Oil & Gas - Maximising Recovery Programme (formerly SHARP) IOR Views

Introduction

Many large oil fields in the UKCS are producing significantly below their early plateau production rates and total oil production has been declining since 1999. Maximising oil recovery from the mature areas of the UKCS represents a huge challenge for the E & P sector.

The only large scale EOR process for light oil reservoirs currently being applied is miscible gas injection in Prudhoe Bay. All other IOR/EOR developments (mainly U.S.) are small scale. Thus, water flooding is still the main recovery method in UKCS fields. Oil recovery factors for UKCS mature fields range from 10% to 70% OOIP, with the average around 45%. Since 1976, the total oil production from UKCS oil fields is 21 billion barrels. Thus, there are more than 20 billion barrels of oil remaining as a target for IOR. In addition, it is estimated that there are about 10 billion barrels of heavy oil in place in the UKCS (less than 25 °API), that remain undeveloped.

Therefore, maximising oil recovery from UKCS ‘Brownfields’ and developing new oil fields are key to sustaining the UKCS as a major oil province. Innovative IOR technologies and advanced EOR processes will play an increasingly important role in this.

Several EOR methods have been considered for UKCS light oil fields, such as hydrocarbon gas injection, polymers and gels, microbial, CO2 flooding and air injection. Only hydrocarbon gas re-injection has been successfully applied in the UKCS. The application of EOR in the UKCS has been reviewed by Jayasekera and Goodyear [1] recently. Here, we will focus on the potential of air injection as an IOR method for the UKCS.

Light Oil Air Injection

Air injection for light oil recovery has been applied successfully in high pressure reservoirs, in the Williston Basin in the Northwest US; Buffalo Red River Unit (BRRU), Medicine Pole Hills Unit (MPHU) and Horse Creek are well-documented [2, 3].

Light oil air injection (LOAI) creates in situ oxidation reactions between the oil and injected oxygen, producing a 'flue gas', for reservoir oil displacement. Only a small amount of oil is consumed, leaving the rest to be displaced and produced. The benefits of LOAI are:

• Air is freely available (everywhere)
• Spontaneous oil ignition occurs in the reservoir, and complete oxygen utilisation is achieved so that there is no safety problem associated with oxygen breakthrough.
• High oil recovery via gravity-stabilised immiscible gas displacement, including rapid reservoir pressurisation, flue gas stripping and oil swelling.

Incremental oil recovery for LOAI in the MPHU project was 14 to 16% OOIP [4]. LOAI projects are continuing to be successful after more than 15 years of operation, but the scale of these projects remains relatively small compared to North Sea operations. Horizontal wells are used as injectors and producers in the current BRRU and MHPU projects, increasing both air injection rate and oil production rate.

Reaction Modelling for Light Oil Air Injection

Accelerating-Rate Calorimeter (ARC) [5] and combustion tube (CT) tests are two well-documented experimental methods for screening and appraisal of LOAI projects. Numerical simulation has also been used for feasibility studies [6,7].

ARC has been used to study the in situ combustion kinetics and obtain reaction parameters, such as Arrhenius activation energy, order of reaction, and pre-exponential factor. It can also provide information on the spontaneous ignition property of crude oils at reservoir pressure.

For any chemical reaction experiment, it is important to identify both the reactants and products. However, the ARC test is only able to provide global reaction kinetic parameters. Because light oil contains a large number of components, many reactions can take place simultaneously. For numerical modelling, the reaction parameters of selected pseudo hydrocarbon components are required, and this remains to be accomplished.

The IOR Group at the University of Bath is seeking funding to develop a new method for kinetic modelling of LOAI. This will provide the kinetic parameters for all of the oxidation reactions needed to develop a new numerical simulation model.

