Enhanced Oil Recovery with CO2 Injection

Transcription

1 Enhanced Oil Recovery with CO2 Injection Wei Yan and Erling H. Stenby Department of Chemical Engineering Technical University of Denmark Contents Overview Mechanism of miscibility Experimental study of gas injection MMP calculation Summary 1

2 Recovery methods Primary recovery by depletion Secondary recovery by water/gas injection for pressure maintenance Tertiary recovery after primary and secondary Enhanced Oil Recovery (EOR): something other than plain water or brine is being injected into the reservoir (Taber et al., SPE 35385) EOR methods A summary by Taber et al. More than 20 methods 2

3 Trends in EOR with CO2 EOR production in the US The percentage of EOR projects continues to increase CO2 injection is the only method that has had a continuous increase CO2 vs. other gases Supercritical extraction at reservoir conditions Easier miscibility than N 2, flue gas, C 1 Cheaper than liquid hydrocarbons Safer to handle and pressurize than hydrocarbon gases Reduction of GHG 3

4 CO2 sequestration + EOR The biggest barrier for CO2 sequestration CO2 sequestration cost: $/ton CO2 credit: 1-20 $/ton CO2 (?) EOR can offset the cost and even make it profitable CO2 injected/extra oil produced (mass): 1:1 to 4:1 3:1 is carbon neutral Net CO2 storage ratio: tons/barrel oil CO2 sequestration + EOR Maximum permissible cost of carbon dioxide in $/Mscf for the North Sea (Blunt et al., 1993.) Displacement efficiency (CO2/extra oil) Oil price ($/barrel) Volume ratio (Mscf/barrel) Mass ratio /1.62/1.07 * 6.17/3.52/ /5.43/ /0.67/ /1.62/ /2.73/ /0.29/ /0.86/ /1.43/0.94 * The three numbers indicate the maximum price for rates of return r = 0/0.1/0.2 A carbon dioxide displacement would be profitable at a 10% rate of return at a gas price of over $3/Mscf (56$/ton). 4

5 Mechanisms of gas injections Swelling of the oil phase Lowering of oil viscosity Reduction of interfacial tension Misciblility (no interfacial tension for miscible displacements) Pseudo ternary system for petroleum mixtures Three components: Light: C1, CO2, N2 Intermediate: C2-C6 Heavy: C7+ Useful to illustrate basic concepts Cannot explain combined mechanism (0.20,0.55,0.25) Single phase region Two phase region A B 1.00 C Critical tie line

6 First contact miscibility (FCM) FCM Single phase at any proportion 2 Minimum Miscibility Pressure (MMP) Fix Comp., change P FCM pressure (FCMP) dilution line Gas A Gas A" Gas A' P Minimum Miscibility Oil B Enrichment (MME) P'>P Fix P, change Comp. 3 1 FCMP and swelling test Experimental/modeling determination of FCMP Easy to perform and provide basic information about gas injection FCMP Psat (atm) Oil Fraction of Gas Gas 6

7 Multicontact miscibility Gas and oil become miscible by multiple contacts, through which (one or both of) their compositions are changed. Easier than FCM For 1D gas injection, 100% recovery if MCM In reality, >90% recovery for swept area Three mechanisms Vaporizing Condensing (No such thing in a real reservoir) Combined (Zick, 1986) Vaporizing mechanism Intermediate components vaporize to gas Oil 2 Miscibility achieved in the displacement front/far from the well critical tie line C Dry gas/oil with sufficient intermediate components 3 1 G 2 G 1 System C1/C4/C10 just above MMP Gas 7

8 Vaporizing mechanism Study using slimtube simulation Gas saturation ln K i Gas region Gas/oil region Methane n-butane n-decane Oil region Density (kg/m 3 ) Liquid Gas Dimensionless distance Condensing mechanism Intermediate components condense to oil 2 Miscibility achieved in the displacement rear/near from the well Heavy oil/enriched gas (with sufficient intermediate components) Oil critical tie line O 2 O 1 System C1/C4/C10 just above MMP C Gas 3 1 8

9 Condensing mechanism Study using slimtube simulation ln K i Gas saturation Gas region Gas/oil region Methane n-butane Oil region -3 n-decane Density (kg/m 3 ) Liquid Gas Dimensionless distance Condensing mechanism? Now it is believed that there is no such mechanism in a real reservoir. Reason: the multicomponent system (reservoir fluid) contains both light intermediate and heavy intermediate. Gas tends to extract heavy intermediate, leaving the oil saturated with light and light intermediates, which are hard to be miscible with the gas. The exchange of components is two-way, both vaporizing/condensing can happen. This leads to the combined mechanism. 9

