Soak Alternating Gas: a New Approach to Carbon Dioxide
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SOAK ALTERNATING GAS: A NEW APPROACH TO CARBON DIOXIDE FLOODING by MALCOLM DAVID MURRAY, B.S.P.E. A THESIS IN PETROLEUM ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN PETROLEUM ENGINEERING Approved Chairperson of the Committee Accepted Dean of the Graduate School August, 2002 ACKNOWLEDGEMENTS "Wisdom is the principle thing; therefore get wisdom and with all your getting get understanding." - Proverbs 4:7 I would like to express my greatest appreciation to those without whom this thesis would not have been possible: first, to Dr. Scott Frailey, my thesis advisor whose expectations of his students have continually challenged me to do my best in both my studies and in this thesis. Although a positive attitude and strong determination are necessary for a student to achieve his greatest possible success, the high standards of his teachers are indispensable to the quality and level of his education: I therefore count it a great privilege to have worked under Dr. Frailey while at Texas Tech University. I am also deeply indebted to Dr. Akanni Lawal who offered me the research assistantship that made studying at Texas Tech University possible: the opportunity to study here and the quality of the friendships that have been formed have made this a rich experience indeed. I would like to thank the Butler family for the funding to make the research for this project possible. I would also like to express my heartfelt thanks to Michelle Doss who helped with the final transcripts of this thesis, as well as to the professors and staff of the Petroleum Engineering department who helped in so many ways. In addition, I would like to thank Dr. Lillian Chou and the people of the International Family Fellowship who became such wonderful friends during my stay in Lubbock, and last but not least, my mother Zennie Fryer who, with my sisters, Carol Hawes and lla Olsen and their families, gave their words and prayers full of love, support and encouragement. Thank you all for what you have given me during this part of my life. TABLE OF CONTENTS ACKOWLEDGEMENTS ii ABSTRACT vii LIST OF FIGURES ix NOMENCLATURE xii CHAPTER 1. INTRODUCTION 1 1.1. An Overview of Petroleum Recovery 1 1.2. The Role of Miscibility in Petroleum Recovery 2 1.3. CO2 Recovery Methods 3 1.4. Soak Alternating Gas: An Alternative Method of Mobility Control 6 2. LITERATURE SURVEY 8 2.1. Introduction 8 2.2. Immiscible Displacement 8 2.3. Miscible Displacement 15 2.3.1. Phase Behavior 16 2.3.2. Diffusion and Dispersion 19 2.3.3. Viscous/Capillary Forces 22 2.3.4. Dead-End Pores 24 2.3.5. Miscibility Development with High Water Saturation 25 IV 2.3.6. Areal Displacement Efficiency 27 2.3.7. Water Injectivity 28 2.4. Displacement Efficiency 29 2.4.1. Microscopic Displacement Efficiency 30 2.4.2 Macroscopic Displacement Efficiency 32 2.5 Field Applications 37 2.5.1. Continuous CO2 Flooding 38 2.5.2. Water-Alternating-Gas (WAG) 42 2.5.2.1. WAG Theoretical and Laboratory Models 42 2.5.2.2. WAG Field Implementations 44 2.5.3. Cyclic CO2 Stimulation (HufTn'Puff) 46 2.5.3.1. CO2 HufTn'Puff in Heavy Oil Reservoirs 46 2.5.3.2. CO2 HufTn'Puff in Conventional Oil Reservoirs 49 2.5.3.2.1. Numerical Simulations 49 2.5.3.2.2. Analyses of Laboratory and Field Data 49 2.5.3.2.3. Full-Scale Field Implementation 52 2.5.4. Summary 54 3. DISCUSSION 80 3.1. Introduction 80 3.2. Miscibility 81 3.3. Phase Behavior 81 3.3.1. The Vaporizing Gas Displacement Process 82 3.3.2. The Condensing Gas Displacement Process 82 3.3.3. The Vaporizing/Condensing Gas Displacement Process 83 3.4. CO2 Recovery Mechanisms 84 3.5. CO2 Recovery Processes 85 3.5.1. Continuous CO2 Flooding 85 3.5.2. CO2 Water Alternating Gas 86 3.5.3. Cyclic CO2 Stimulation (HufTn'Puff) 87 3.6. SAG: A New Alternative 88 3.6.1. SAG Parameters 90 3.6.2. Miscibility Conditions for SAG 93 4. CONCLUSIONS 97 5. RECOMMENDATIONS 99 BIBLIOGRPAHY 103 VI ABSTRACT Carbon dioxide (CO2) flooding as a method of enhanced oil recovery (EOR) has been used successfully for many years to increase the recovery of the original oil in place (OOIP) of a reservoir. The two most common methods of using CO2 to accomplish this are the continuous CO2 flood and the cyclic CO2 flood, or CO2 hufTn'puff. In the continuous CO2 flood, CO2 is injected continuously into a wellbore while fluids are recovered continuously at the adjacent wellbores. A frequently used variation of this process is the water- alternating-gas (WAG) flood where CO2 and water are injected alternately to combine the solvent properties of CO2 with the mobility properties of water and further optimize the recovery. However, one limitation to the WAG process is that of water shielding where the relatively high water saturation in the pore spaces prevents much of the potential contact between the crude oil and the CO2 from occurring and causes a significant portion of the oil to be bypassed. In the cyclic CO2 flood, CO2 