Combustion Tube Tests

Combustion tube experiments are used to investigate how a high temperature combustion front propagates through a reservoir medium, at reservoir conditions. The results have been used as a reference data set for validation of numerical simulation models. However, the combustion tube results are not scaleable, since it is one-dimensional experiment. Combustion tube experiments are unable to provide any information regarding the effect of oil column thickness and lateral displacement [8]. High air fluxes are normally used in light oil combustion tube experiments, ranging up to very high values; 30 to 160 Sm³/m²hr [9,10]. Such high air fluxes can only be achieved in the region close to the injector. When the combustion front moves away from the injector towards the producer, the air flux decreases as a function of radial distance. During the lifetime of an LOAI project, the average air flux is likely to be less than 1 Sm³/m²hr. Thus, at such low air fluxes, high temperature oxidation, or full combustion (>350°C), as observed during combustion tube tests, is very unlikely to occur in the reservoir. Thus, in reality, the process is one of low temperature oxidation (LTO), occurring at temperatures significantly less than 350°C. The effect of low air injection flux was investigated at Bath University in a series of special combustion tube tests (actually isothermal tube tests, at reservoir temperature), ranging from 0.34 to 1.37 Sm³/m²hr, at reservoir temperature [7].

 

Numerical Simulation

A number of numerical simulation studies have been carried out for appraisal of light oil air injection projects [3, 6, 7, 11-15]. Different approaches have been used to represent the oil components for modelling the 'in situ' combustion reactions.

• Prior to 1990, 2D numerical models were used to assess air injection process in a waterflooded North Sea reservoir, and also the West Heidelberg In-Situ Combustion Project, respectively by Hughes [11] and Kumar [12].
• During 1995 to 1997, 3D numerical simulations of air injection into light oil reservoirs were conducted. Sakthikumar et al [13] used nitrogen injection to represent air injection. Others employed black oil simulators [3,14, 15].
• Most recently, Surguchev et al [6] and Ren et al [7] have used the STARS simulator to simulate LOAI in North Sea reservoirs.

The LOAI simulation models developed by Surguchev et al and Ren et al, are compared in Table 1.

Table 1: Comparison of the Field-Scale Simulation Models

Surguchev’s model is very similar to models which have been used for in situ combustion-HTO for heavy oil recovery, and hence involve thermal cracking reactions. Ren's model uses LTO kinetics. Thus, these models represent two different understandings of how light oil air injection works. Which one is correct? Because no field production data is available to test the predictions against, the answer is unknown.

UKCS Potential

Is LOAI in UKCS reservoirs likely to be a major IOR technology? Air injection is a very effective IOR method, and has been proven successful in the US.

The light oil reservoirs in the UKCS have much thicker oil columns than those at West Hackberry (21m, steep dipping), MPHU (5.5m), BRRU (3m) and Horse Creek (6m) LOAI projects. Gas overriding occurs when air is injected into a thick oil reservoir [11, 12], and this has a negative impact on recovery.

LOAI - Major, New Technology - What Needs To Be Addressed?

How can the air injection technique be adapted for UKCS light oil reservoirs? The UK industry needs to do its 'homework'.

• Long term planning is needed to develop innovative IOR/EOR techniques, since it usually takes more than 10 year from screening of recovery processes to start-up of a commercial IOR project.
• New methods are needed to obtain more correct kinetic models for the in situ oxidation reactions, occurring in LO reservoirs during air injection.
• Numerical simulation is ultimately the key tool for the reservoir engineer to assess all of the details of LOAI projects. If it is not possible to predict the temperature in the reservoir resulting from oxidation reactions, then the simulation predictions will be invalid.

As pointed out by Jayasekera and Goodyear [1], there is only a very limited window of opportunity for the application of IOR in UKCS mature oil fields, perhaps 20 years at the most. Therefore, the development of IOR projects for UKCS oil fields should be undertaken as soon as possible.

Finally, LOAI is a sustainable process, since the CO2 produced can be kept in the reservoir (no gas breakthrough), or, if production is continued beyond gas breakthrough, it can be separated and reinjected into neighbouring reservoirs (cascaded from one to another), and finally sequestrated. CCS (Carbon Capture and Storage) is not required, saving $0.5/tonne CO2/100km [16]. Sustainability is also achieved because air is available in whatever quantity required. Thus, large-scale, integrated air injection is the way forward for obtaining future incremental oil production from the UKCS. The North Sea could be a world test-bed, and a world leader.

