Air Injection Studies For Enhanced Oil Recovery
Conventionally, air injection has been used for recovery of heavy crude oil in the production field, but studies have shown that depletion of light crude oil in the reservoir leads to abandonment of such wells. Hence, this work studied the kinetics and combustion of light crude oil in-situ the reservoir to understand their potentials for high-pressure air injection (HPAI) enhanced oil recovery (EOR). Advanced thermo-kinetic simulation and Pressure-Volume-Temperature tools (AKTS and PVTsim) were coupled with non-isothermal Differential Scanning Calorimetry (DSC) measurements and Accelerating Rate Calorimeter (ARC) for the studies.
The combustion and kinetics of three (3) light crude oils obtained from Offshore of Newfoundland, Canada were precisely described by the methods. It was observed that the crude with the lowest API of 30.214 had the lowest enthalpy change of 10.9 J/g and the highest onset oxidation temperature of 220 oC, while the crude with the highest API gravity of 46.963 had the highest enthalpy of 24.6 J/g and the lowest onset oxidation temperature of 140 oC. Effect of 10% water saturation of one of the crude samples (Sample A) was studied and it was observed that there was increase in the onset oxidation temperature by 40 oC and lowering of the enthalpy change by 9 J/g. These findings provided evidence that the versatile Differential Scanning Calorimetry thermograms when coupled with kinetic simulation technique can yield reliable results with respect to oil recovery with high correlation coefficient (r > 0.9).
This reliable information such as onset, peak and endset temperatures with their respective heat flow patterns, could then be used to provide precise thermo-kinetic parameters. Kinetic triplets such as activation energy, pre-exponential and the reaction model necessary for reservoir screening in an air injection EOR process can also be accurately determined. Mine tailings containing high pyrrhotite content were then used as catalyst to study its effect on the onset oxidation temperature of the crude oils using ARC.
An amount of 20% tailings in crude oil lowered the average onset oxidation temperature from 148 oC to 116 oC. It also had the widest oxidation temperature range of 63 oC between the onset and endset temperature, as well as the highest pressure drop of 2.4 bar, which signifies high conversion in the crude oil oxidation reaction as well as production of miscible flue gas which favoured enhanced oil recovery process. Products of air combustion products in-situ was studied as an injectant in a light oil Nigerian reservoir using a simulated slim tube experiment and was observed than flue gas products from air oxidation at high temperature and pressure favoured enhanced oil recovery.
Enhanced Oil Recovery (EOR) is a tertiary recovery process which is normally applied after primary and secondary recovery, to mobilize oil trapped in pores by vicious capillary forces. Thermal, chemical, solvent and gases are the most common form of various EOR process (Isco, 2007). Due to the decline of oil reserves caused by the rising oil production, and clamours for environmentally friendly practice in EOR techniques, petroleum engineers are currently driving EOR projects towards more efficient techniques. One of such efficient technique is the Air/Flue gas injection which is motivated by inexpensive source of air as well as environmentally friendly carbon-dioxide sequestration. The motivation for the use of air as an injectant in the EOR project is because of its abundance, availability and low cost. It can simply be supplied by the use of a compressor, with overall project having low initial and operating cost in comparison to other EOR methods (JOGMEC, 2011).
Air for increasing oil recovery from reservoirs dates back to the 1940’s and early 1950’s (Hvizdos et al., 1983) and by the 1960s and 1970, about forty (40) in-situ full field or pilot projects had been undertaken throughout the world with North America topping such projects (Pwaga et al., 2010). This technique, apart from laboratory studies has been implemented in fields such as West Hackberry in Louisiana, Horse Creek North and South Dakota, Zhongyuan and Liaoche oil fields in China, H field in Indonesia, South Bridge in California and other countries such as Romania, United Kingdom, Japan, Canada, India, Argentina, Venezuela have maintained laboratory and field studies too (Sakthikumar et al., 1996; Ren et al., 1999; Mendoza et al., 2011; Niu et al., 2011; Iwata et al., 2001; Xia et al., 2004; Zhu et al., 2001). Air has also been used in heavy oil recovery and enhancement of this technique can lead to significant light oil production (Surguchev et al., 1998).
An alternative to air injection is the flue gas (which contains nitrogen and carbon-dioxide) produced from the combustion of oxygen contained in the air to sweep oil. This EOR technique, when applied to light oil is known as light oil air injection while in heavy oil reservoir, it is called in-situ combustion. (Kuhlman, 2004; Teramoto et al., 2006; Turta et al., 2007; Li et al., 2009).
Some studies carried out to describe criteria as well as performance of air injection projects gave positive results even though experimental condition could not mimic the adiabacity of the reservoir (Sakthikumar et al., 1996). Temperature regimes, heat energy content, pressure and temperature dependence during oil combustion were also studied using simple Arhennius type model which assumes constant kinetic parameters throughout the reaction (Hvizdos et al., 1983; Elgibaly, 1998; Niu et al., 2011; Li, et al., 2009). There have been few researches which reports on the complexities of combustion reaction of crude oils where kinetic parameters fluctuate or the alteration of the oxidations zones.
