Improving Energy Efficiency in Thermal Oil Recovery Surface FacilitiesN.M. NADELLA SNC Lavalin Inc. SummaryThermal oil recovery methods such as Cyclic Steam Stimulation (CSS), Steam Assisted Gravity Drainage (SAGD) and In-situ Combustion are being used for recovering heavy oil and bitumen. These processes expend energy to recover oil. The process design of the surface facilities requires optimization to improve the efficiency of oil recovery by minimizing the energy consumption per barrel of oil produced. Optimization involves minimizing external energy use by heat integration. This paper discusses the unit processes and design methodology considering thermodynamic energy requirements and heat integration methods to improve energy efficiency in the surface facilities. A design case study is presented. Introduction As primary oil production declines, enhanced oil recovery (EOR) methods will be increasingly deployed. For the recovery of heavy oil and bitumen, thermal recovery methods have become standard methods of recovery. For bitumen resources in Alberta, Canada, thermal recovery and mining are the main recovery methods. Thermal oil recovery methods involve use of heat to improve the oil recovery from petroleum reservoirs. These methods are, Hot water flood Steam methods like CSS, SAGD, steam flood In-situ Combustion There are several variations of the above methods1 like co-injection of solvents, gases and air as shown in Figure 1. As shown in Figure 2, 98.1% of the thermal EOR production is currently based on Steam, while 1.7% is based on in-situ combustion and 0.2% based on hot water flooding2. Surface facilities for the steam based thermal production requires steam generation plants, water treatment for boiler feed water generation, produced water recycle and wastewater treatment units in addition to well pads, gathering systems, pipelines, oil treatment, gas treatment units and other utilities and offsite units. Surface facilities for in-situ combustion methods require air compression units, steam generation on a smaller scale, produced gas treatment, oil treatment, water treatment and other utilities and offsite units. This paper discusses surface facilities for steam based oil recovery and in-situ combustion processes. The surface facilities may also include cogeneration units for electric power, sour gas treatment, sulfur recovery, carbon capture and sequestration units as part of the overall project. Therefore, the process design of surface facilities involves process integration and energy optimization to minimize overall costs of steam and/or power generation, maximize heat recovery recognizing trade-offs between capital and operating costs, and minimizing the overall waste heat loss and utility cooling or heating. Description of Surface Facilities The process units in the surface facilities for steam based thermal oil recovery and in-situ combustion are described below and compared

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in Table 1. In addition, wastewater treatment, campsites and other infrastructure facilities will be required depending on the project location. Steam Based Thermal Facilities Steam based thermal processes like CSS, SAGD or steam flood have very similar surface facilities. Main process units in such surface facilities are shown in Figure 3. The surface facilities consist of the following main process units, Well Pad facilities Pump Stations Central Plant and Pipelines Well pad facilities include well controls, steam distribution and control, production control, well testing and gathering systems. The produced fluids are sent directly to the Central Plant if the well pads are located close by. If the well pads are located far from the Central Plant, intermediate pumping stations may be required. Alternatively, Central Plant may be combined with well pad if there is only one well pad in the facility. Currently, the CSS and SAGD surface facilities are being designed for capacities of 5,000 barrels/day to 100,000 barrels/day of oil production. The Central Plant consists of oil processing, produced water de-oiling, water treatment, steam generation, product storage and pumping, utilities and off-sites. There are several process options for each unit of the surface facilities. These options are listed in Table 1, column 2. In-situ Combustion Surface Facilities The main surface process units for In-situ combustion are shown in Figure 4. The surface facilities for in-situ combustion also consist of the following main process units, Well Pad facilities Pump Stations Central Plant and Pipelines Well pad facilities include well controls, air and steam distribution and control, production control, gas separation, sour gas handling, free2

water knockout, de-sanding and emulsion pumping. The produced fluids are sent directly to the Central Plant if the well pads are located close by. If the well pads are located far from the Central Plant, intermediate pumping stations may be required. Alternatively, Central Plant may be combined with well pad if there is only one well pad in the facility. Currently the design capacity of the in-situ combustion projects is less than 10,000 barrels/day. The process units are oil processing, produced water de-oiling, water treatment, steam and power co-generation, product storage and pumping, air compression, sour gas treatment, sulfur recovery, utilities and off-sites. There are several process options for each unit of the surface facilities. These options are listed in Table 1 column 3. Surface Facility Process Selection The process option for each of the surface units is selected based on the overall economics for the project and are linked to factors like production capacity, well-head operating conditions, requirements and availability of diluent, sales oil quality etc. The process selection will be done during the conceptual phase of the project. There will be more than one process option that may be suitable for the given design conditions. In such cases, comparison of the capital and operating costs for different processes will enable selection of the economic design for the surface facilities. This design will be refined through detailed engineering phases. Energy Consumption Energy is consumed in the thermal oil recovery surface facilities to generate steam or compress air to support the oil recovery from the reservoir. Steam generation consumes major amount of energy in the steam based processes while air compression requires the most energy for in-situ combustion processes at the surface facilities. In this paper, energy consumed in the form of fuel for steam generation, electricity for moving