References

1. A. J. Jayasekera and S. G. Goodyear, Improved Hydrocarbon Recovery in the United Kingdom Continental Shelf: Past, Present and Future, SPE 75171, Presented at the SPE/DOE Improved Oil Recovery Symposium, Tulsa, Oklahoma, April 13-17 (2002)
2. M. R. Fassihi, D. V. Yannimaras, E. E. Westfall, and T. H. Gillham, Economics of Light Oil Air Injection Projects, SPE 35393, presented at the SPE/DOE tenth Symposium on Improved Oil Recovery, Tulsa, Oklahoma, April 21-24 (1996)
3. Clara, V. Zelenko, P. Schirmer, T. Wolter, Appraisal of Horse Creek Air Injection Project performance, SPE 49519, presented at 8th Abu Dhabi International Petroleum Exhibition & Conference, Abu Dhabi, United Arab Emirates, October 11-14 (1998)
4. M. R. Fassihi, D. V. Yannimaras, and V. K. Kumar, Estimation of Recovery Factor in Light Oil Air Injection Projects, SPE 28733, presented at the SPE International Petroleum Conference & Exhibition of Mexico, Veracruz, Mexico, October, 10-13 (1994)
5. V. Yannimaras, and T. H. Gillham, Screening of Oils for In-Situ Combustion at Reservoir Conditions by Accelerating-Rate Calorimetry, SPE Reservoir Engineering, February (1995), pp36-39.
6. L. M. Surguchev, A. Koundin and D. V. Yannimaras: Air Injection – Cost Effective IOR Method to Improve Oil Recovery from Depleted and Waterflooded Fields. SPE 57296, 1999 Asia Pacific Improved Oil Recovery Conference, 25-26, Oct (1999)
7. S. R. Ren, M. Greaves and R. R. Rathbone: Air Injection LTO Process: An IOR Technique for Light- Oil Reservoirs, SPE Journal, March (2002), pp90-100.
8. M. R. Islam and S. M. Farouq Ali: Scaling Criteria for In- Situ Combustion Experiments, Paper SS89-01, presented at the Third Technical Meeting of South Saskathchewan Section, the Petroleum Society of CIM, Regina, Canada, September (1989)
9. E.S. Juan, A. Sanchez, A del Monte, R. G. Moore, S. A. Mehta and M. G. Ursenbach: Laboratory Screening for Air Injection-Based IOR in Two Waterflooded Light Oil Reservoirs, Paper CIPC 2003-215, presented at Petroleum Society’s Canadian International Petroleum Conference, Calgary, Alberta, June 10-12 (2003)
10. D.L. Tiffin and D. V. Yannimaras: The In-Situ Combustion Performance of Light Oil as a Function of Pressure (1000 to 6000 psig), IN SITU, (1997) 21, No. 1, pp47-64
11. D S Hughes: A Numerical Assessment of Oxygen-Supported In-Situ Combustion as an EOR Process in a Waterflooded North Sea Reservoir, the 3rd European Symposium on IOR, Rome, April 1985
12. Kumar: A Cross-Sectional Simulation of West Heidelberg In-Situ Combustion Project, SPE Reservoir Engineering, February (1991) pp46-54.
13. S. Sakthikumar, K. Madaoui and J. Chastang: An Investigation of the Feasibility of Air Injection into a Waterflooded Light Oil Reservoir, SPE 29806, presented at SPE Middle East Oil Show, March 11-14 (1995)
14. T. H. Gillham, B. W. Cerveny, E. A. Turek and D. V. Yannimaras: Keys to Increase Production via Air Injection in Gluf Coast Light Oil Reservoirs, SPE 38848, presented at 1997 SPE Annual Technical Conference and Exhibition, Texas, 5-8, October (1997)
15. M. L. Fraim, P. D. Moffitt nad D. V. Yannimaras: Laboratory Testing and Simulation Results for High Pressure Air Injection in a Waterflooded North Sea Oil Reservoir, SPE 38905, presented at the SPE Annual Technical Conference & Exhibition, San Antonio, October, 5-8 (1997)
16. http://www.ieagreen.org.uk/