulations demonstrate that slug injections as small as 10% PV of enriched gas in depleted gascondensate systems can develop miscibility at the same conditions as continuous enriched-gas injection.

RecomendationMMP in depleted reservoirs

For depleted rich gas condensate reservoirs:

- Perform 3D compositional simulations to evaluate miscible gas (slug) injection versus traditional dry gas injection.

- Measure the MMP by traditional slimtube displacement

Key Observation

Miscibility variation with depth due to gravity-induced compositional gradients can be significant.

The miscibility variation depends strongly on the mechanism of developed miscibility:

- Condensing/Vaporizing Mechanism ( C/V )- Vaporizing Gas Drive ( VGD )

NOTE

If the condensing/vaporizing mechanism exists, then the true C/V MMP will always be less than the VGD (vaporizing) MMP.

MMP variation with enrichmentat a specific depth in an oil zone

0.0 0.2 0.4 0.6 0.8 1.0

250300

350400

450500

Enrichment Level, fraction

EO

S-C

alculated MM

P, bara Bubble Point Pressure

E*

VGD MMP

C/V MMP

Vaporizing MMP variation with depth (dry gas injection in SVO reservoir )

200 300 400 500 600

-2900

-2800

-2700

-2600

-2500

-2400

-2300

-2200

EOS-Calculated MMP, bara

Depth, m

SS

L

GOC

Reservoir Pressure

Saturation Pressure

E = 0.0 (dry gas)

MMP versus DepthExample 2

Gas Condensate Reservoirs – Summary

• MMP on the gas side of the GOC is less than or equal to the MMP on the oil side of the GOC.

• MMP may decrease slightly at depths above the GOC until a minimum is reached

• MMP increases until the condensing part of the mechanism disappears and the MMP equals the

dewpoint (VGD MMP) variation with depth.