We have identified a CO2 and water injection strategy to recover moderately heavy oil and store
carbon dioxide (CO2) simultaneously. We propose the use of counter-current injection of gas
and water to improve reservoir sweep and trap CO2; water is injected in the upper portion of
the reservoir and gas is injected in the lower portion. This process is referred to as water over
gas injection or modified simultaneous water alternating gas injection (SWAG). This thesis is
based on the results of quasi-validated compositional reservoir simulations in that exact
matches were not obtained for the disparate fluids and reservoirs properties but the trends of
oil recovery and water cut were accepted as representative of comparative physical
mechanisms of displacement. We have compared oil recovery and water cut trends of the
compositional simulation model to the displacement experiments conducted by Dyer and
Farouq Ali[1] where varying injection rates, number of WAG cycles and size of CO2 slug were
investigated. Dyer and Farouq Ali’s displacement experiments used an Aberfeldy crude mixed
with liquid petroleum to obtain an oil viscosity of 1055 mPa.s at standard conditions to
represent viscosity reservoir conditions. The fluid description used in our compositional
simulations are based on PVT-matched properties of oil found in an unconsolidated deltaic,
sandstone deposit in the Gulf of Paria, offshore Trinidad. At standard conditions the crude
viscosity is 1175mPa.s and at reservoir conditions (81° C and 27.9 MPa) 8 mPa.s. In this region
oil density ranges between 940 and 1010 kg/m3 (9-18 degrees API). The PVT properties were
matched by regressing: the 3-parameter Peng-Robinson[2] equation of state to the oil relative
volume, total relative volume and; the coefficients of the Lohrenz Bray Clark [3]correlation to
the viscosity of the crude between 0 and 20MPa at 81.7 °C.
The reservoir simulation model was scaled to the length to width ratio of the displacement
experiment and, the ratio of gravitational to viscous forces of injected water used in
displacement experiments. From this we study we identified the limitations of WAG and the
injection parameters favourable to oil recovery, gas trapping and gas storage capacity.
We have then used a synthetic reservoir to represent an unconsolidated sand measuring 1000m
× 150m × 100m with average porosity of 26% and initial water saturation of 20% to investigate
with representative parameters, determined from the comparison with the displacement
experiments, to investigate the efficacy of water over gas injection. The original oil in place
(OOIP) is 3.12 × 106 m3 (19 MMbbl).The two water injection rates investigated, 100 and
200m3/day(630 and 1260 bbl/day). These rates correspond to water gravity numbers
(dimensionless ratio of viscous to gravity forces) 6.3 to 3.1 for our reservoir properties. The gas
injection surface rate was 50 000 sm3/day (1.8 Mscf/day) in both instances corresponding to
gas gravity numbers ranging between 150 and 200 with varying reservoir flow rates .We have
applied this injection strategy using vertical producers with two injection configurations: single
vertical injector and a pair of horizontal parallel laterals. The producer was vertical in each
case.
The impact of miscibility was investigated by varying the injection gas composition by
comparing the effect of using pure CO2 and a mixture of CO2 and C2-C6 in a 2:1 ratio, on oil
recovery, carbon storage and field performance. Eight simulation runs were conducted varying
injection gas composition for miscible and immiscible gas drives, water injection rate and
injection well orientation. Our results show that water over gas injection can realize oil
recoveries ranging from 17 to 30% of original oil in place (OOIP). In each instance more than
50% of the injected CO2 remains in the reservoir with less than 15% of retained CO2 in the
mobile phase. The remaining CO2 is distributed in oil, water and trapped gas phases.
Our reservoir simulations show that water over gas injection can be applied successfully to
recover heavy oil and trap CO2 in an unconsolidated sand. This injection design has also shown
immiscible and miscible oil recovery can be improved with horizontal injection. Water injection
over gas injection increases contact between injected CO2by dispersing the injected gas over a
wider volume in the reservoir, hindering gas override and providing reservoir pressure
support. Gas storage is inversely proportional to the water gravity number because of the effect
the injected water has on gas saturation distribution. In combination with established industry
reservoir management techniques such as pressure control and gas cycling, it may be possible
to further improve the oil recovery and carbon storage of water over gas injection.