1.2 Problem Statements
The following are the problems, research gaps in literature and previous studies on enhanced oil recovery with reference to light oil air injection projects.
- Despite the several thermal and kinetic studies carried out on Enhanced oil recovery, there has been no research that addresses how the kinetic parameters fluctuate during the combustion process for the benefit of oil recovery.
- Arrhenius type of equation and simple nth order model have been used to study oxidation of oil and these do not adequately capture the complexity of the reaction (such as the trend of activation energy as reaction progresses), therefore, the need to study other models and techniques.
- Very few and scanty literatures exist that captures the various oxidation reaction zones of crude oil.
- Interaction parameters of temperature, pressure, extent of air oxidation and sulphur content on crude oil recovery have received low discussions in literatures, hence the need to focus attention on them for the benefit of enhanced oil recovery especially in countries like Nigeria where EOR is yet to be fully practiced.
1.3 Justifications of Research
This sections highlights the benefits of this research.
- This study will provide insight into improved oil recovery from low producing/ abandoned wells.
- The iso-conversional approach will help capture the complexity of crude oil oxidation reaction.
- The findings will benefit upstream companies operating in Nigeria currently at the secondary production stage and at verge of abandoning the wells.
- This will also help the federal Government of Nigeria during negotiations of oil well sales.
- It will open up researches and studies on catalytic potential of Nigeria’s mine tailings.
1.4 Aim and Objectives of Research
The aim of this research work was to investigate the kinetics, combustion of air and combustion air products for enhanced oil recovery.
This aim was achieved by the following objectives:
- Using differential iso-conversional method to describe oxidation of crude oils in-situ reservoir.
- Studying oxidation behaviours of crude oil in an adiabatic environment to verify non-isothermal differential scanning calorimetry (DSC) results.
- Using tailing as catalyst to alter oxidation characteristics of crude oils.
- Studying the combined effect of parameters of reservoir conditions such as temperature, pressure, gas content on oil recovery.
1.5 Scope of Research
The scope of this research are focussed on:
- Combustion behaviours and kinetics of the oxidation reaction of low pressure reservoir of light crude oils.
- The use of mine tailings to improve the reactions for the benefit of improved oil recovery.
- Modelling of air injection process with respect to the progress of oxidation kinetics with temperature.
- The use of crude oil databank and PVT tests to analyse the enhanced oil recovery process for Nigerian crude oils.
5.0 Conclusions and Recommendations
Based on the thermal and mineral analyses carried out using the light oils, mine tailings, and air combustion product in-situ the reservoir at high temperature and pressures, the following conclusions and recommendations can be made.
- The superiority of the iso conversional studies can easily be observed based on the activation energy determination. Sample A when analysed using the iso-conversional method, had a non-constant activation energy between 50 – 235 kJ/mol as the reaction progressed to completion while with the simple Arrhenius model analysis, a constant activation energy of 133.6 kJ/mol was determined for the reaction. This is evident in the other samples analysed.
- The detected onset oxidation temperatures of the reactions for sample A was in the low temperature oxidation zones (140 oC) while samples B (215 oC) and C (220 oC) recorded their onset oxidation temperature in the medium temperature oxidation zones respectively.
- The estimated activation energies using the ASTM E-698 method were observed to be higher for the lower API gravity crude samples. Sample A which high API gravity of 46.963 had a low activation of 133.6 kj/mol while sample C with low API gravity of 30.214 recorded the highest activation energy of 176.5 kj/mol.
- The apparent activation energy using the iso-conversional method increased as the reaction progressed with sample A recording a peak at α = 0.58 due to production of flue gases which hampers oxidation reaction hence the larger activation barrier.
- The oxidation reaction using 20 wt% tailings caused the oxidation reaction to span from the low temperature oxidation zone to medium temperature oxidation zone (where the onset of 106 oC to enset up to 200oC), thereby increasing its suitability in an EOR project involving a low and medium temperature oxidation reactions.
- In the study of the use of tailings to lower oxidation temperature of light crude oil in-situ the reservoir, 20% tailings on the crude oil recorded the lowest onset oxidation and highest endset temperature making it a best tailing amount required for an Air injection EOR project of the crude oil.
- Air injection EOR is favoured by high reservoir temperature, pressure as well as the oxidation of injected air.
- Future work should investigate the risk analysis of Air injection projects operating on catalytic enhancements.
- A comparison of historical data vs simulated data should be carried out for future reliability analysis.
- A study of Coking problem during air injection projects should be carried out to see effect on overall oil recovery.
- Although 20% tailings showed the highest temperature lowering of the onset temperature, screening different crude oils for their response to specific tailings is highly recommended before field project.
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