25th International Meeting on Organic Geochemistry IMOG 2011

P-343
Effective diffusivities of CO2 and associated noble gases in
impermeable shale caprocks of an EOR-CO2 field
Caroline Magnier 1 , Alain Prinzhofer 1 , Eric Flauraud 1 , Sophie Giannesini 1,2
1 IFP New Energy, Rueil Malmaison, France, 2 IPGP, Paris, France (corresponding
author:caroline.magnier@ifpen.fr)
Present research on CCS (Carbon Capture and
Storage) is concerned on mitigation options for the
increasing atmospheric CO2 concentrations. The
petroleum industry is studying different liabilities for
the injection of acid gases in geological structures,
deep saline aquifers or depleted petroleum reservoirs.
Some of the options are thought for permanent
storage if and when a good efficiency of seal rocks is
proven to acid gas mobility. In that respect, recent
experimental studies have been described that
estimate gas migration distances in overlying low
permeability cap rocks when CO2 gas, dissolved or
not, plumes upward in geological sediments by
diffusion or advection flow, through faults or leaking
wells.
If shales and limestones sediments are mostly
encountered as seals above hydrocarbon
accumulations and over CO2 potential storage
reservoirs, it is because shales are impermeable
natural barriers. This is one of the reasons why it is
important to assess their capacity to remain
impermeable to fluids, regardless of what is injected
in the underlying cavities.
Experiments on clays were conducted on different
rock sediments of an EOR-CO2 field to investigate
the transport by diffusion of CO2 and noble gases in
water saturated pore networks. Effective diffusivities
for each gases were obtained by a model and
compared as a function of the rock type.
pressure
gauge
pressure
gauge
CO 2 +
noble
gases
parent
other gas
daughter
sampling
zone
Rock sample
sampling
zone
sample
primary / secondary
vacuum
sample
primary / secondary
vacuum
Figure 1: The experimental setup for measuring effective
diffusivities of gases in saturated pores
The experimental setup consists of a stainless steel
reactor with separate gas reservoirs (parent/daughter)
with volumes of 175 cm 3 filled and put at near equal
pressures.
A gas is mobile with a same flow rate in a given rock
type regardless of the porosity while a gas production
is inversely proportional to the thickness of the
sample (Figure 2).A gas is mobile with a same flow
rate in a given rock type regardless of the porosity
while a gas production is inversely proportional to the
thickness of the sample (Figure 2).
The dissolved CO2 gas migrated differently in the
natural clays. The lowest CO2 retention was found in
caprocks 1 with a Dw/Deff ratio between 15-18 while
caprock 2 or caprocks 3 (B-45) have greater CO2
retention, with Dw/Deff ratios at 30 and 45 respectively.
Noble gases followed a decreasing mobility from
helium, neon, argon and krypton respectively,
krypton being very much more retained.
% CO2 PRODUCTION
20
15
10
5
0
Caprock 1 (B-419)
B-419 5 mm
B-419 10.2 mm
B- 419 12.6 mm
0 500 1000 1500 2000
TIME (HRS)
Figure 2: CO2 production vs. time in a shaly caprock sample
The effective diffusivity of helium was found to
be equal to the rocks tortuosity.
From such results, larger scale simulations of
gas flows of CO2 associated with inert tracers
with effective diffusion rates or ratios of diffusion
rates between gas compounds in a given rock
type are now considered in order to attempt to
quantify leakage amounts CO2 in aquifers.
% CO2 PRODUCTION
20
15
10
5
0
0 20 40 60 8
t Dw /L 2 (ADIMENSIONAL)
474
B-419 5 mm
B-419 10.2 mm
B- 419 12.6 mm