10 Combined mechanism 15 comp. (N 2, C 1, CO 2, C 2, C 3, ic 4, nc 4, ic 5, nc 5, C 6 and 5 C 7+ comps). Gas saturation Gas region Gas/oil region Oil region ln K i Density (kg/m 3 ) 1000 Liquid Near miscible zone Gas 400 Vaporizing segment Condensing 200 segment Dimensionless distance Experimental study Swelling test Easy to perform Forward- and backward-contact Slimtube experiment Rising bubble apparatus 10

11 Forward contact Simulate vaporizing process Provide phase and volumetric data for the process Miscibility can be achieved if P>MMP Gas Gas1 Oil1 Gas1 Oil Removed Oil Backward contact Simulate condensing process Injection gas Removed Injection gas Oil Gas1 Oil1 Oil1 11

12 Slimtube experiment Physically simulates gas injection into a 1D reservoir Standard method to determine MMP 1.2 Pore Volume Injection (PVI) at different pressures Recoveries measured Time consuming Slimtube experiment MMP is determined as the pressure corresponding to the break point Recovery % MMP Pressure (atm) 12

13 Rising bubble apparatus Quick but only for vaporizing mechanism Pressure Gauge Windowed Pressure Vessel Flat Glass Tube Gas Bubble Needle G A S P U M P O I L Air Bath MMP calculation method Empirical correlations Limiting tieline method Single cell simulation Slimtube simulation (multicell/cell-to-cell simulation) Global approach by key tieline identification (semianalytical method based on intersecting tie lines) 13

14 Experimental correlations Many suggestions found in the literature Expressed, e.g., as functions of pseudo critical properties of gas, specific gravity of gas Easy to use, fast predictions Accurate for reference system Inaccurate for other systems Limiting tie line method Negative flash to find the P when the injection tie line or the initial tie line become critical C 2 -C 6 Initial tie-line Fast, but without stability analysis only for pure vaporizing /condensing Gas Critical point Oil Injection tie-line C 1 C 7+ 14

15 Single cell simulation Jensen and Michelsen, 1990 Correponding to forward/backward contact (vaporizing/condensing mechanisms) L Onecell simulation Initial tie-line P < MMP L = Number of contacts nc i= x i y i Multicell (slimtube) simulation Multicell (cell-to-cell) simulation physical description Injection gas Batch i Cell 1 Cell 2 Cell n Production Slimtube simulation mathematical description t z n n ( F F ) n+ 1 n Ci, k = Ci, k i, k i, k 1 C i F i n = time step k = grid block Overall molar composition Overall molar flux 15

16 Assumptions in slimtube simulation The porous medium is homogenous and incompressible Instantaneous thermodynamic equilibrium Small pressure gradient compared to total pressure Capillary forces and gravity are neglected The flow is isothermal and linear Mass transfer by diffusion/dispersion is neglected Slimtube (multicell) simulation Directly simulate slimtube experiment Give correct MMP Time consuming Numerical dispersion if grids are too few Simulation time proportional to N 2 grid Extrapolation to infinity N grid needed, for example, determine RF (P) by plotting RF(P) vs. 1/sqrt(N grid ) and extrapolating to zero. 16

17 Slimtube simulation (example) ln (K) Vapor molefraction Grid number Recovery curves from slimtube simulations (numerical dispersion) RF at 1.2 PVI FD (100 grid blocks, 1200 time steps) FD (500 grid blocks, 6000 time steps) FD (5000 grid blocks, time steps) Pressure (atm) 17

18 A MMP calculation method is needed Can correctly account for the injection mechanism Wrong mechanism leads to overestimation Fast Unlike slimtube No numerical artifacts like numerical dispersion Global approach by key tieline identification Fast, semi-analtyical based on intersecting key tielines Based on the analysis of 1D multicomponent two-phase dispersion free flow using the Method Of Characteristics (MOC) Ci t Fi + x = 0 i =1,.., nc 18

19 Main results from the analysis (I) In the composition space, the analytical solution forms a composition path starting from the injection gas composition to the initial oil composition. The composition path must travel through a sequence of key tielines. For a nc component system, there are nc-1 key tielines, including The initial tie line and the injection tie line nc-3 crossover tielines Main results from the analysis (II) At MMP, one of the key tie lines become critical vaporizing and condensing mechanisms are special cases when the initial key tie line and the injection key tie line become critical The composition path can have discontinuities known as shocks. When the path consists ONLY of shocks (the usual case), the key tie lines will intersect pairwise. For other situations (solution consisting of not only shocks but also rarefactions), intersection of key tielines is a good approximation 19