is injected into the reservoir, shut in for a soak period, then produced from the same wellbore until the economic limits of production are reached and the cycle must be repeated. During the injection phase of a CO2 hufTn'puff, CO2 is distributed over as great an area as possible throughout the reservoir. Next, during the soak period CO2 is allowed to disperse through the water to contact as much oil as possible and make full use of the CO2 recovery mechanisms, which allows production to be optimized in the production phase of the process. vii In this work, a new concept in CO2 flooding is introduced as "soak- alternating-gas," or SAG, which incorporates the soak period of a CO2 hufTn'puff into the continuous CO2 flood to provide additional mobility control and a viable alternative to a WAG process in cases where water injectivity is too low to allow WAG to be feasible. Since SAG does not depend on water injectivity the prospect of greater recovery in such cases could be quite significant. In addition, the mobility control provided by SAG may offer advantages over those of WAG, even where water injectivity is adequate. Thus, the integration of continuous CO2 flooding techniques with those of the CO2 hufTn'puff appears to offer greater recovery potential than those of either method used separately. The concepts behind SAG appear to be supported by previous literature on the research, testing, and implementation of the continuous CO2, the CO2 hufTn'puff, and the WAG processes. In order to ascertain its potential, it is recommended that the SAG process be investigated with respect to miscibility conditions as well as the parameters of the injection stage, soak period and production stage, then verified experimentally using slim tube experiments, coreflood experiments and pilot tests prior to full-scale field implementation. VIII LIST OF FIGURES 2.1 Relative permeability characteristics of porous media in (a) strongly water-wet rock, and (b) strongly oil-wet rock 57 2.2 Theoretical fractional water flow of water versus water saturation 57 2.3 Actual fractional water flow versus water saturation 58 2.4 Water saturation versus dimensionless distance at a dimensionless time (i.e., pore volumes injected) of 0.2 58 2.5 Dimensionless time versus Dimensionless at various degree of water saturation 59 2.6 Saturation history at XD=1 (Outlet of System) 59 2.7 Interfacial forces at an interface between two immiscible fluids and a solid 60 2.8 Correlation of recoveries of residual phases as a function of Nca 60 2.9 Experimental and simulated densities from multi-contact miscibility 61 2.10 Holm-Josendal correlation of CO2 MMP as a function of temperature 61 2.11 Correlation of CO2 MMP as a function of extractable C5-C30 hydrocarbons present 62 2.12 Correlation of CO2 MMP as a function of the molecular weight of the crude oil 62 2.13 Schematic representation of viscous finger growth in unstable linear miscible displacements 63 2.14 Idealized impact of transverse dispersion on ultimate crude oil recovery by CO flooding 63 2.15 Image obtained from a microvisualization experiment 64 2.16 Miscible displacement of Fluid A by Fluid B, step change in concentration at inlet 65 IX 2.17 Concentration profiles for the injection of a slug of Fluid B to displace Fluid A 65 2.18 Dispersion resulting from laminar flow in a straight capillary 66 2.19 Dispersion based on porous media being viewed as a series of mixing tanks 66 2.20 Dispersion resulting from bypassing of fluid trapped in stagnant pockets 67 2.21 Dispersion caused by variation in the flow paths in porous media 67 2.22 Flow regimes for miscible displacement in a vertical cross-section 68 2.23 Flow regimes in a two-dimensional, uniform linear system 69 2.24 Effect of mobile water on oil recovery for CO2 MCM displacements of reservoir crude oil in a water-wet core 69 2.25 Effect of mobile water on oil recovery for CO2 MCM displacements of reservoir crude oil in an oil-wet core 70 2.26 Solvent cut as a function of pore volumes injected with K as a parameter 70 2.27 Claridge correlation for areal sweep efficiency as a function of the mobility ratio 71 2.28 Schematic of displacement process with multiple phases 72 2.29 Gravity segregation in displacement processes 72 2.30 Standard flooding patterns 73 2.31 Fractional flow of CO2 and crude oil of well 2-26, Joffre Viking field 74 2.32 Plot of CO2PIUS water injection versus recovery 74 2.33 Calculation of fractional water flow in a tertiary WAG process 75 2.34 Production rates of oil, water, and CO2 before and after CO2 injection in a WAG process (Four-Pattern Area) 75 2.35 Production rates before and after CO2 injection in a WAG process (Seventeen-Pattern Area) 76 2.36 Utilization of CO2 with incremental oil production and CO2 production in a WAG process (Seventeen-Pattern Area) 76 2.37 Comparison between cumulative recoveries on a continuous CO2 flood, 1:1 WAG, and Denver Unit WAG processes 11 2.38 Yearly cumulative production versus soak times, West Sak Field.