fluids and treatment processes will be considered for review and optimization. Subsurface heat generation and energy consumption for in-situ combustion in the reservoir is not in the scope of this paper. The selection of the enhanced oil recovery process and screening parameters for a given oil reservoir are described Green3 et al. The fuel gas consumed in the thermal EOR surface facilities is mainly to generate steam. The amount of steam used per barrel of oil production determines the overall energy efficiency. In steam based processes, the commonly used parameters reflecting energy consumption are the steam to oil ratio (SOR) and oil to steam ratio (OSR). Steam can be injected continuously as in steam flood, or SAGD or intermittently as in CSS process. Also, the amount of steam injected varies during the life of the project. Hence, cumulative steam to oil ratio (CSOR) over the period of steam injection is more reflective of the energy consumption of the recovery process. This parameter is dependent on reservoir characteristics, development strategy and is always optimized based on impact on oil production. The steam to oil ratios for various reservoir locations6 are given as, Location Steam Floods, California CSS, California CSS, Alberta CSS, Venezuela SAGD, Alberta OSR ~ 0.25 0.5 - 1.0 0.3 0.5 ~ 3.0 0.3 0.5 SOR ~ 4.0 1.0 2.0 2.0 3.3 ~ 0.33 2.0 3.3

EOR Method Hot water flood Steam Drive Dry Combustion Wet Combustion Steam soak CSS SAGD

Typical Design Parameter 9 m3 water/m3oil 1.66 6.29 ton steam/m3oil 3000 sm3air/m3oil 170 1000 sm3 air/sm3oil 0.16 2.0 ton steam/m3 0.3 3.3 ton steam/m3 oil 2.0 3.3 ton steam/m3 oil

Surface facilities are designed to provide the required steam or air for the thermal oil recovery processes. Given the design air or steam flow rates, the goal is to minimize energy losses and minimize the fuel gas or other utilities required in the surface facilities. Energy Optimization Energy optimization is an important part of surface facilities process design. Some of the general strategies to optimize the energy consumption are, Evaluate and quantify the thermodynamic limitations of the treatment processes. Actual energy consumption has to be higher than the thermodynamic minimum. Select processes with lower thermodynamic minimum energy requirements. Select the surface process unit operating conditions that match with the reservoir operating conditions. Thus heat exchange will be minimized. Any heat exchange will have efficiency limitation due to entropy changes. Minimize transportation of hot fluids for treatment to avoid insulation losses Evaluate if direct contact heat exchange is possible as this will be more efficient than indirect heat exchange. If cogeneration is required, maximize fuel efficiency through heat recovery steam generation. Avoid excess generation of low level heat. Due to seasonal variations of3

The impact of SOR on energy consumed per barrel of oil produced and the amount of heat in the produced fluids6 is given in Figure 5. In the in-situ combustion, the amount of air injected per barrel of oil produced determines the overall energy efficiency. A cumulative air to oil ratio determines the overall project economics. This quantity is also dependent on reservoir characteristics. Typical design parameters for each of the thermal oil recovery processes have been summarized from literature5 as,

ambient temperatures, low level heat from the process cooling will have to be removed expending energy in air or water cooling. Maximize heat integration between hot and cold process streams to minimize external heating or cooling. Select equipment like boilers, steam turbines, heaters and pumps with higher efficiencies. If low level heat generation could not be avoided, consider waste heat energy recovery units. Some energy transfer processes specific to thermal oil recovery processes and their impacts are listed below, Energy transfer process Heat recovery from produced liquids Heat recovery from produced gas Heat recovery from boiler blow down Waste heat available for winterization Flue gas heat recovery Steam generation Air compression Cogeneration of power Impact on Steam based Oil Recovery High Impact on insitu Combustion Low

When there is more than one suitable process for a separation unit, energy consumption will be important for process selection as this impacts the operating costs for the unit. Minimum Energy Thermodynamics provides minimum energy requirements and maximum thermodynamic efficiency for a separation process, The minimum thermodynamic work required for separating a homogeneous mixture in to pure products at constant temperature is given by7,8 the increase of Gibbs free energy of the products over the feed. This can be expressed as,

