Modelling the Chemistry of In-Situ Combustion

Transcription

1 Modelling the Chemistry of In-Situ Combustion Norman Freitag Saskatchewan Research Council 32 nd Annual Symposium and Workshop IEA Collective Project on Enhanced Oil Recovery Vienna, Austria October 16-19, 2011

2 Summary of In-Situ Combustion in Canada At least 18 field projects or pilots were started 14 by 1990 Most oil viscosities ,000 cp None still operating in 2011 Only one (Battrum) completely produced the field Some were halted during periods of low oil prices, but many more were unnecessary failures Main reasons for failures: Loss of combustion front (or failure to ignite one) Difficult to produce mobilized oil (Moore, R.G. et al., AOSTRA/PETROM R.A. ICPT Cimpina 1993 Joint Canada/Romania Heavy Oil Symp., Mar. 7-13, Sinaia, Romania)

3 Requirements for Success 1) Maintain a stable combustion front! Deep (warm) light-oil reservoirs ignite spontaneously (most of the time) In heavy oils, combustion front can go out 2) Predict operating conditions (injection rate, well locations, etc.) at which the front will be stable No proven approach for untested field conditions Will require numerical simulation Copyright SRC 2011

4 Generic Combustion Front Air Compressor Water Combustion Pyrolysis/Coking Steam Zone Burnt formation Undisturbed

5 Chemical Reactions Three major types of reactions: 1) Pyrolysis/coking Ultimate products: volatile hydrocarbons & coke 2) Combustion Generally requires temperature > 370 C In heavy oils, fuel is usually pyrolysis coke 3) Low-temperature oxidation (LTO) Produces acids (corrosion), surfactants (stable emulsions) and increases oil viscosity Final products are H 2 O, CO 2, CO and, with heavy oils, an oxygen-rich solid residue

6 Conventional Choices of Reactions for Simulation Often based on Belgrave et al. (Belgrave, J.D.M. et al., SPE/DOE Seventh Symp. On Enhanced Oil Recovery, Tulsa, Oklahoma, April 22-25, 1990; SPE/DOE 20250) Oil divided into maltenes and asphaltenes Pyrolysis: Maltenes Asphaltenes Asphaltenes Coke Gas Low-Temperature Oxidation: Maltenes + O 2 Asphaltenes + O 2 Coke Combustion: Coke + O 2 CO 2 + H 2 O

7 Conventional Reaction Kinetics and Stoichiometry Kinetics: Usually first-order in reactant concentrations: Rate i = k i (C reactant 1 ) m X (C reactant n ) m Arrhenius parameters for rate constant (k i ): k i = A exp(-e a /RT) Stoichiometry: Conserve total mass, but not individual elements Example: Asphaltenes formed by low-temperature oxidation of maltenes have higher oxygen content, but are treated as asphaltenes in crude oil

8 SARA-Based Reaction Model Retain asphaltenes as a pseudocomponent Divide maltenes into fractions with increasing polarity: Saturates Aromatics Resins Can describe pyrolysis and LTO better

9 Pyrolysis Reactions (Freitag and Exelby, J. Can. Pet. Tech. 2006, 45 (3) 38-44) Steam-distillable hydrocarbons Saturates Aromatics Resins Asphaltenes Coke One reaction per fraction E a = 190 to 230 kj/mol Rate ~ X faster for asphaltenes than for saturates in Canadian heavy oils (Arrow width roughly proportional to yield for Canadian oils)

10 Combustion Reactions Exactly what is burning? Two scenarios: 1) Oil pyrolyzes before it burns fuel is coke Occurs with most heavy oils Single Arrhenius equation gives satisfactory kinetics - E a ~ 120 kj/mol for Canadian heavy oils (Ren, Y. et al., J. Can. Pet. Tech. 46 (4), 47-53) 2) Oil fractions burn before they are pyrolyzed In light oils (?) Maximum temperatures always lower - Is this just rapid LTO?

11 LTO in Oils & SARA Fractions Differential Scanning Calorimetry (DSC) shows: 1) Pure saturates oxidize much faster than other fractions 2) Saturates LTO is repressed by other fractions

12 Chemical Literature on LTO Most literature deals with LTO of n-alkanes in gas phase Chemically, saturates are nearest to n-alkanes Therefore, the reaction most studied in literature is the one that is repressed in crude oils Different phase (liquid vs. gas) too Conclusion: Most literature on LTO kinetics is for a different reaction than seen in crude oil Literature on kinetics may be mostly irrelevant Mechanisms may still apply (?)

13 Oxygen Consumption Rates of Aromatics Air injected into isothermal tubular reactor outlet gas concentrations indicate rate of oxidation of aromatics on sand Short period of slower reaction which delays CO x production

14 Oxygen Consumption Rates: Isolated Saturates Versus Mixture Pure saturates show strong induction period Caused by naturally occurring oxygen inhibitors Note how 50/50 mixture resembles pure aromatics

15 Effect of Oxygen Concentration on LTO Rates of Non-Saturates Below 160 C, half-order in O 2 concentration Approaches first-order reaction by 230 C Conclusions: - Change in control mechanism! - Minimum two LTO reactions for simulation Dependency on O 2 partial press. to at least 1200 kpa Different from literature (?)

16 Effect of Negative Temperature Coefficient (NTC) Region Temperature range over which oxidation rate decreases with rising temperature Range varies with pressure and chemical structure Occurs when alkyl (saturated) groups exist Does not occur with benzene or methane NTC region of C suggested for heavy oils (Ursenbach et al., Paper , 8 th Can. Int. Pet. Conf., June 12-14, 2007) NTC of pure alkanes has been simulated Used dozens to hundreds of compounds and reactions Simulation of NTC in heavy oils still untried

17 Current Status Many reactions in light oils still uncertain Reaction chemistry for heavy oils better defined Can model pyrolysis Can model fuel (coke) combustion Low-temperature oxidation model incomplete Copyright SRC 2011

18 Required Improvements for Comprehensive Reaction Model 1) Inclusion of oxidation inhibitors for LTO Repression of saturates LTO by other fractions Feasibility has been demonstrated (Freitag, N., J. Can. Pet. Tech. 2010, 49 (7), 36-41) What do inhibitors form as they are consumed? - Inhibitors have upper temperature limits how to model? - Another pseudocomponent needed 2) Mechanism for change in order of reaction for oxygen with temperature (LTO) Requires addition of free-radical intermediate(s) Copyright SRC 2011

19 Required Improvements (Cont d.) 3) Pyrolysis reactions of LTO residue Does the coke from this pyrolysis burn like other coke? 4) Inclusion of gas-phase combustion reactions Chemical literature may apply 5) A fourth type of reaction to govern NTC Probably requires even more intermediate(s) as pseudofractions Copyright SRC 2011

20 Acknowledgements Funding through the PTRC (Petroleum Technology Research Centre) is gratefully acknowledged

21 Chemical Literature on LTO Free radical mechanism RH is a hydrocarbon, R is a free radical) Begins with hydroperoxide formation R + O 2 RO 2 RO 2 + RH ROOH + R Branching reaction accelerates reaction rate Half-order in ROOH conc. ROOH RO + OH RO + RH ROH + R OH + RH H 2 O + R