Oil & Gas Journal - April 03, 2017

FIG. 5 CO2 TRANSPORT PATTERN

1,000

100

10

1

0.1 1.0 10 100 1,000 10,000

Darcyfow

Tem
pe
rat
ure
/
Pre
ss
ure
,
K
./
M
Pa
Pore diameter, nm
Kn = 0.001

Kn = 0.01

Kn = 0.1

Kn = 10

Kn + 10 with lower mean free path

Knudsendiffusion Transitiondiffusion Fickdiffusion Slippagefow

FIG. 6 CO2 TRANSPORT IN CORES

Kn udson number

1.0000

0.1000

0.0100

0.0010

0.0001

0.0000

Temperature = 45° C.

Pressure = 5 MPa

1 10 100 1,000 10,000

Pore diameter, nm

FIG. 7 OIL-RECOVERY MECHANISMS

CO2

CO2 + oil Macroporosity Microporosity Oil CO2

MICP technology also was used to analyze gas flow through different pore sizes. Core-flooding experiments observed the response of pore characteristics to CO2 injection and oil recovery.

Fig. 1 depicts the core flooding experiment. Samples were dried at 80° C. for 24 hr and placed in a core holder for vacuum treatment. Cores were saturated with 3.5% sodium chloride, stored for 10 hr and then flooded with 1 ml/min of decane until no brine exited the outlet, establishing water saturation as the difference between entered brine and displaced brine.

After soaking, workers injected CO2 at 0.15 ml/min and monitored oil production.

Workers closed the oil outlet once CO2 broke through, maintained 5 MPa for 12 hr, and then slowly reopened the outlet for more CO2 injection until breakthrough resumed.

The experiment indicated pores of 10 nanometer (nm) to 100 µm. Fig. 2 shows that all four limestone samples demonstrated typical bimodal-pore characteristics.

Fig. 3 shows cumulative pore volume for four samples having different permeability. Cumulative pore volume was divided using a critical 1 µm pore size. Higher-permeability samples had larger cumulative pore volume above the critical pore size as did high-po-rosity samples.

Permeability was controlled primarily by macroporosity (pore sizes above 1 µm). Pore sizes less than 1 µm contributed little to permeability. Greater explanation of microporosity’s quantitative effect on permeability in carbonates lies outside the scope of this article, but can be found in previous research. 6-8

Each sample, especially the Wisconsin sample, was low permeability. Researchers used the unsteady-state method to measure permeability, monitoring CO2 pressure in the inlet and outlet.