Wmin = H TS = F .......... ......(1)Where, H represents the change in enthalpy between final and initial stages, S represents the change in entropy, and F is the change of the free energy. The free energy can be expressed in terms of molal concentration of the salt in water as,

Low High High High High Low low

High Low High High High High high

Wmin = Fdn = RT ln a w dn = RT lnn1 n2

p dn.......... .......... .......( 2 ) p0

Where n represents the number of water moles in the solution, R is the gas constant, aw is the water activity in the solution, P is the water vapor pressure assumed as an ideal gas. The minimum work or energy can also be expressed in terms of chemical potentials as,

Wmin = F = c + p f .......... ..( 3)

Energy and Separation Processes Energy is required for different separation processes used in surface facilities. The selection of these processes depends on their suitability for treating the produced fluids i.e., meeting sales oil specification, and water recycled as boiler feed water and waste water to disposal wells.4

Where, the subscripts c, p, and f are concentrate, product and feed, respectively. Expressing chemical potential to activity coefficients will result in an equation of the form,

Wmin = RT x Fi ln Fi x Fi .......... ....( 4)i =1

n

The activity coefficients for salt mixtures have been published as relations of osmotic constants8 and molality or as empirical relations with temperatures for seawater desalination.

This minimum work estimation allows one to evaluate various separation processes and also signifies the difficulty of separation. Practical Energy Consumption In practice, the actual energy consumption will be much higher due to, Fluid flow frictional pressure drops Heat transfer due to fluids at different temperatures Non ideal mixing of fluids and mass transfer Non ideal chemical reactions taking place in the process Practical energy consumptions for the separation processes used in thermal oil recovery surface facilities are given below, Separation Recovery) Process (% Energy Consumption, kWh/1000Sm3 53 819 21 26 264 1,057 26 40 ~ 431 ~ 18,494 1,057 4,227 3,963 1,849 2,642

Steam to oil ratio is 3.0 Bitumen is produced using gas lift Well head production temperature is 179C Warm lime softening and once through steam generators are used Boiler blow down will be recycled and make-up water rate is limited to 10% of boiler feed water rate. Low level heat generation and heat rejection to utilities will be minimized Ambient temperatures vary between 45C to 35C. Heat losses through insulation will be neglected.

Electrostatic oil-water separation (> 99) Gas Floatation (>90) Media Filtration (>99) Warm Lime Softening (>90) Ion Exchange for hardness removal (>99) Mechanical vapor compression9 for evaporation (97) Reverse Osmosis10 (35-55) Multistage flash10 (10-20) Multiple-effect distillation10 (>60) Case Study

In order to illustrate energy optimization methods described above, a case study for the design of a 30,000 barrels/day SAGD facility in Alberta is presented. The design parameters and assumptions for the case study and optimization results are as follows, 30,000 BBL/day SAGD Facility The steps in energy optimization of the surface facilities are given in Figure 6. The design parameters for this case are,5

The optimized flow sheet with main process parameters are shown in figure 7. Pinch analysis results are shown in figures 8-10. The results indicate, The only external heat required is for steam generation. The heat from produced fluids is recovered to boiler feed water, makeup water and remaining heat is recovered to ethylene glycol. Hot ethylene glycol is used for building heating, heat tracing and process heat requirements. Residual heat is then used to preheat combustion air to the steam generators. Any remaining heat will be dissipated through air coolers. Some waste heat will be rejected during summer when utility heat requirements are reduced. Thermal efficiency of the surface facilities is governed by the efficiency of steam generators, while the efficiency of the SAGD process is governed by the steam to oil ratio used. The fuel gas energy input is estimated at about 0.9 to 1.3 GJ/BBL of bitumen produced.

Conclusion Thermal EOR processes and surface facilities require high energy input to produce, treat and transport the heavy oil from the reservoir. In order to minimize the energy expended per barrel of oil produced, process integration and selection of suitable processes for surface facilities is required. Heat integration and Pinch analysis allows quantification of the minimum energy requirements and optimization of the heat exchange networks. Separation processes can be screened based on energy consumption in addition to meeting the process requirements. Acknowledgement The author wishes to acknowledge the support from SNC Lavalin management in the preparation and presentation of this paper. ABBREVIATIONS EOR: Enhanced oil recovery OSR: Oil to steam ratio CSOR: Cumulative steam to oil ratio CSS: Cyclic steam stimulation SAGD: Steam assisted gravity drainage SOR: Steam to oil ratio NOMENCLATURE H S F a P R T W x = = = = = = = = = = = enthalpy difference entropy difference change in free energy activity vapor pressure gas constant, energy/moltemperature temperature, K or C work, energy/mol mol fraction of component activity coefficient chemical potential