20 Illustration of the concepts CO 2 Semi-analytical 1D Solutions Injection gas Injection tie line Crossover tie line S 0 z 1 Solution path nc-1 key tie lines T,P fixed CH 4 C 10 Initial oil Initial tie line C 4 Details: find intersection key tielines C 4 Tie-line extending through injected Gas True point of intersection Wang and Orr (1997) Critical point Gas Oil CO 2 Jessen et al. (1998) Tie-line extending through initial Oil C 10 20

21 Details: mathematical models (I) Intersection equations j+ 1 x α i j j j+ 1 ( ) + y α x (1 α ) y α = j 1 i 2 j 1 i 2 j i 2 j i = 1, nc 1 j = 1, nc 2 Isofugacity criterion j l j v i = 1, nc x ˆ y ˆ i ϕi i ϕi = 0, j = 1, nc 1 Specification of Initial and Injection composition Oil 1 1 z (1 ) 0 i xi βoil yi βoil = = 1, i nc Inj nc nc zi xi (1 β Inj ) yi βinj = 0 Details: mathematical models (II) Summation of mole fractions nc i= 1 x j i y j i = 0, j = 1, nc 1 Total number of equations N equations = 2( nc 2 1) Newton-Raphson iteration scheme. J + F = 0 21

22 Details: structure of Jacobian matrix (nc=4) X... X... X X... X.. X X... X. X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X... X.... X... X... X X... X... X.... X... X.. X X.... X... X.... X... X. X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X... X.... X... X... X. X X... X.... X... X.. X. X X... X.... X... X. X. X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X... X X X... X..... X X... X.... X Details: search for MMP Displacement of Zick[1] Oil by Gas A Displacement of Zick [1] Oil by Gas B Tie-line Length Tie-line Length Pressure (atm) Tie-line length d = nc ( x i y i ) i= Pressure (atm) equals 0 at MMP 22

23 Details: validation of the algorithm Method / Oil Zick-A Zick-B SVOC SVOD SVOC+D Multicell [2] Slimtube [2]* ± ± ± 10 Slimtube [1]** Louis Bleriot*** Key tie line Time (seconds) Comparison of different results from literature. P (atm) *Eclipse simulation, ** Experimental, *** Multicell [1] Zick, 1986; [2] Høier, 1997 Details: validation of the algorithm 550 Calculated MMP (atm) Multicell Simulator MMP (atm) 23

24 Influence of gas composition on MMP Gas enrichment study when two gases are available The rich gas is treated as solvent Monotonic Non-monotonic yinj = ygas (1 E) + y solvent E Extension: semi-analytical solution to 1D two-phase gas injection Identification of key tielines MOC 1D solution to fullly self-sharpening systems (only of shocks) MOC 1D solution to systems also having rarefactions Streamline method 3D streamline based compositional reservoir simulation 24

25 Example Volume fraction of gas (S) MOC Numerical (100,450) Numerical (1000, 4500) Numerical (10000, 45000) 0.9 sec 4.4 sec 5.4 min 7.8 hr Wave velocity (z/t) A near miscible displacement at 365 atm and K. Besides phase equilibrium... Viscosity instability CO2 viscosity: cp Reservoir fluids: cp Inherently unstable Gravity segregation CO2 desnity: 1/2-3/4 water density, close to oil Reservoir heterogeneity Channeling 25

26 Summary EOR with CO 2 provides double benefits in terms of sequestering CO 2 and improving oil recovery EOR with CO 2 injection is mainly attributed to multicontact miscibility. Three mechanisms for MCM are discussed, only two of them (the vaporizing and the combined) are realistic In experimental study of CO2 injection, swelling test is the easiest one to perform while only the slimtube experiment can correctly determine MMP (also the standard method). Summary Many MMP calculation methods are available, but only two (the slimtube simulation and the intersecting tieline method) can capture the correct mechanism. The first one is time consuming and needs extrapolation, while the second one gives quick and correct solution. A useful extension of the intersecting tie line method is the semianalytical solution to 1D two-phase gas injection, which can be further used in streamlined based reservoir simulation MMP (phase equilibrium) only determines local displacement efficiency, sweep efficiency are related to other aspects (viscosity, gravity, rock heterogeneity) which must be taken into consideration. 26