CO 2 Sequestration during Heavy Oil Production using CO 2 EOR

Following successful investigation of heavy oil recovery using CO2-EOR, the

effort was shifted to exploring any occurrence of CO2 sequestration.

The investigation focussed on the following areas:

 CO2 sequestration during Miscible and Immiscible conditions;

 CO2 sequestration using the integrated surface and sub-surface modelling;

REVEAL, the 3D reservoir simulator by Petroleum Experts, was used to model the reservoir in 3D with a grid block of dimension 25, 25, 15 in X, Y and Z directions respectively. A block size of 500 ft x 500 ft x 200 ft, grid depth of 10000 ft and a single porosity was considered. Two wells, one producer and an injector, and both horizontal were investigated. The model was homogenous as shown in Figure 3.5 below.

Figure 3.5: Block grid (Horizontal Well) – Illustration Only

Tables 3.6 and 3.7 present the reservoir and fluid properties used in the

Table 3.6: Fluids Properties & Rock Properties

Data Units

Rock Compressibility 3 x 10-5 _1/psi

Permeability 100 mD

Reservoir Porosity 0.2 Fraction

Well Control: Constant injection Pressure 3000 psig

Water Compressibility 2.9 x 10-6 _1/psi

Heavy Oil Specific Gravity 15 o_API

Heavy Oil Viscosity 523 - 2188 cP

Heavy Oil FVF 1.19 RB/STB

Water FVF 0.99 RB/STB

Gas FVF 0.0034 RB/STB

Gas Oil Ratio, GOR 500 scf/STB

Reservoir Temperature 122 - 200 o_F

Water gravity 1.068 Sp. gravity

Gas Gravity 0.7 Sp. gravity

Table 3.7: Residual Saturation used for the Simulation

Data Units

Critical Oil / Gas Residual Saturation, Sogc 0.05 Fraction Critical Oil / water Residual Saturation, Sowc 0.2 Fraction Critical water Residual Saturation, Swc 0.2 Fraction Critical Gas Residual Saturation, Sgc 0.2 Fraction End Point Oil / water Relative Permeability, Krow 1 Fraction End Point Oil / Gas Relative Permeability, Krog 1 Fraction End Point water Relative Permeability, Krw 1 Fraction End Point Gas Relative Permeability, Krg 1 Fraction

Corey Exponent for Oil-water 2 -

Corey Exponent for Oil-Gas 2 -

Table 3.8: Aquifer Properties

Data Units

Aquifer Model Infinite Linear -

Aquifer Porosity 0.2 Fraction

Aquifer Permeability 1000 mD

Aquifer Compressibility 3 x 10-6 _1/psi

Thickness 300 feet

Encroachment Angle 90 degree

Width 300 feet

Region 1 X_West, From (1, 1, 1) to (1, 25, 15) - Region 2 X_West, From (25, 1, 1) to (25, 25, 15) -

The initial pressure used in this analysis was 2500 psig, with the temperature of 200 o_{F. The CO}

2 was injected into the reservoir through a horizontal well, 8 km

long and completed over a length of approximately 492.13 ft (150 m). The reservoir gas was modelled as CO2. With a critical pressure of 1073 psi and

critical temperature of 87.8 °F, CO2 will be in a supercritical state at bottom-hole

injection and reservoir conditions; hence CO2 was defined in the model as gas

Both Black Oil and Compositional Models were used. The Peng-Robinson (PR) EOS was selected to generate the VLP (Vertical Lift Performance) files for the injection and production system using PROSPER. The production system was modelled as a black oil model while the injection system remained compositional to take into account the properties of CO2.

The following two methods were used to interpret the REVEAL results in order to quantify the CO2 sequestration during CO2-EOR:

 Mass conservation of CO2 around the reservoir loop;

 Production profiles evaluation.

3.2.5.1 Mass Conservation

This approach considered the mass of CO2 entering (

m

CO2inj) and leaving (

m

_CO2out) the reservoir and the mass of CO2 retention (

m

_CO2Seq) within the

reservoir, which is conveyed in the following expression:

 

_m

_CO2inj



_m

_CO2out

_m

_{CO2 Seq}  (3.1)  Where:

inj CO

m

₂ : Mass flowrate of CO2 entering the reservoir;

out CO

m

2 : Mass flowrate of CO2 exiting the reservoir (CO2 produced);

Seq CO

m

₂ : Mass flowrate of CO2 retained in the reservoir (CO2

Sequestration).

The density of CO2 changes as its pressure (P) changes and using the ideal gas

Equation-of-State (EOS), the CO2 density (CO2) can be calculated at the

appropriate pressure, and hence the volumetric flowrate of CO2 (

Q

_CO2Seq) can be

established using the expression below. “T” stands for temperature and “Mw” for the molecular weight of CO2 and the other terms have their usual meanings.

               T P R M m Q W Seq CO Seq CO 2 2    (3.2)  Where:

Seq CO

Q

2 : Volumetric flowrate of CO2 sequestrated;

CO2 : Density of CO2;

P : Pressure at reference point; T : Temperature at reference point; MW : Molecular weight of CO2.

3.2.5.2 Production Evaluation

The CO2 sequestration (

Q

CO2Seq) is estimated as the difference between the injected CO2 (

Q

CO2inj) and the produced CO2 (

Q

CO2out), taking into account the rates of CO2 production during steady or quasi-steady state since the reservoir

gas was modelled as CO2.

The term

Q

_CO2outWI, represents the produced CO2 when there is no CO2 injection.

 

 

Q

_{CO2 Seq}



Q

_CO2inj



Q

_CO2out



Q

_{CO2out WI}



  (3.3)  Where,

WI out CO

Q

2 : Produced CO2 (original gas in place) when there is no CO2

injection.

In case where the reservoir gas is modelled differently other than CO2, the

WI out CO

Q

2 in the equation 3.3 may be omitted.

Q

CO2out WI was found to be less than 1% of that produced during CO2 injection, hence the impact on the overall

results was negligible, as far the simulations are concerned.

The CO2 retention as a function of barrel of heavy oil produced (Seqco2) was

calculated using the volumetric flow rate of heavy oil produced (Qoil prod) and the

CO2 sequestration by the following expression:

    prod oil Seq CO CO Q Q Seq 2 2      (3.4)   Where.

SeqCO2 : CO2 retention / sequestration per barrel of produced heavy oil;

Qoil prod : Volumetric flowrate of heavy oil produced.

The CO2 requirement / utilisation per barrel of heavy oil produced (CO2(Req))

was obtained using the required CO2 injection as follows: 

    prod oil inj CO Q Q q CO₂(Re )  2   _(3.5)  Where,

In document Cold heavy oil production using CO2-EOR technique. (Page 84-89)