Subscripts c = F, f = i = min = max = n = p = w =

concentrate feed component minimum maximum number of components in feed product water

REFERENCES1. S. Thomas, Enhanced Oil Recovery An Overview, Oil & Gas Science and Technology Rev. IFP, Vol. 63(2008), No. 1, pp 9-19. Leena Kottungal, 2010 Worldwide EOR Survey, Oil &Gas Journal, April 19, 2010; 108, 14,pp 4153 . Don W. Green, G. Paul Willhite, Enhanced Oil Recovery, SPE Textbook Series Vol. 6, Richardson, Texas, 1998, Chapter 8, Table 8.1, p 302. S.M. Farouq Ali, Heavy Oil Ever Mobile, Journal of Petroleum Science and Engineering 37 (2003) 5-9. Daniel N. Dietz, Paper SPE-5558, Review of Thermal Recovery Methods, 1975. N.M. Nadella, Heat Integration and Energy Optimization in SAGD Surface Facilities, Paper 2008-317, Proceedings of the World Heavy Oil Congress, Edmonton, Alberta, Canada, March 2008. Jimmy L. Humphrey, George E. Keller II, Separation Process Technology, 1st Edition 1997, pp 296-297, McGraw-Hill, New York. Raphael Semiat, Energy Issues in Desalination Processes, Environmental Science & Technology, Vol. 42, No. 22, 2008, pp 8193-8201. Heins, W.F., Start-up, Commissioning, and Operational Data from the Worlds First SAGD Facilities using Evaporators to Treat Produced Water for Boiler Feed Water, Paper 2006-183, Canadian International Petroleum Conference, June 13-15, 2006. Srinivas (Vasu) Veerapaneni, Bruce Long, Scott Freeman, Rick Bond, Reducing Energy Consumption for Seawater Desalination, AWWA Journal, June 2007, 99, 6; pp 95-106.

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Table 1. Process Options for Thermal EOR Surface FacilitiesProcess Unit Wells Process Options (Steam based EOR) Gas Lift Electric Submersible pumps (ESP) Pump jacks Well-Test Skid Group separator Emulsion pumping Separate gas and emulsion pipelines Multiphase pumps Options for heat recovery or heat integration with Central Plant Produced Gas processing Produced water de-oiling Blend treatment using a diluent, free water knockout drum and electrostatic oil treaters. High temperature and low-pressure separators. Supply as fuel gas Excess gas compressed and used as lift gas or dehydrated and sent to offsite utility Sulfur removal unit for sour gases several technologies Skim tanks, induced gas floatation and oil removal filters with crushed walnut shell media. Ceramic membranes Silica and hardness removal using hot lime softeners or warm lime softeners followed by ion exchange Mechanical Vapor compression for evaporation Once through steam generators (OTSG) Drum type boilers Combined steam and power generation Process Options (In-situ Combustion) Natural Lift Steam Lift Emissions control Wastewater treatment Low NOx burners Flue gas desulfurization CO2 capture and sequestration Scale inhibition and disposal to injection wells Membranes for waste reduction and water recycle. Evaporation and crystallization for zero liquid discharge Gas separator Free water knockout Desanding tank and system Vapor recovery on the tanks Emulsion pumping Separate gas and emulsion pipelines Options for heat recovery or heat integration with Central Plant Blend treatment using a diluent, free water knockout drum and electrostatic oil treaters. High temperature and low-pressure separators. Heat recovery from hot produced gas Water vapor condensation H2S and CO2 removal Sulfur Removal Unit with sour gas flaring/incineration Skim tanks, induced gas floatation and oil removal filters with crushed walnut shell media. Ceramic membranes Silica and hardness removal using hot lime softeners or warm lime softeners followed by ion exchange Mechanical Vapor compression for evaporation Ion exchange for TDS removal from condensed water Once through steam generators (OTSG) for injection steam Drum type boilers for superheated steam generation Combined steam and power generation Low NOx burners Flue gas desulfurization CO2 capture and sequestration Scale inhibition and disposal to injection wells Membranes for waste reduction and water recycle. Evaporation and crystallization for zero liquid discharge

Well Pads & Pump Stations

Oil Processing

Water treatment

Steam generation

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EOR METHODS

THERMAL

NON-THERMAL

HOT WATER

STEAM

IN-SITU COMBUSTION

ELECTRICAL

STEAM FLOOD

THAI

CSS: Cyclic Steam Stimulation LASER: Liquid addition to Steam for Enhanced Recovery SAGD: Steam assisted Gravity Drainage Vapex: Vapor Extraction Process SAGP: Steam Assisted Gas Push THAI: Toe to Heel Air Injection

