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Date • Location of Event

Development of Gas/Oil Miscibility in Water and Gas Injection

Tara LaForce, and Franklin M. Orr, Jr. (Stanford)Second Nature Sample Poster

Dec. 3, 2008

Abstract

We use analytical solutions for a one-dimensional model to study water and gas injection strategies into an oil reservoir. The model accounts for three-phase flow of four components through porous rocks and uses a cubic equation of state to model gas/oil phase behavior. In oil reservoirs super-critical CO2 may generate a multi-contact miscible displacement of hydrocarbons, so both miscible and immiscible injection of CO2 is studied. We consider the implications of using CO2 in order to improve oil recovery, and find that this is often a competing goal with storing CO2 far from production wells. –Mixtures which create efficient miscible displacements result in earlier breakthrough of injected CO2, while mixtures that have delayed arrival of CO2 in production wells have less favorable oil recovery.

Background

Depleted oil reservoirs are excellent candidates for CO2 storage.

When CO2 is stored in oil reservoirs it is of critical importance to maximize the reservoir volume containing CO2.

CO2 can behave as a miscible gas in many reservoirs, which means displacement efficiency of hydrocarbons is optimal

Injecting water simultaneously with CO2 in aquifers has been shown to increase the volume of CO2 that can be stored as a trapped phase because injected water inhibits the ability of CO2 to rise under buoyancy forces (SPE109905).

In an oil reservoir, a similar result is likely. Moreover, if there are active wells in the reservoir then injection of water to trap CO2 will prevent excessive gas cycling.

But can we store CO2 securely and improve oil recovery at once?

Displacement Efficiency

Can be thought of as flow along a streamline in the reservoirThis is a 1D problem

From SPE 97270

D

Gas

S

atur

atio

n

1

0

CO2 Injection in 2D and 3D

The drawback in injecting CO2 alone is that the sweep efficiency is poor

By injecting water and gas (WAG) simultaneously, it is possible to get good sweep efficiency AND optimal displacement efficiency

… or is it?

From SPE 97270

From Al-Shuraiqi et al. 12th European Symposium on IOR

Phase behavior in ternary subsystems at MMP(three-phase region shown in light blue)

Phase behavior in full system at MMP(only three-phase regions are shown)

Vaporizing Gas Drive Phase Behavior

Peng-Robinson EOS is used to model the gas/oil phase behaviorHenry’s Law is used to model hydrocarbon/aqueous phase partitioningWater does not partition into gas or oil

AB

CD

E

Initial ConditionInjection C

onditions

shock

rarefaction

Vaporizing Gas Drive at MMPInjection of CO2 into a mixture of C3 and C16 results in a miscible vaporizing gas drive. Water/CO2 mixtures C-E behave like miscible gas drives, but A and B are immiscible.Injection of water/CO2 mixtures have miscible displacements as long as injected CO2, is more mobile than water and flows ahead of injected water.Mobility of the CO2 is determined by the relative permeabilities of the water and CO2 phases.

Volume of CO2 injected: E=100% D=25% C=19.84% B=15% A=0%

A B

CDE

shock

rarefaction

D/tD

Tertiary Vaporizing Gas Drive

JC = 75%

water

Water in the initial condition causes the miscible front to accelerate:If no water is present initially then the MGF = 19.84%If 12% residual water is present initially then the MGF = 20.77%If 25% water is present initially then the MGF is about 22.17%

Initial Conditions

Results and Conclusions

In SWAG displacements a minimum gas fraction of CO2 must be injected to create a miscible gas bank

Miscible displacements that have optimal displacement efficiency result in an unstable displacement and early gas breakthrough

Relative permeabilities determine whether miscibility will be achieved

The failure to achieve miscibility will allow the design for injection strategies that will effectively trap the CO2 in the reservoir

A high fraction of CO2 can be injected in a water-flooded reservoir without creating a fast-moving miscible gas bank that will quickly reach production wells

This represents directly competing goals between optimizing oil recovery with a miscible gas flood and storing the CO2 far from production wells

Many Thanks To:

Grantham Institute for Climate Change at Imperial College London

Shell Grand Challenge on Clean Fossil Fuels

UK Engineering and Physical Science Research Council