CSS

LASER

SAGD

VAPEX VAPEX+STEAM SAGP

Figure 1. Thermal EOR Methods

Total EOR Total Thermal Production BPD 1,624,044 % of Total 100 1,016,972 63

Steam 997,453 61

In-Situ Combustion 17,203 1.06

Hot Water 2,316 0.14

Figure 2. Production from Thermal Oil Recovery

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LIFT GAS STEAM TO WELL PADS

WELL PAD FACILITIES

EMULSION OIL TREATMENT

DILBIT STORAGE

DILBIT TO PIPELINE STEAM GENERATION BFW

PRODUCED GAS TREATMENT (SRU)

DILUENT

OIL OIL-WATER SEPARATION

WATER TREATMENT

BLOW DOWN

BRACKISH WATER MAKE-UP PRODUCED GAS FG SYSTEM NATURAL GAS WASTE TO INJ. WELL

FIGURE 3. SURFACE FACILITIES FOR STEAM BASED THERMAL EOR (SF,CSS,SAGD)

AIR

STEAM

WELL PAD FACILITIES PROD. GAS

EMULSION OIL TREATMENT

DILBIT STORAGE

DILBIT TO PIPELINE STEAM GENERATION + CO-GEN BFW AIR COMPR.

DILUENT PRODUCED GAS TREATMENT (SRU) WATER MAKE-UP

OIL OIL-WATER SEPARATION

WATER TREATMENT

BLOW DOWN

WASTE TO INJ. WELL NATURAL GAS

FIGURE 4. SURFACE FACILITIES FOR IN-SITU COMBUSTION

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SOR vs Heat Content of Produced Fluids2,300,000 2,100,000 1,900,000 1,700,000 1,500,000 1,300,000 1,100,000 900,000 700,000 500,000 300,000 1.5 2.0 2.5 3.0 SOR 3.5 4.0 4.5 5.0 % of Input Heat 5.5 37.40% 36.85% 36.30% 35.75% 35.20% 34.65% 34.10% 33.55% 33.00% 32.45% 31.90% Fraction of Input Heat

Heat Content, kJ/BBL Bitumen Produced

Heat Content of Produced Fluids, KJ

Total heat input to Reservoir, kJ

Figure 5. Heat Content of Produced Fluids

UTILITY COSTS

UTILITY SYSTEMS DESIGN

HEAT AND MATERIAL BALANCES

PINCH ANALYSIS AND HEAT INTEGRATION

WASTE HEAT AND LOW LEVEL HEAT RECOVERY OPTIONS

HEX NETWORK OPTIONS

PROCESS CONFIGURATION

FIGURE 6. FLOW CHART FOR ENERGY OPTIMIZATION

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POWER (1 MW) Lift Gas Produced Gas Treatment 110C Produced gas 23 GJ/hr Gas Lift (RESERVOIR) Emulsion 179 C Sulphur 131C Diluent

5oC

POWER (1.2 MW)

120C

OIL TREATMENT FWKO + TREATER Produced Water

Dilbit 120oC

38 GJ/hr Dilbit 45oC

Pipeline/Storage

120oC

90 C

1653 GJ/hr

BFW 90C 7 MPag, 286 C 213 GJ/hr 100 GJ/hr Flue Gas 144 GJ/hr 80oC 98C

64 GJ/hr 5oC

Makeup Water

Steam

160 C

POW ER (7.5 MW)

STEAM GENERATION (OTSG)

DEOILING & WATER TREATMENT (WLS)

80oC Waste Water 70oC

Disposal Well

BFW 180 C

Recycle 50 GJ/hr Blow down 1444 GJ/hr 286oC 165C 80 GJ/hr 70C High TDS Water 70oC Disposal Well

75C

145 C 90 C Glycol S/U Heater Natural Gas LP Steam Sep 6 GJ/hr 70C

HEAT TRACING, BLDG. HEAT, PROCESS HEAT

Glycol Pumps

70 C

93 GJ/hr 30 C -40C

Combustion Air

0 GJ/hr 40C 40C

NOTES 1. Bitumen Production: 30,000 BPD 2. Naphtha Diluent used to produce Dilbit 3. Gas Lift used for well production 4. Natural Gas used in OTSG burners 5. Heat transferred from steam to bitumen at 8 C 6. Boiler efficiency = 90% 7. The heat duty shown for boilers includes produced gas

Figure 7. Overall Heat Integration for SAGD Surface Facilities

Figure 8. Composite Curves

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Figure 9. Temperature difference vs. Heat Exchanger network Area

Figure 10. Overall costs vs. T minimum

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